valw/docs/superpowers/plans/2026-09-29-valw-phase-0-1.md

# valw Phase 0 + 1 Implementation Plan

> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.

**Goal:** Ship `valw screen`, `valw region` and `valw doctor`: pixel-exact PNG screenshots on niri that are saved, copied to the clipboard, logged, and explained when they fail. Everything is packaged as a dendritic Nix flake.

**Architecture:** One short-lived Rust process per command. The process captures every needed output up front through wlr-screencopy and keeps the pixels in the compositor's own byte order (B, G, R, X), so the overlay appears fast even on a slow machine. For `region` it shows the frozen frames in layer-shell overlays and crops in memory; conversion to RGBA happens only for what gets saved. It then writes the PNG and forks a small child that serves the clipboard. Pure logic (pixel formats, selection math, overlay pixels, file names, config, errors) is kept apart from the SCTK/Wayland glue so it can be unit tested without a compositor.

**Tech Stack:** Rust 2024 (stable, from nixpkgs), smithay-client-toolkit 0.21, wayland-protocols(-wlr), niri-ipc 26.4.0, wl-clipboard-rs, image (png), clap, serde/toml, tracing, rustix/libc, anyhow; Nix flake-parts with `*.ulu.nix` auto-discovery; jj.

**Spec:** `docs/superpowers/specs/2026-09-29-valw-phase-0-1-design.md` (read it first; this plan implements it).

## Global Constraints

- Code, comments, error/log messages: English. Comments are plain and short and say *why*.
- Version control is **jj** (the repo is non-colocated; there is no `.git`). Commit with `jj commit -m "<msg>"`. Never use git commands.
- Use `cargo clippy` (never `cargo check`) and `cargo nextest run` (never `cargo test`).
- Run every cargo command inside the devShell. direnv loads it automatically once `.envrc` is allowed; otherwise prefix the command with `nix develop -c`.
- `CARGO_TARGET_DIR` is `${XDG_CACHE_HOME:-$HOME/.cache}/valw/target` (set by the devShell). A `target/` directory must never appear in the repo, because the `path:` flake would copy it into the store on every evaluation.
- The flake discovers `nix/**/*.ulu.nix` automatically. Never write a manual `imports = [ ... ]` list.
- Prefer nix3 commands (`nix build`, `nix develop`, `nix flake check`). Don't `find` over `/nix/store`.
- `.gitignore` is a whitelist. Any new top-level file or directory must be added to it, or jj silently leaves it out of commits.
- Dependency versions are exactly those in Task 1's `Cargo.toml`; `niri-ipc` stays pinned at `=26.4.0`.
- Layer-shell namespace: `valw-overlay`. Exit codes: 0 success, 1 error, 2 usage (clap), 3 cancelled.
- Until Task 11, `src/main.rs` starts with `#![allow(dead_code)]` because modules land before the CLI uses them. Task 11 removes it, and clippy must then be clean without it.

## Review Focus

These are the input classes most likely to bite a real user that no unit test fully exercises. Each one has a check in the task that owns the code.

1. **Rotated or flipped outputs** (`transform != Normal`): `screen` and `region` must produce an upright image identical to grim's. Unit test: `frame::tests::transforms_match_grim` (Task 6). Real check: Task 13, step 4.
2. **Fractional scale (1.25)**: the overlay must look sharp (not resampled), and the crop must be pixel-exact against grim. Unit tests: `selection::tests::fractional_scale_rounds_outwards` (Task 7). Real check: Task 13, step 2.
3. **A drag that leaves the anchor output** (press on one monitor, release on another, or outside every monitor): the result must be clipped to the monitor where the drag began and never panic. Unit tests: `selection::tests::drag_across_outputs_is_clipped_to_the_output` and `output_offsets_including_negative` (Task 7). Real check: Task 11, step 7.
4. **Hotkey pressed again while an overlay is open**: the second process exits 1 with "valw is already running" and a hint, and the first keeps working. The same must hold after the first one has exited while its clipboard child is still alive. Unit tests: `lock::tests::second_acquire_fails_until_release`, `detach::tests::child_holds_neither_lock_nor_stdio` (Task 5). Real check: Task 10, step 8.
5. **Unwritable log or save locations**: an unwritable log dir must not block a screenshot (stderr-only fallback). An unwritable save dir must fail with a message that names the path. Unit tests: `log::tests::open_run_log_fails_on_unwritable_dir` (Task 4), `output::tests::write_atomic_reports_unwritable_dir` (Task 8). Real check: Task 13, step 6.

---

## File Structure

| File | Responsibility |
|---|---|
| `flake.nix` | flake-parts entry; auto-discovers `nix/**/*.ulu.nix` |
| `nix/package.ulu.nix` | `packages.default` via `rustPlatform.buildRustPackage` |
| `nix/devshell.ulu.nix` | `devShells.default`; sets `CARGO_TARGET_DIR` outside the repo |
| `nix/formatter.ulu.nix` | `formatter` (nixfmt over all `.nix` files) |
| `nix/checks.ulu.nix` | `checks`: clippy, fmt, nextest, headless sway |
| `.envrc`, `.gitignore` | direnv hookup; whitelist ignore file |
| `Cargo.toml` | crate manifest, all Phase 1 dependencies |
| `src/main.rs` | clap CLI, run/exit-code plumbing, capture → deliver flow |
| `src/error.rs` | `Hint`, `HintExt`, `Cancelled`, three-part `render` |
| `src/config.rs` | config struct, defaults, TOML parse with unknown-key report |
| `src/log.rs` | per-run log file, pruning, stderr warnings, panic hook |
| `src/lock.rs` | single-instance `flock` |
| `src/detach.rs` | fork a child that outlives valw with no lock and no stdio |
| `src/frame.rs` | `OutputGeom`, `PixelRect`, `Frame` (B,G,R,X pixels), shm format → BGRX, RGBA for saving, output transforms |
| `src/selection.rs` | drag state machine; logical → physical pixel rectangles |
| `src/render.rs` | overlay pixels: dim, paste undimmed selection, outline |
| `src/output.rs` | file names, unique paths, PNG encode, atomic write, stdout, clipboard |
| `src/niri.rs` | niri IPC: focused output, version, version parsing |
| `src/wayland.rs` | SCTK connection, state, handlers, dispatch helpers |
| `src/capture.rs` | `Capturer` trait; wlr-screencopy implementation |
| `src/region.rs` | layer-shell overlay surfaces and pointer/keyboard handling |
| `src/doctor.rs` | `valw doctor` checks and report |

Unit tests live in a `#[cfg(test)] mod tests` block at the bottom of each file.

---

### Task 1: Flake, devShell and crate skeleton (Phase 0)

**Files:**
- Create: `flake.nix`, `nix/package.ulu.nix`, `nix/devshell.ulu.nix`, `nix/formatter.ulu.nix`, `.envrc`, `.gitignore`, `Cargo.toml`, `src/main.rs`
- Generated: `flake.lock`, `Cargo.lock`

**Interfaces:**
- Consumes: nothing.
- Produces: `nix build` → `result/bin/valw`; `nix develop` shell with `cargo`, `clippy`, `rustfmt`, `cargo-nextest`, `rust-analyzer`, `grim`, `imagemagick`, `wayland-info`; `CARGO_TARGET_DIR` set outside the repo.

- [ ] **Step 1: Write `flake.nix`**

```nix
{
  description = "valw, macOS-style screenshots for niri";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";

    flake-parts = {
      url = "github:hercules-ci/flake-parts";
      inputs.nixpkgs-lib.follows = "nixpkgs";
    };
  };

  outputs =
    inputs@{ flake-parts, ... }:
    flake-parts.lib.mkFlake { inherit inputs; } (
      { lib, ... }:
      let
        inherit (lib.filesystem) listFilesRecursive;
        inherit (lib.lists) filter;
        inherit (lib.strings) hasSuffix;
      in
      {
        systems = [
          "x86_64-linux"
          "aarch64-linux"
        ];
        imports = filter (hasSuffix ".ulu.nix") (listFilesRecursive ./nix);
      }
    );
}
```

- [ ] **Step 2: Write `nix/package.ulu.nix`**

```nix
{ ... }:
{
  perSystem =
    { pkgs, lib, ... }:
    {
      packages.default = pkgs.rustPlatform.buildRustPackage {
        pname = "valw";
        version = "0.1.0";

        src = lib.cleanSourceWith {
          src = ../.;
          filter =
            path: _type:
            !(builtins.elem (baseNameOf path) [
              ".direnv"
              ".jj"
              "result"
              "target"
            ]);
        };

        cargoLock.lockFile = ../Cargo.lock;

        nativeBuildInputs = [ pkgs.pkg-config ];
        buildInputs = [
          pkgs.libxkbcommon
          pkgs.wayland
        ];

        # Tests run through `nix flake check` instead.
        doCheck = false;

        meta = {
          description = "macOS-style screenshots for niri";
          mainProgram = "valw";
          platforms = lib.platforms.linux;
        };
      };
    };
}
```

- [ ] **Step 3: Write `nix/devshell.ulu.nix`**

```nix
{ ... }:
{
  perSystem =
    { pkgs, self', ... }:
    {
      devShells.default = pkgs.mkShell {
        inputsFrom = [ self'.packages.default ];

        packages = with pkgs; [
          cargo-nextest
          clippy
          rust-analyzer
          rustfmt
          # Manual verification: grim is the pixel oracle, magick compares.
          grim
          imagemagick
          wayland-utils
        ];

        # The repo is a path: flake, so Nix copies the whole tree on every
        # evaluation. Keep the multi-gigabyte target dir out of it.
        shellHook = ''
          export CARGO_TARGET_DIR="''${XDG_CACHE_HOME:-$HOME/.cache}/valw/target"
        '';
      };
    };
}
```

- [ ] **Step 4: Write `nix/formatter.ulu.nix`**

```nix
{ ... }:
{
  perSystem =
    { pkgs, ... }:
    {
      formatter = pkgs.writeShellApplication {
        name = "nixfmt-tree";
        runtimeInputs = [
          pkgs.nixfmt
          pkgs.fd
          pkgs.findutils
        ];
        text = ''
          if [ "$#" -eq 0 ]; then
            set -- .
          fi
          fd --type f --extension nix --hidden --exclude .jj . "$@" \
            | xargs -r nixfmt
        '';
      };
    };
}
```

- [ ] **Step 5: Write `.envrc` and `.gitignore`**

`.envrc`:

```sh
watch_file nix/*.ulu.nix
use flake
```

`.gitignore`:

```gitignore
/*
!/.gitignore
!/.envrc
!/flake.nix
!/flake.lock
!/Cargo.toml
!/Cargo.lock
!/nix/
!/src/
!/docs/
!/README.md
```

- [ ] **Step 6: Write `Cargo.toml`**

```toml
[package]
name = "valw"
version = "0.1.0"
edition = "2024"
description = "macOS-style screenshots for niri"
license = "MIT"

[dependencies]
anyhow = "1.0.104"
chrono = "0.4.45"
clap = { version = "4.6.7", features = ["derive"] }
image = { version = "0.25.10", default-features = false, features = ["png"] }
libc = "0.2.189"
niri-ipc = "=26.4.0"
rustix = { version = "1.1.5", features = ["event", "fs", "pipe", "process", "stdio"] }
serde = { version = "1.0.229", features = ["derive"] }
serde_ignored = "0.1.14"
smithay-client-toolkit = "0.21.1"
toml = "1.1.6"
tracing = "0.1.44"
tracing-subscriber = { version = "0.3.23", features = ["env-filter"] }
wayland-client = "0.31.15"
wayland-protocols = { version = "0.32.13", features = ["client", "staging"] }
wayland-protocols-wlr = { version = "0.3.12", features = ["client"] }
wl-clipboard-rs = "0.9.4"

[dev-dependencies]
tempfile = "3.27.0"

# Image encoding is painfully slow unoptimised; keep dev builds usable.
[profile.dev.package."*"]
opt-level = 3
```

- [ ] **Step 7: Write `src/main.rs`**

```rust
// Modules land before the CLI uses them; Task 11 removes this.
#![allow(dead_code)]

use clap::Parser;

#[derive(Parser)]
#[command(version, about = "macOS-style screenshots for niri")]
struct Cli {}

fn main() {
    Cli::parse();
}
```

- [ ] **Step 8: Generate the lock files**

The devShell evaluates the package, and the package reads `Cargo.lock`, so `Cargo.lock` has to exist before `nix develop` can run:

```sh
nix shell nixpkgs#cargo -c cargo generate-lockfile
nix flake lock
```

Expected: `Cargo.lock` and `flake.lock` exist at the repo root.

- [ ] **Step 9: Enter the devShell and build**

```sh
direnv allow
cargo clippy --all-targets -- -D warnings
cargo run -- --version
```

Expected: clippy prints no warnings, and the last command prints `valw 0.1.0`. `ls` shows **no** `target/` directory in the repo, and `echo $CARGO_TARGET_DIR` prints a path under `~/.cache/valw/`.

- [ ] **Step 10: Build with Nix and check the flake copy stays small**

```sh
nix build
./result/bin/valw --version
du -sh "$(nix flake metadata --json | jq -r .path)"
```

Expected: `valw 0.1.0`, and the flake source is a few MB at most. If it is much larger, something like `target/` or `result` has leaked into the tree.

- [ ] **Step 11: Commit**

```sh
jj commit -m "build: dendritic flake, devShell and crate skeleton"
```

---

### Task 2: Error type and rendering

**Files:**
- Create: `src/error.rs`
- Modify: `src/main.rs` (add `error` to the `mod` list)

**Interfaces:**
- Consumes: nothing.
- Produces: `pub struct Hint(pub String)`; `pub struct Cancelled` (implements `std::error::Error`); `pub trait HintExt<T> { fn hint(self, hint: impl Into<String>) -> anyhow::Result<T>; }` implemented for every `Result<T, E: Into<anyhow::Error>>`; `pub fn render(err: &anyhow::Error, log: Option<&Path>) -> String`.

- [ ] **Step 1: Write the failing tests for `error`**

Create `src/error.rs` with only the test module:

```rust
#[cfg(test)]
mod tests {
    use super::*;
    use anyhow::{Context, anyhow};
    use std::path::PathBuf;

    #[test]
    fn renders_all_parts() {
        let err = Err::<(), _>(anyhow!("compositor offered only XBGR2101010"))
            .context("could not convert the frame")
            .hint("run `valw doctor` and include its output in a bug report")
            .context("could not capture output HDMI-A-1")
            .unwrap_err();
        let log = PathBuf::from("/tmp/valw.log");

        assert_eq!(
            render(&err, Some(&log)),
            "error: could not capture output HDMI-A-1\n\
             \x20 cause: could not convert the frame\n\
             \x20 cause: compositor offered only XBGR2101010\n\
             \x20 hint:  run `valw doctor` and include its output in a bug report\n\
             \x20 log:   /tmp/valw.log\n"
        );
    }

    #[test]
    fn renders_without_hint_or_log() {
        let err = anyhow!("disk full");
        assert_eq!(render(&err, None), "error: disk full\n");
    }

    #[test]
    fn hint_as_outermost_layer_is_not_the_headline() {
        let err = Err::<(), _>(anyhow!("valw is already running"))
            .hint("wait for the other capture to finish")
            .unwrap_err();
        assert_eq!(
            render(&err, None),
            "error: valw is already running\n\
             \x20 hint:  wait for the other capture to finish\n"
        );
    }

    #[test]
    fn cancelled_is_detectable() {
        let err = anyhow::Error::new(Cancelled).context("region selection");
        assert!(err.downcast_ref::<Cancelled>().is_some());
    }
}
```

Add a `mod` list to `src/main.rs`, right after the `#![allow(dead_code)]` line, containing `mod error;`.

- [ ] **Step 2: Run the tests and watch them fail**

Run: `cargo nextest run error::`
Expected: compilation fails, e.g. `` cannot find type `Hint` in this scope ``.

- [ ] **Step 3: Implement `error`**

Put this above the test module in `src/error.rs`:

```rust
use std::fmt;
use std::path::Path;

/// A concrete next step for the user, attached to an error with [`HintExt::hint`].
#[derive(Debug)]
pub struct Hint(pub String);

impl fmt::Display for Hint {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(&self.0)
    }
}

/// The user cancelled the capture. Not a failure; maps to exit code 3.
#[derive(Debug)]
pub struct Cancelled;

impl fmt::Display for Cancelled {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str("cancelled")
    }
}

impl std::error::Error for Cancelled {}

pub trait HintExt<T> {
    fn hint(self, hint: impl Into<String>) -> anyhow::Result<T>;
}

impl<T, E> HintExt<T> for Result<T, E>
where
    E: Into<anyhow::Error>,
{
    fn hint(self, hint: impl Into<String>) -> anyhow::Result<T> {
        self.map_err(|e| e.into().context(Hint(hint.into())))
    }
}

/// Renders an error as `error:` / `cause:` / `hint:` / `log:` lines.
pub fn render(err: &anyhow::Error, log: Option<&Path>) -> String {
    let hint = err.downcast_ref::<Hint>().map(|h| h.0.as_str());
    // Hints live in the context chain too; skip them so they print once.
    let mut messages = err
        .chain()
        .map(|e| e.to_string())
        .filter(|m| Some(m.as_str()) != hint);

    let mut out = format!("error: {}\n", messages.next().unwrap_or_default());
    for cause in messages {
        out.push_str(&format!("  cause: {cause}\n"));
    }
    if let Some(hint) = hint {
        out.push_str(&format!("  hint:  {hint}\n"));
    }
    if let Some(log) = log {
        out.push_str(&format!("  log:   {}\n", log.display()));
    }
    out
}
```

- [ ] **Step 4: Run the tests and watch them pass**

Run: `cargo nextest run error::`
Expected: all 4 tests in `error::tests` pass.

- [ ] **Step 5: Lint**

Run: `cargo fmt && cargo clippy --all-targets -- -D warnings && cargo nextest run`
Expected: no warnings; every test passes.

- [ ] **Step 6: Commit**

```sh
jj commit -m "feat: three-part error rendering with hints"
```

---


### Task 3: Config file

**Files:**
- Create: `src/config.rs`
- Modify: `src/main.rs` (add `config` to the `mod` list)

**Interfaces:**
- Consumes: `error::HintExt` (Task 2).
- Produces: `pub struct Config { pub save: Save, pub capture: Capture }` (Default, Deserialize, PartialEq); `pub struct Save { pub directory: String, pub filename: String, pub copy_to_clipboard: bool }`; `pub struct Capture { pub show_cursor: bool }`; `impl Config { pub fn save_dir(&self) -> PathBuf }`; `pub fn home() -> PathBuf`; `pub fn default_path() -> PathBuf`; `pub fn load(path: &Path) -> Result<Config>`; `pub fn parse(text: &str) -> Result<(Config, Vec<String>)>`; `pub fn expand_home(path: &str, home: &Path) -> PathBuf`.

- [ ] **Step 1: Write the failing tests for `config`**

Create `src/config.rs` with only the test module:

```rust
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn empty_file_gives_defaults() {
        let (config, unknown) = parse("").unwrap();
        assert_eq!(config, Config::default());
        assert!(unknown.is_empty());
    }

    #[test]
    fn partial_file_keeps_other_defaults() {
        let (config, _) = parse("[save]\ncopy_to_clipboard = false\n").unwrap();
        assert!(!config.save.copy_to_clipboard);
        assert_eq!(config.save.directory, "~/Pictures/Screenshots");
        assert!(!config.capture.show_cursor);
    }

    #[test]
    fn unknown_keys_are_reported_not_rejected() {
        let text = "[preview]\nenabled = true\n\n[save]\nfolder = \"x\"\n";
        let (config, unknown) = parse(text).unwrap();
        assert_eq!(config, Config::default());
        assert_eq!(
            unknown,
            vec!["preview".to_string(), "save.folder".to_string()]
        );
    }

    #[test]
    fn invalid_toml_names_the_line() {
        let err = parse("[save]\ndirectory = \n").unwrap_err();
        assert!(format!("{err:#}").contains("line 2"), "{err:#}");
    }

    #[test]
    fn wrong_type_is_an_error() {
        assert!(parse("[capture]\nshow_cursor = \"yes\"\n").is_err());
    }

    #[test]
    fn missing_file_gives_defaults() {
        let dir = tempfile::tempdir().unwrap();
        assert_eq!(
            load(&dir.path().join("nope.toml")).unwrap(),
            Config::default()
        );
    }

    #[test]
    fn expands_home() {
        let home = Path::new("/home/u");
        assert_eq!(
            expand_home("~/Pictures", home),
            Path::new("/home/u/Pictures")
        );
        assert_eq!(
            expand_home("$HOME/Pictures", home),
            Path::new("/home/u/Pictures")
        );
        assert_eq!(expand_home("~", home), Path::new("/home/u"));
        assert_eq!(expand_home("/srv/shots", home), Path::new("/srv/shots"));
        assert_eq!(expand_home("~other/x", home), Path::new("~other/x"));
    }
}
```

Add `mod config;` to the `mod` list at the top of `src/main.rs` (keep it sorted).

- [ ] **Step 2: Run the tests and watch them fail**

Run: `cargo nextest run config::`
Expected: compilation fails, e.g. `` cannot find function `parse` in this scope ``.

- [ ] **Step 3: Implement `config`**

Put this above the test module in `src/config.rs`:

```rust
use std::path::{Path, PathBuf};

use anyhow::{Context, Result};
use serde::Deserialize;

use crate::error::HintExt;

#[derive(Debug, Clone, PartialEq, Deserialize, Default)]
#[serde(default)]
pub struct Config {
    pub save: Save,
    pub capture: Capture,
}

#[derive(Debug, Clone, PartialEq, Deserialize)]
#[serde(default)]
pub struct Save {
    pub directory: String,
    pub filename: String,
    pub copy_to_clipboard: bool,
}

#[derive(Debug, Clone, PartialEq, Deserialize, Default)]
#[serde(default)]
pub struct Capture {
    pub show_cursor: bool,
}

impl Default for Save {
    fn default() -> Self {
        Self {
            directory: "~/Pictures/Screenshots".into(),
            filename: "Screenshot %Y-%m-%d at %H.%M.%S.png".into(),
            copy_to_clipboard: true,
        }
    }
}

impl Config {
    /// The save directory with `~` and `$HOME` expanded.
    pub fn save_dir(&self) -> PathBuf {
        expand_home(&self.save.directory, &home())
    }
}

pub fn home() -> PathBuf {
    std::env::var_os("HOME")
        .map(PathBuf::from)
        .unwrap_or_else(|| "/".into())
}

/// `$XDG_CONFIG_HOME/valw/config.toml`, falling back to `~/.config`.
pub fn default_path() -> PathBuf {
    std::env::var_os("XDG_CONFIG_HOME")
        .map(PathBuf::from)
        .unwrap_or_else(|| home().join(".config"))
        .join("valw/config.toml")
}

/// Loads the config. A missing file means defaults.
pub fn load(path: &Path) -> Result<Config> {
    let text = match std::fs::read_to_string(path) {
        Ok(text) => text,
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => return Ok(Config::default()),
        Err(e) => return Err(e).with_context(|| format!("could not read {}", path.display())),
    };
    let (config, unknown) = parse(&text)
        .with_context(|| format!("invalid config file {}", path.display()))
        .hint("fix the file or delete it to use the defaults")?;
    for key in unknown {
        tracing::warn!("ignoring unknown config key `{key}` in {}", path.display());
    }
    Ok(config)
}

/// Parses config text. Returns the config and the keys valw doesn't know.
pub fn parse(text: &str) -> Result<(Config, Vec<String>)> {
    let de = toml::Deserializer::parse(text)?;
    let mut unknown = Vec::new();
    let config = serde_ignored::deserialize(de, |path| unknown.push(path.to_string()))?;
    Ok((config, unknown))
}

pub fn expand_home(path: &str, home: &Path) -> PathBuf {
    if path == "~" || path == "$HOME" {
        return home.to_path_buf();
    }
    for prefix in ["~/", "$HOME/"] {
        if let Some(rest) = path.strip_prefix(prefix) {
            return home.join(rest);
        }
    }
    PathBuf::from(path)
}
```

- [ ] **Step 4: Run the tests and watch them pass**

Run: `cargo nextest run config::`
Expected: all 7 tests in `config::tests` pass.

- [ ] **Step 5: Lint**

Run: `cargo fmt && cargo clippy --all-targets -- -D warnings && cargo nextest run`
Expected: no warnings; every test passes.

- [ ] **Step 6: Commit**

```sh
jj commit -m "feat: config file with defaults and unknown-key warnings"
```

---


### Task 4: Per-run logs and panic hook

**Files:**
- Create: `src/log.rs`
- Modify: `src/main.rs` (add `log` to the `mod` list)

**Interfaces:**
- Consumes: `config::home` (Task 3); `error::{Hint, render}` (Task 2).
- Produces: `pub fn default_dir() -> PathBuf`; `pub fn init(dir: &Path, now: DateTime<Local>) -> Option<PathBuf>` (installs the global subscriber, returns this run's log path or `None` if it fell back to stderr); `pub fn since_start() -> Duration`; `pub fn install_panic_hook(log: Option<PathBuf>)`.

- [ ] **Step 1: Write the failing tests for `log`**

Create `src/log.rs` with only the test module:

```rust
#[cfg(test)]
mod tests {
    use super::*;
    use chrono::TimeZone;

    #[test]
    fn log_names_sort_by_time() {
        let a = Local.with_ymd_and_hms(2026, 9, 29, 9, 5, 1).unwrap();
        let b = Local.with_ymd_and_hms(2026, 9, 29, 19, 0, 0).unwrap();
        assert_eq!(log_name(a), "2026-09-29T09-05-01.000.log");
        assert!(log_name(a) < log_name(b));
    }

    #[test]
    fn prune_keeps_newest() {
        let dir = tempfile::tempdir().unwrap();
        for i in 0..12 {
            File::create(dir.path().join(format!("2026-09-29T10-00-{i:02}.000.log"))).unwrap();
        }
        File::create(dir.path().join("notes.txt")).unwrap();

        prune(dir.path(), 10).unwrap();

        let mut left: Vec<String> = fs::read_dir(dir.path())
            .unwrap()
            .map(|e| e.unwrap().file_name().into_string().unwrap())
            .collect();
        left.sort();
        assert_eq!(left.len(), 11);
        assert_eq!(left[0], "2026-09-29T10-00-02.000.log");
        assert!(left.contains(&"notes.txt".to_string()));
    }

    #[test]
    fn open_run_log_fails_on_unwritable_dir() {
        let dir = tempfile::tempdir().unwrap();
        let blocker = dir.path().join("file");
        File::create(&blocker).unwrap();
        // A regular file where the log dir should be.
        assert!(open_run_log(&blocker, Local::now()).is_err());
    }
}
```

Add `mod log;` to the `mod` list at the top of `src/main.rs` (keep it sorted).

- [ ] **Step 2: Run the tests and watch them fail**

Run: `cargo nextest run log::`
Expected: compilation fails, e.g. `` cannot find function `log_name` in this scope ``.

- [ ] **Step 3: Implement `log`**

Put this above the test module in `src/log.rs`:

```rust
use std::fs::{self, File};
use std::io::IsTerminal;
use std::path::{Path, PathBuf};
use std::sync::{Mutex, OnceLock};
use std::time::{Duration, Instant};

use anyhow::{Context, Result};
use chrono::{DateTime, Local};
use tracing::Level;
use tracing_subscriber::filter::{EnvFilter, filter_fn};
use tracing_subscriber::prelude::*;

use crate::error::{Hint, render};

/// Number of run logs kept on disk, including the current one.
const KEEP: usize = 10;

static START: OnceLock<Instant> = OnceLock::new();

/// Time since logging was set up, i.e. roughly since the process started.
pub fn since_start() -> Duration {
    START.get_or_init(Instant::now).elapsed()
}

/// `$XDG_STATE_HOME/valw/logs`, falling back to `~/.local/state/valw/logs`.
pub fn default_dir() -> PathBuf {
    std::env::var_os("XDG_STATE_HOME")
        .map(PathBuf::from)
        .unwrap_or_else(|| crate::config::home().join(".local/state"))
        .join("valw/logs")
}

/// Sets up tracing: this run's log file at `VALW_LOG` level (default info),
/// plus warnings and errors on stderr. Returns the log file path.
///
/// If the log file can't be created, logging falls back to stderr only and
/// the returned path is `None`; a broken log dir must not block a screenshot.
pub fn init(dir: &Path, now: DateTime<Local>) -> Option<PathBuf> {
    START.get_or_init(Instant::now);
    let file = open_run_log(dir, now);

    // Errors reach stderr through error::render, so only warnings go here.
    let stderr = tracing_subscriber::fmt::layer()
        .with_writer(std::io::stderr)
        .with_ansi(std::io::stderr().is_terminal())
        .without_time()
        .with_target(false)
        .with_filter(filter_fn(|m| *m.level() == Level::WARN));

    let (file_layer, path, failure) = match file {
        Ok((file, path)) => {
            let filter =
                EnvFilter::try_from_env("VALW_LOG").unwrap_or_else(|_| EnvFilter::new("info"));
            let layer = tracing_subscriber::fmt::layer()
                .with_writer(Mutex::new(file))
                .with_ansi(false)
                .with_filter(filter);
            (Some(layer), Some(path), None)
        }
        Err(e) => (None, None, Some(e)),
    };

    tracing_subscriber::registry()
        .with(stderr)
        .with(file_layer)
        .init();

    if let Some(e) = failure {
        tracing::warn!("logging to stderr only: {e:#}");
    }
    path
}

fn open_run_log(dir: &Path, now: DateTime<Local>) -> Result<(File, PathBuf)> {
    fs::create_dir_all(dir).with_context(|| format!("could not create {}", dir.display()))?;
    let path = dir.join(log_name(now));
    let file =
        File::create(&path).with_context(|| format!("could not create {}", path.display()))?;
    prune(dir, KEEP)?;
    Ok((file, path))
}

/// Log names sort chronologically, so pruning can compare names.
fn log_name(now: DateTime<Local>) -> String {
    now.format("%Y-%m-%dT%H-%M-%S%.3f.log").to_string()
}

/// Deletes all but the `keep` newest `.log` files in `dir`.
fn prune(dir: &Path, keep: usize) -> Result<()> {
    let mut logs: Vec<PathBuf> = fs::read_dir(dir)?
        .filter_map(|e| e.ok().map(|e| e.path()))
        .filter(|p| p.extension().is_some_and(|x| x == "log"))
        .collect();
    logs.sort();
    let excess = logs.len().saturating_sub(keep);
    for old in &logs[..excess] {
        fs::remove_file(old)?;
    }
    Ok(())
}

/// Sends panics to the log and prints them in the usual error format.
pub fn install_panic_hook(log: Option<PathBuf>) {
    std::panic::set_hook(Box::new(move |info| {
        let backtrace = std::backtrace::Backtrace::force_capture();
        tracing::error!("panic: {info}\n{backtrace}");
        let err = anyhow::anyhow!("{info}").context(Hint(
            "this is a bug in valw; please report it with the log".into(),
        ));
        eprint!("{}", render(&err, log.as_deref()));
    }));
}
```

- [ ] **Step 4: Run the tests and watch them pass**

Run: `cargo nextest run log::`
Expected: all 3 tests in `log::tests` pass.

- [ ] **Step 5: Lint**

Run: `cargo fmt && cargo clippy --all-targets -- -D warnings && cargo nextest run`
Expected: no warnings; every test passes.

- [ ] **Step 6: Commit**

```sh
jj commit -m "feat: per-run log files and panic hook"
```

---


### Task 5: Single-instance lock and detached child

**Files:**
- Create: `src/lock.rs`
- Create: `src/detach.rs`
- Modify: `src/main.rs` (add `lock`, `detach` to the `mod` list)

**Interfaces:**
- Consumes: `error::HintExt` (Task 2).
- Produces: `pub struct Lock` with `pub fn acquire(path: &Path) -> Result<Lock>` (busy → error `valw is already running` + hint) and `pub fn release(self)`; `pub fn lock::default_path() -> PathBuf`; `pub fn detach::spawn(lock: Lock, work: impl FnOnce()) -> Result<()>` (releases the lock, forks; the child gets `setsid` and /dev/null stdio, runs `work`, then `_exit(0)`).

- [ ] **Step 1: Write the failing tests for `lock`**

Create `src/lock.rs` with only the test module:

```rust
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn second_acquire_fails_until_release() {
        let dir = tempfile::tempdir().unwrap();
        let path = dir.path().join("valw.lock");

        let first = Lock::acquire(&path).unwrap();
        let err = Lock::acquire(&path).unwrap_err();
        assert_eq!(
            err.to_string(),
            "wait for the other capture to finish, or press Esc in it"
        );
        assert_eq!(err.root_cause().to_string(), "valw is already running");

        first.release();
        Lock::acquire(&path).unwrap();
    }

    #[test]
    fn stale_lock_file_is_fine() {
        let dir = tempfile::tempdir().unwrap();
        let path = dir.path().join("valw.lock");
        File::create(&path).unwrap();
        Lock::acquire(&path).unwrap();
    }
}
```

Add `mod lock;` to the `mod` list at the top of `src/main.rs` (keep it sorted).

- [ ] **Step 2: Run the tests and watch them fail**

Run: `cargo nextest run lock::`
Expected: compilation fails, e.g. `` failed to resolve: use of undeclared type `Lock` ``.

- [ ] **Step 3: Implement `lock`**

Put this above the test module in `src/lock.rs`:

```rust
use std::fs::File;
use std::path::{Path, PathBuf};

use anyhow::{Context, Result, bail};
use rustix::fs::{FlockOperation, flock};
use rustix::io::Errno;

use crate::error::HintExt;

/// Held for the whole capture so two captures never overlap. The kernel
/// drops the lock when the process dies, so a crash can't leave it stuck.
#[derive(Debug)]
pub struct Lock {
    _file: File,
}

/// `$XDG_RUNTIME_DIR/valw.lock`, falling back to the temp dir.
pub fn default_path() -> PathBuf {
    std::env::var_os("XDG_RUNTIME_DIR")
        .map(PathBuf::from)
        .unwrap_or_else(std::env::temp_dir)
        .join("valw.lock")
}

impl Lock {
    pub fn acquire(path: &Path) -> Result<Lock> {
        let file = File::options()
            .create(true)
            .truncate(false)
            .write(true)
            .open(path)
            .with_context(|| format!("could not open lock file {}", path.display()))?;
        match flock(&file, FlockOperation::NonBlockingLockExclusive) {
            Ok(()) => Ok(Lock { _file: file }),
            Err(Errno::WOULDBLOCK) => Err(anyhow::anyhow!("valw is already running"))
                .hint("wait for the other capture to finish, or press Esc in it"),
            Err(e) => {
                bail!("could not lock {}: {e}", path.display())
            }
        }
    }

    /// Releases the lock. Must happen before forking, or the child would
    /// inherit the file and keep the lock held.
    pub fn release(self) {}
}
```

- [ ] **Step 4: Run the tests and watch them pass**

Run: `cargo nextest run lock::`
Expected: all 2 tests in `lock::tests` pass.

- [ ] **Step 5: Write the failing tests for `detach`**

Create `src/detach.rs` with only the test module:

```rust
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn child_holds_neither_lock_nor_stdio() {
        let dir = tempfile::tempdir().unwrap();
        let path = dir.path().join("valw.lock");
        let lock = Lock::acquire(&path).unwrap();
        let (report_r, report_w) = rustix::pipe::pipe().unwrap();
        let (go_r, go_w) = rustix::pipe::pipe().unwrap();

        spawn(lock, move || {
            // Report whether stdout is /dev/null, then wait for the parent.
            let null = rustix::fs::stat("/dev/null").unwrap();
            let out = rustix::fs::fstat(rustix::stdio::stdout()).unwrap();
            let is_null = out.st_rdev == null.st_rdev && out.st_ino == null.st_ino;
            rustix::io::write(&report_w, &[is_null as u8]).unwrap();
            let _ = rustix::io::read(&go_r, &mut [0u8; 1]);
        })
        .unwrap();

        // The child is still running, yet the lock is free.
        Lock::acquire(&path).unwrap();

        let mut is_null = [0u8; 1];
        rustix::io::read(&report_r, &mut is_null).unwrap();
        assert_eq!(is_null, [1], "child stdout should be /dev/null");

        // Closing our end lets the child exit.
        drop(go_w);
    }
}
```

Add `mod detach;` to the `mod` list at the top of `src/main.rs` (keep it sorted).

- [ ] **Step 6: Run the tests and watch them fail**

Run: `cargo nextest run detach::`
Expected: compilation fails, e.g. `` cannot find function `spawn` in this scope ``.

- [ ] **Step 7: Implement `detach`**

Put this above the test module in `src/detach.rs`:

```rust
use std::fs::File;
use std::io;

use anyhow::{Context, Result};

use crate::lock::Lock;

/// Runs `work` in a forked child that outlives valw, like `wl-copy` does.
/// The parent returns right away.
///
/// The child drops the capture lock (so the next capture isn't blocked) and
/// points stdio at /dev/null (so `valw ... -o - | x` sees EOF without waiting
/// for the clipboard to be replaced).
pub fn spawn(lock: Lock, work: impl FnOnce()) -> Result<()> {
    lock.release();
    // SAFETY: valw is single-threaded at this point, and the child only
    // touches its own state before exiting with _exit.
    match unsafe { libc::fork() } {
        -1 => Err(io::Error::last_os_error()).context("could not fork the clipboard server"),
        0 => {
            let _ = rustix::process::setsid();
            let _ = detach_stdio();
            work();
            // SAFETY: skips atexit handlers and destructors that belong to the parent.
            unsafe { libc::_exit(0) }
        }
        _ => Ok(()),
    }
}

fn detach_stdio() -> io::Result<()> {
    let null = File::options().read(true).write(true).open("/dev/null")?;
    rustix::stdio::dup2_stdin(&null)?;
    rustix::stdio::dup2_stdout(&null)?;
    rustix::stdio::dup2_stderr(&null)?;
    Ok(())
}
```

- [ ] **Step 8: Run the tests and watch them pass**

Run: `cargo nextest run detach::`
Expected: all 1 tests in `detach::tests` pass.

- [ ] **Step 9: Lint**

Run: `cargo fmt && cargo clippy --all-targets -- -D warnings && cargo nextest run`
Expected: no warnings; every test passes.

- [ ] **Step 10: Commit**

```sh
jj commit -m "feat: single-instance lock and detached background child"
```

---


### Task 6: Frames, pixel formats and output transforms

**Files:**
- Create: `src/frame.rs`
- Modify: `src/main.rs` (add `frame` to the `mod` list)

**Interfaces:**
- Consumes: nothing.
- Produces: `pub struct OutputGeom { pub name: String, pub x: i32, pub y: i32, pub width: i32, pub height: i32 }` (logical, global coordinates); `pub struct PixelRect { pub x: u32, pub y: u32, pub width: u32, pub height: u32 }` (a frame's physical pixels); `pub type Bgrx = RgbaImage` (container whose bytes are B, G, R, X, i.e. XRGB8888 in memory); `pub struct Frame { pub output: OutputGeom, pub pixels: Arc<Bgrx> }` with `full(&self) -> PixelRect` and `to_rgba(&self, rect: PixelRect) -> RgbaImage` (opaque); `pub const SUPPORTED: [Format; 4]` (preference order); `pub fn to_bgrx(format: Format, width: u32, height: u32, stride: u32, data: &[u8], y_invert: bool) -> Result<Bgrx>`; `pub fn apply_transform(img: Bgrx, transform: Transform) -> Bgrx`. `Format` is `wayland_client::protocol::wl_shm::Format`, `Transform` is `wayland_client::protocol::wl_output::Transform`.

- [ ] **Step 1: Write the failing tests for `frame`**

Create `src/frame.rs` with only the test module:

```rust
#[cfg(test)]
mod tests {
    use super::*;

    /// 2x1 buffer: a red pixel then a blue one, in the given byte order.
    fn two_pixels(format: Format) -> Vec<u8> {
        match format {
            Format::Xrgb8888 | Format::Argb8888 => vec![0, 0, 255, 0, 255, 0, 0, 0],
            _ => vec![255, 0, 0, 0, 0, 0, 255, 0],
        }
    }

    fn bgr(img: &Bgrx, x: u32, y: u32) -> [u8; 3] {
        let p = img.get_pixel(x, y).0;
        [p[0], p[1], p[2]]
    }

    #[test]
    fn reads_all_supported_formats_as_bgrx() {
        for format in SUPPORTED {
            let img = to_bgrx(format, 2, 1, 8, &two_pixels(format), false).unwrap();
            assert_eq!(bgr(&img, 0, 0), [0, 0, 255], "red, {format:?}");
            assert_eq!(bgr(&img, 1, 0), [255, 0, 0], "blue, {format:?}");
        }
    }

    #[test]
    fn honours_stride_padding() {
        // One pixel per row, 8-byte stride.
        let data = [0, 0, 255, 0, 9, 9, 9, 9, 255, 0, 0, 0, 9, 9, 9, 9];
        let img = to_bgrx(Format::Xrgb8888, 1, 2, 8, &data, false).unwrap();
        assert_eq!(img.as_raw().len(), 8);
        assert_eq!(bgr(&img, 0, 0), [0, 0, 255]);
        assert_eq!(bgr(&img, 0, 1), [255, 0, 0]);
    }

    #[test]
    fn y_invert_flips_rows() {
        let data = [0, 0, 255, 0, 255, 0, 0, 0];
        let img = to_bgrx(Format::Xrgb8888, 1, 2, 4, &data, true).unwrap();
        assert_eq!(bgr(&img, 0, 0), [255, 0, 0]);
        assert_eq!(bgr(&img, 0, 1), [0, 0, 255]);
    }

    #[test]
    fn rejects_unknown_format_by_name() {
        let err = to_bgrx(Format::Xbgr2101010, 1, 1, 4, &[0; 4], false).unwrap_err();
        assert_eq!(err.to_string(), "unsupported buffer format Xbgr2101010");
    }

    #[test]
    fn rejects_short_buffer() {
        assert!(to_bgrx(Format::Xrgb8888, 2, 2, 8, &[0; 8], false).is_err());
    }

    fn frame(width: u32, height: u32, data: Vec<u8>) -> Frame {
        Frame {
            output: OutputGeom {
                name: "A".into(),
                x: 0,
                y: 0,
                width: width as i32,
                height: height as i32,
            },
            pixels: Arc::new(Bgrx::from_raw(width, height, data).unwrap()),
        }
    }

    #[test]
    fn to_rgba_swaps_channels_and_makes_opaque() {
        // Red with alpha 0, as an ARGB8888 buffer may deliver it.
        let f = frame(1, 1, vec![0, 0, 255, 0]);
        assert_eq!(f.to_rgba(f.full()).get_pixel(0, 0).0, [255, 0, 0, 255]);
    }

    #[test]
    fn to_rgba_crops() {
        // 3x2: pixel value = its index, in the blue byte.
        let data = (0u8..6).flat_map(|i| [i, 0, 0, 0]).collect();
        let f = frame(3, 2, data);
        let rect = PixelRect {
            x: 1,
            y: 1,
            width: 2,
            height: 1,
        };
        let out = f.to_rgba(rect);
        assert_eq!(out.dimensions(), (2, 1));
        assert_eq!(out.get_pixel(0, 0).0, [0, 0, 4, 255]);
        assert_eq!(out.get_pixel(1, 0).0, [0, 0, 5, 255]);
    }

    fn marked() -> Bgrx {
        // 3x2, top-left pixel marked.
        let mut img = Bgrx::new(3, 2);
        img.put_pixel(0, 0, image::Rgba([255, 255, 255, 255]));
        img
    }

    fn marked_at(img: &Bgrx) -> (u32, u32, u32, u32) {
        let (x, y, _) = img
            .enumerate_pixels()
            .find(|(_, _, p)| p.0[0] == 255)
            .unwrap();
        (img.width(), img.height(), x, y)
    }

    #[test]
    fn transforms_match_grim() {
        use Transform::*;
        let cases = [
            (Normal, (3, 2, 0, 0)),
            (_90, (2, 3, 1, 0)),
            (_180, (3, 2, 2, 1)),
            (_270, (2, 3, 0, 2)),
            (Flipped, (3, 2, 2, 0)),
            (Flipped90, (2, 3, 0, 0)),
            (Flipped180, (3, 2, 0, 1)),
            (Flipped270, (2, 3, 1, 2)),
        ];
        for (t, want) in cases {
            assert_eq!(marked_at(&apply_transform(marked(), t)), want, "{t:?}");
        }
    }
}
```

Add `mod frame;` to the `mod` list at the top of `src/main.rs` (keep it sorted).

- [ ] **Step 2: Run the tests and watch them fail**

Run: `cargo nextest run frame::`
Expected: compilation fails, e.g. `` cannot find function `to_bgrx` in this scope ``.

- [ ] **Step 3: Implement `frame`**

Put this above the test module in `src/frame.rs`:

```rust
use std::sync::Arc;

use anyhow::{Result, bail, ensure};
use image::{RgbaImage, imageops};
use wayland_client::protocol::{wl_output::Transform, wl_shm::Format};

/// An output's place in the global logical coordinate space.
#[derive(Debug, Clone, PartialEq)]
pub struct OutputGeom {
    pub name: String,
    pub x: i32,
    pub y: i32,
    pub width: i32,
    pub height: i32,
}

/// A rectangle in a frame's physical pixels.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct PixelRect {
    pub x: u32,
    pub y: u32,
    pub width: u32,
    pub height: u32,
}

/// Pixels in wl_shm XRGB8888 memory order: B, G, R, X. This is what niri
/// hands out and what the overlay shows, so nothing on the way to the first
/// overlay frame has to reorder bytes. `RgbaImage` is only the container;
/// its channels do not mean R, G, B, A here.
pub type Bgrx = RgbaImage;

/// One captured output: upright pixels at physical resolution.
#[derive(Debug, Clone)]
pub struct Frame {
    pub output: OutputGeom,
    /// Shared with the overlay, which shows it as the undimmed selection.
    pub pixels: Arc<Bgrx>,
}

impl Frame {
    /// The whole frame as a rectangle.
    pub fn full(&self) -> PixelRect {
        PixelRect {
            x: 0,
            y: 0,
            width: self.pixels.width(),
            height: self.pixels.height(),
        }
    }

    /// The `rect` part as opaque RGBA, for saving.
    ///
    /// Alpha is always 255: a screenshot of what's on screen is opaque, even
    /// where the compositor's buffer has alpha 0.
    pub fn to_rgba(&self, rect: PixelRect) -> RgbaImage {
        let stride = self.pixels.width() as usize * 4;
        let raw = self.pixels.as_raw();
        let mut out = RgbaImage::new(rect.width, rect.height);
        for (y, row) in out.rows_mut().enumerate() {
            let start = (rect.y as usize + y) * stride + rect.x as usize * 4;
            let (src, _) = raw[start..start + rect.width as usize * 4].as_chunks::<4>();
            for (px, s) in row.zip(src) {
                px.0 = [s[2], s[1], s[0], 255];
            }
        }
        out
    }
}

/// Shm formats valw can read, in order of preference.
pub const SUPPORTED: [Format; 4] = [
    Format::Xrgb8888,
    Format::Argb8888,
    Format::Xbgr8888,
    Format::Abgr8888,
];

/// Copies a screencopy buffer into B, G, R, X order, dropping stride padding.
pub fn to_bgrx(
    format: Format,
    width: u32,
    height: u32,
    stride: u32,
    data: &[u8],
    y_invert: bool,
) -> Result<Bgrx> {
    let swap = match format {
        Format::Xrgb8888 | Format::Argb8888 => false,
        Format::Xbgr8888 | Format::Abgr8888 => true,
        other => bail!("unsupported buffer format {other:?}"),
    };
    ensure!(
        stride >= width * 4,
        "stride {stride} is too small for width {width}"
    );
    ensure!(
        data.len() >= (stride * height) as usize,
        "buffer is {} bytes, expected at least {}",
        data.len(),
        stride * height
    );

    let row_len = width as usize * 4;
    let mut out = Bgrx::new(width, height);
    for (y, dst) in out.chunks_exact_mut(row_len).enumerate() {
        let src_y = if y_invert { height as usize - 1 - y } else { y };
        let src = &data[src_y * stride as usize..][..row_len];
        dst.copy_from_slice(src);
        if swap {
            for px in dst.as_chunks_mut::<4>().0 {
                px.swap(0, 2);
            }
        }
    }
    Ok(out)
}

/// Turns a buffer in the output's native orientation upright, the same way
/// grim does: rotate clockwise by the transform's angle, then mirror.
pub fn apply_transform(img: Bgrx, transform: Transform) -> Bgrx {
    let rotated = match transform {
        Transform::_90 | Transform::Flipped90 => imageops::rotate90(&img),
        Transform::_180 | Transform::Flipped180 => imageops::rotate180(&img),
        Transform::_270 | Transform::Flipped270 => imageops::rotate270(&img),
        _ => img,
    };
    match transform {
        Transform::Flipped
        | Transform::Flipped90
        | Transform::Flipped180
        | Transform::Flipped270 => imageops::flip_horizontal(&rotated),
        _ => rotated,
    }
}
```

- [ ] **Step 4: Run the tests and watch them pass**

Run: `cargo nextest run frame::`
Expected: all 8 tests in `frame::tests` pass.

- [ ] **Step 5: Lint**

Run: `cargo fmt && cargo clippy --all-targets -- -D warnings && cargo nextest run`
Expected: no warnings; every test passes.

- [ ] **Step 6: Commit**

```sh
jj commit -m "feat: screencopy frames, pixel formats and output transforms"
```

---


### Task 7: Selection math and overlay pixels

**Files:**
- Create: `src/selection.rs`
- Create: `src/render.rs`
- Modify: `src/main.rs` (add `selection`, `render` to the `mod` list)

**Interfaces:**
- Consumes: `frame::{OutputGeom, PixelRect}` (Task 6).
- Produces: `selection::{MIN_PIXELS, Point { x: f64, y: f64 }, Selection { Idle, Dragging { start, end } }}` with `press`, `motion`, `release(p) -> Option<(Point, Point)>`; `selection::clip(a, b, out: &OutputGeom, frame_w: u32, frame_h: u32) -> Option<PixelRect>`; `selection::resolve(...)` (same, but `None` under `MIN_PIXELS`); `render::{DIM, dim(pixels: &[u8], brightness: u8) -> Vec<u8>, compose(dst: &mut [u8], dark: &[u8], bright: &[u8], width: u32, sel: Option<PixelRect>)}` (all buffers are B, G, R, X).

- [ ] **Step 1: Write the failing tests for `selection`**

Create `src/selection.rs` with only the test module:

```rust
#[cfg(test)]
mod tests {
    use super::*;

    fn out(name: &str, x: i32, y: i32, width: i32, height: i32) -> OutputGeom {
        OutputGeom {
            name: name.into(),
            x,
            y,
            width,
            height,
        }
    }

    fn p(x: f64, y: f64) -> Point {
        Point { x, y }
    }

    fn rect(x: u32, y: u32, width: u32, height: u32) -> PixelRect {
        PixelRect {
            x,
            y,
            width,
            height,
        }
    }

    #[test]
    fn scale_one() {
        let o = out("A", 0, 0, 1920, 1080);
        assert_eq!(
            clip(p(10.0, 20.0), p(110.0, 70.0), &o, 1920, 1080),
            Some(rect(10, 20, 100, 50))
        );
    }

    #[test]
    fn drag_direction_does_not_matter() {
        let o = out("A", 0, 0, 1920, 1080);
        assert_eq!(
            clip(p(110.0, 70.0), p(10.0, 20.0), &o, 1920, 1080),
            clip(p(10.0, 20.0), p(110.0, 70.0), &o, 1920, 1080)
        );
    }

    #[test]
    fn fractional_scale_rounds_outwards() {
        // 1920x1080 at 1.25 is 1536x864 logical.
        let o = out("A", 0, 0, 1536, 864);
        // 10.5 * 1.25 = 13.125 -> 13; 20.3 * 1.25 = 25.375 -> 26.
        assert_eq!(
            clip(p(10.5, 0.0), p(20.3, 8.0), &o, 1920, 1080),
            Some(rect(13, 0, 13, 10))
        );
    }

    #[test]
    fn scale_one_and_a_half() {
        let o = out("A", 0, 0, 1280, 720);
        assert_eq!(
            clip(p(100.0, 100.0), p(200.0, 150.0), &o, 1920, 1080),
            Some(rect(150, 150, 150, 75))
        );
    }

    #[test]
    fn output_offsets_including_negative() {
        let left = out("L", -1366, 0, 1366, 768);
        let right = out("R", 0, 0, 1920, 1080);
        assert_eq!(
            clip(p(-1356.0, 10.0), p(-1256.0, 60.0), &left, 1366, 768),
            Some(rect(10, 10, 100, 50))
        );
        assert_eq!(
            clip(
                p(1930.0, 10.0),
                p(2030.0, 60.0),
                &out("R2", 1920, 0, 800, 600),
                800,
                600
            ),
            Some(rect(10, 10, 100, 50))
        );
        assert_eq!(
            clip(p(-10.0, 10.0), p(10.0, 20.0), &right, 1920, 1080),
            Some(rect(0, 10, 10, 10))
        );
    }

    #[test]
    fn drag_across_outputs_is_clipped_to_the_output() {
        let left = out("L", 0, 0, 1366, 768);
        // From (1300, 100) on the left output to (1500, 900) on the right one.
        assert_eq!(
            clip(p(1300.0, 100.0), p(1500.0, 900.0), &left, 1366, 768),
            Some(rect(1300, 100, 66, 668))
        );
    }

    #[test]
    fn nothing_left_on_other_output() {
        let o = out("A", 0, 0, 100, 100);
        assert_eq!(clip(p(200.0, 200.0), p(300.0, 300.0), &o, 100, 100), None);
    }

    #[test]
    fn click_is_not_a_selection() {
        let o = out("A", 0, 0, 1920, 1080);
        assert_eq!(resolve(p(10.0, 10.0), p(10.0, 10.0), &o, 1920, 1080), None);
        assert_eq!(resolve(p(10.0, 10.0), p(11.0, 30.0), &o, 1920, 1080), None);
        assert_eq!(
            resolve(p(10.0, 10.0), p(12.0, 12.0), &o, 1920, 1080),
            Some(rect(10, 10, 2, 2))
        );
    }

    #[test]
    fn state_machine() {
        let mut s = Selection::default();
        assert_eq!(s.release(p(1.0, 1.0)), None);

        s.press(p(1.0, 2.0));
        s.motion(p(5.0, 6.0));
        assert_eq!(
            s,
            Selection::Dragging {
                start: p(1.0, 2.0),
                end: p(5.0, 6.0)
            }
        );
        assert_eq!(s.release(p(7.0, 8.0)), Some((p(1.0, 2.0), p(7.0, 8.0))));
        assert_eq!(s, Selection::Idle);

        s.motion(p(9.0, 9.0));
        assert_eq!(s, Selection::Idle);
    }
}
```

Add `mod selection;` to the `mod` list at the top of `src/main.rs` (keep it sorted).

- [ ] **Step 2: Run the tests and watch them fail**

Run: `cargo nextest run selection::`
Expected: compilation fails, e.g. `` cannot find struct, variant or union type `Point` in this scope ``.

- [ ] **Step 3: Implement `selection`**

Put this above the test module in `src/selection.rs`:

```rust
use crate::frame::{OutputGeom, PixelRect};

/// Selections smaller than this (in physical pixels) count as a click.
pub const MIN_PIXELS: u32 = 2;

/// A point in global logical coordinates.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct Point {
    pub x: f64,
    pub y: f64,
}

#[derive(Debug, Clone, Copy, Default, PartialEq)]
pub enum Selection {
    #[default]
    Idle,
    Dragging {
        start: Point,
        end: Point,
    },
}

impl Selection {
    pub fn press(&mut self, p: Point) {
        *self = Selection::Dragging { start: p, end: p };
    }

    pub fn motion(&mut self, p: Point) {
        if let Selection::Dragging { end, .. } = self {
            *end = p;
        }
    }

    /// Ends the drag and returns its start and end. Back to `Idle` either way.
    pub fn release(&mut self, p: Point) -> Option<(Point, Point)> {
        match std::mem::take(self) {
            Selection::Dragging { start, .. } => Some((start, p)),
            Selection::Idle => None,
        }
    }
}

/// The part of the drag from `a` to `b` that falls on `out`, in the physical
/// pixels of a `frame_w` x `frame_h` frame. `None` if nothing is left.
///
/// Edges round outwards so the rectangle covers every pixel the user touched.
pub fn clip(a: Point, b: Point, out: &OutputGeom, frame_w: u32, frame_h: u32) -> Option<PixelRect> {
    let sx = frame_w as f64 / out.width as f64;
    let sy = frame_h as f64 / out.height as f64;
    let to_px = |v: f64, origin: i32, s: f64, max: u32, round: fn(f64) -> f64| {
        round((v - origin as f64) * s).clamp(0.0, max as f64) as u32
    };
    let x0 = to_px(a.x.min(b.x), out.x, sx, frame_w, f64::floor);
    let x1 = to_px(a.x.max(b.x), out.x, sx, frame_w, f64::ceil);
    let y0 = to_px(a.y.min(b.y), out.y, sy, frame_h, f64::floor);
    let y1 = to_px(a.y.max(b.y), out.y, sy, frame_h, f64::ceil);
    (x1 > x0 && y1 > y0).then(|| PixelRect {
        x: x0,
        y: y0,
        width: x1 - x0,
        height: y1 - y0,
    })
}

/// Like [`clip`], but treats anything under [`MIN_PIXELS`] as a click.
pub fn resolve(
    a: Point,
    b: Point,
    out: &OutputGeom,
    frame_w: u32,
    frame_h: u32,
) -> Option<PixelRect> {
    clip(a, b, out, frame_w, frame_h).filter(|r| r.width >= MIN_PIXELS && r.height >= MIN_PIXELS)
}
```

- [ ] **Step 4: Run the tests and watch them pass**

Run: `cargo nextest run selection::`
Expected: all 9 tests in `selection::tests` pass.

- [ ] **Step 5: Write the failing tests for `render`**

Create `src/render.rs` with only the test module:

```rust
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn dims_every_byte() {
        assert_eq!(dim(&[200, 100, 50, 0], 255), vec![200, 100, 50, 0]);
        assert_eq!(dim(&[200, 100, 50, 255], DIM), vec![120, 60, 30, 153]);
        assert_eq!(dim(&[255, 0, 1, 2], 0), vec![0, 0, 0, 0]);
    }

    fn px(buf: &[u8], width: u32, x: u32, y: u32) -> [u8; 4] {
        let i = ((y * width + x) * 4) as usize;
        buf[i..i + 4].try_into().unwrap()
    }

    #[test]
    fn compose_outline_and_inside() {
        let (w, h) = (6, 6);
        let dark = vec![1u8; (w * h * 4) as usize];
        let bright = vec![2u8; (w * h * 4) as usize];
        let mut dst = vec![0u8; dark.len()];
        let sel = PixelRect {
            x: 1,
            y: 1,
            width: 4,
            height: 4,
        };

        compose(&mut dst, &dark, &bright, w, Some(sel));

        assert_eq!(px(&dst, w, 0, 0), [1; 4], "outside");
        assert_eq!(px(&dst, w, 1, 1), [255; 4], "corner");
        assert_eq!(px(&dst, w, 4, 2), [255; 4], "right edge");
        assert_eq!(px(&dst, w, 2, 4), [255; 4], "bottom edge");
        assert_eq!(px(&dst, w, 2, 2), [2; 4], "inside");
        assert_eq!(px(&dst, w, 5, 5), [1; 4], "outside");
    }

    #[test]
    fn compose_without_selection_is_all_dark() {
        let dark = vec![1u8; 16];
        let mut dst = vec![0u8; 16];
        compose(&mut dst, &dark, &[2u8; 16], 2, None);
        assert_eq!(dst, dark);
    }

    #[test]
    fn compose_one_pixel_selection() {
        let dark = vec![1u8; 16];
        let mut dst = vec![0u8; 16];
        let sel = PixelRect {
            x: 1,
            y: 1,
            width: 1,
            height: 1,
        };
        compose(&mut dst, &dark, &[2u8; 16], 2, Some(sel));
        assert_eq!(px(&dst, 2, 1, 1), [255; 4]);
    }
}
```

Add `mod render;` to the `mod` list at the top of `src/main.rs` (keep it sorted).

- [ ] **Step 6: Run the tests and watch them fail**

Run: `cargo nextest run render::`
Expected: compilation fails, e.g. `` cannot find function `dim` in this scope ``.

- [ ] **Step 7: Implement `render`**

Put this above the test module in `src/render.rs`:

```rust
use crate::frame::PixelRect;

/// Overlay brightness outside the selection (60%, i.e. darkened by 40%).
pub const DIM: u8 = 153;

/// B, G, R, X pixels with every byte scaled by about `brightness / 255`.
///
/// This runs on every output before the overlay can appear, so it is one
/// branch-free pass the compiler can vectorise. Scaling X too is harmless:
/// the overlay buffer is XRGB8888, which ignores it.
pub fn dim(pixels: &[u8], brightness: u8) -> Vec<u8> {
    let factor = brightness as u16 + 1;
    pixels
        .iter()
        .map(|&b| ((b as u16 * factor) >> 8) as u8)
        .collect()
}

/// Draws one overlay frame into `dst`: `dark` everywhere, `bright` inside
/// `sel`, and a 1 px white outline on the edge of `sel`.
pub fn compose(dst: &mut [u8], dark: &[u8], bright: &[u8], width: u32, sel: Option<PixelRect>) {
    dst.copy_from_slice(dark);
    let Some(r) = sel else { return };
    let stride = width as usize * 4;
    let (x0, x1) = (r.x as usize * 4, (r.x + r.width) as usize * 4);

    for y in r.y..r.y + r.height {
        let row = y as usize * stride;
        dst[row + x0..row + x1].copy_from_slice(&bright[row + x0..row + x1]);
        if y == r.y || y == r.y + r.height - 1 {
            dst[row + x0..row + x1].fill(255);
        } else {
            dst[row + x0..row + x0 + 4].fill(255);
            dst[row + x1 - 4..row + x1].fill(255);
        }
    }
}
```

- [ ] **Step 8: Run the tests and watch them pass**

Run: `cargo nextest run render::`
Expected: all 4 tests in `render::tests` pass.

- [ ] **Step 9: Lint**

Run: `cargo fmt && cargo clippy --all-targets -- -D warnings && cargo nextest run`
Expected: no warnings; every test passes.

- [ ] **Step 10: Commit**

```sh
jj commit -m "feat: selection geometry and overlay compositing"
```

---


### Task 8: Saving, stdout and clipboard

**Files:**
- Create: `src/output.rs`
- Modify: `src/main.rs` (add `output` to the `mod` list)

**Interfaces:**
- Consumes: `error::HintExt` (Task 2); `lock::Lock`, `detach::spawn` (Task 5).
- Produces: `pub fn file_name(template: &str, now: &DateTime<Local>, output: Option<&str>) -> Result<String>`; `pub fn unique_path(dir: &Path, name: &str) -> PathBuf`; `pub fn encode_png(img: &RgbaImage) -> Result<Vec<u8>>`; `pub fn write_atomic(path: &Path, bytes: &[u8]) -> Result<()>`; `pub fn write_stdout(bytes: &[u8]) -> Result<()>`; `pub fn copy_to_clipboard(png: Vec<u8>, lock: Lock) -> Result<()>`; `pub enum Target { Default, Path(PathBuf), Stdout, ClipboardOnly }` with `pub fn from_args(output: Option<PathBuf>, clipboard_only: bool) -> Target`.

- [ ] **Step 1: Write the failing tests for `output`**

Create `src/output.rs` with only the test module:

```rust
#[cfg(test)]
mod tests {
    use super::*;
    use chrono::TimeZone;

    fn now() -> DateTime<Local> {
        Local.with_ymd_and_hms(2026, 9, 29, 19, 4, 5).unwrap()
    }

    #[test]
    fn default_template() {
        let name = file_name("Screenshot %Y-%m-%d at %H.%M.%S.png", &now(), None).unwrap();
        assert_eq!(name, "Screenshot 2026-09-29 at 19.04.05.png");
    }

    #[test]
    fn output_suffix() {
        let name = file_name("shot.png", &now(), Some("HDMI-A-1")).unwrap();
        assert_eq!(name, "shot (HDMI-A-1).png");
    }

    #[test]
    fn invalid_pattern_is_an_error_not_a_panic() {
        let err = file_name("shot %Q.png", &now(), None).unwrap_err();
        assert!(
            format!("{err:#}").contains("invalid strftime pattern"),
            "{err:#}"
        );
    }

    #[test]
    fn rejects_slashes_and_empty_names() {
        assert!(file_name("%Y/%m.png", &now(), None).is_err());
        assert!(file_name("", &now(), None).is_err());
    }

    #[test]
    fn suffix_placement() {
        assert_eq!(with_suffix("a.png", " (2)"), "a (2).png");
        assert_eq!(with_suffix("a.b.png", " (2)"), "a.b (2).png");
        assert_eq!(with_suffix("noext", " (2)"), "noext (2)");
        assert_eq!(with_suffix(".hidden", " (2)"), ".hidden (2)");
    }

    #[test]
    fn unique_path_counts_up() {
        let dir = tempfile::tempdir().unwrap();
        assert_eq!(unique_path(dir.path(), "a.png"), dir.path().join("a.png"));
        fs::write(dir.path().join("a.png"), b"").unwrap();
        fs::write(dir.path().join("a (2).png"), b"").unwrap();
        assert_eq!(
            unique_path(dir.path(), "a.png"),
            dir.path().join("a (3).png")
        );
    }

    #[test]
    fn write_atomic_creates_dirs_and_leaves_no_temp() {
        let dir = tempfile::tempdir().unwrap();
        let path = dir.path().join("new/sub/a.png");
        write_atomic(&path, b"png").unwrap();
        assert_eq!(fs::read(&path).unwrap(), b"png");
        let names: Vec<_> = fs::read_dir(path.parent().unwrap()).unwrap().collect();
        assert_eq!(names.len(), 1);
    }

    #[test]
    fn write_atomic_reports_unwritable_dir() {
        let dir = tempfile::tempdir().unwrap();
        let file = dir.path().join("file");
        fs::write(&file, b"").unwrap();
        let err = write_atomic(&file.join("a.png"), b"png").unwrap_err();
        assert!(format!("{err:#}").contains("could not create"), "{err:#}");
    }

    #[test]
    fn write_atomic_relative_path() {
        let dir = tempfile::tempdir().unwrap();
        std::env::set_current_dir(dir.path()).unwrap();
        write_atomic(Path::new("a.png"), b"png").unwrap();
        assert_eq!(fs::read(dir.path().join("a.png")).unwrap(), b"png");
    }

    #[test]
    fn png_round_trip() {
        let img = RgbaImage::from_raw(1, 1, vec![1, 2, 3, 255]).unwrap();
        let png = encode_png(&img).unwrap();
        let back = image::load_from_memory(&png).unwrap().to_rgba8();
        assert_eq!(back, img);
    }

    #[test]
    fn targets() {
        use Target::*;
        assert_eq!(Target::from_args(None, false), Default);
        assert_eq!(Target::from_args(None, true), ClipboardOnly);
        assert_eq!(Target::from_args(Some("-".into()), false), Stdout);
        assert_eq!(
            Target::from_args(Some("a.png".into()), false),
            Path("a.png".into())
        );
    }
}
```

Add `mod output;` to the `mod` list at the top of `src/main.rs` (keep it sorted).

- [ ] **Step 2: Run the tests and watch them fail**

Run: `cargo nextest run output::`
Expected: compilation fails, e.g. `` cannot find function `file_name` in this scope ``.

- [ ] **Step 3: Implement `output`**

Put this above the test module in `src/output.rs`:

```rust
use std::fmt::Write as _;
use std::fs;
use std::io::{Cursor, Write as _};
use std::path::{Path, PathBuf};

use anyhow::{Context, Result, anyhow};
use chrono::{DateTime, Local};
use image::{ImageFormat, RgbaImage};
use wl_clipboard_rs::copy::{MimeType, Options, Source};

use crate::error::HintExt;
use crate::lock::Lock;

/// Renders the strftime `template`. With `output`, appends ` (NAME)` before
/// the extension, for `valw screen --all`.
pub fn file_name(template: &str, now: &DateTime<Local>, output: Option<&str>) -> Result<String> {
    let mut name = String::new();
    write!(name, "{}", now.format(template))
        .map_err(|_| anyhow!("invalid strftime pattern in save.filename: {template:?}"))
        .hint("see https://docs.rs/chrono/latest/chrono/format/strftime for the syntax")?;
    if name.is_empty() || name.contains('/') {
        return Err(anyhow!(
            "save.filename must render to a plain file name, got {name:?}"
        ))
        .hint("put folders in save.directory, not save.filename");
    }
    if let Some(output) = output {
        name = with_suffix(&name, &format!(" ({output})"));
    }
    Ok(name)
}

/// Inserts `suffix` before the extension: `a.png` + ` (2)` = `a (2).png`.
fn with_suffix(name: &str, suffix: &str) -> String {
    match name.rsplit_once('.') {
        Some((stem, ext)) if !stem.is_empty() => format!("{stem}{suffix}.{ext}"),
        _ => format!("{name}{suffix}"),
    }
}

/// `dir/name`, or `dir/name (2)`, `(3)`, … if taken.
pub fn unique_path(dir: &Path, name: &str) -> PathBuf {
    let first = dir.join(name);
    if !first.exists() {
        return first;
    }
    (2..)
        .map(|n| dir.join(with_suffix(name, &format!(" ({n})"))))
        .find(|p| !p.exists())
        .expect("some suffix is free")
}

pub fn encode_png(img: &RgbaImage) -> Result<Vec<u8>> {
    let mut png = Vec::new();
    img.write_to(&mut Cursor::new(&mut png), ImageFormat::Png)
        .context("could not encode PNG")?;
    Ok(png)
}

/// Writes via a temp file in the same directory and a rename, so a partial
/// screenshot never appears under the final name.
pub fn write_atomic(path: &Path, bytes: &[u8]) -> Result<()> {
    // "a.png" has the parent "", which means the current directory.
    let dir = match path.parent() {
        Some(p) if !p.as_os_str().is_empty() => p,
        _ => Path::new("."),
    };
    fs::create_dir_all(dir)
        .with_context(|| format!("could not create {}", dir.display()))
        .hint("check save.directory in the config")?;
    let name = path.file_name().context("path has no file name")?;
    let tmp = dir.join(format!(".{}.tmp", name.to_string_lossy()));
    let result = fs::write(&tmp, bytes).and_then(|()| fs::rename(&tmp, path));
    if result.is_err() {
        let _ = fs::remove_file(&tmp);
    }
    result.with_context(|| format!("could not write {}", path.display()))
}

pub fn write_stdout(bytes: &[u8]) -> Result<()> {
    let mut out = std::io::stdout().lock();
    out.write_all(bytes)
        .and_then(|()| out.flush())
        .context("could not write to stdout")
}

/// Copies `png` to the clipboard. The data is served by a forked child that
/// lives until something else is copied, so this consumes the lock.
pub fn copy_to_clipboard(png: Vec<u8>, lock: Lock) -> Result<()> {
    let mut options = Options::new();
    options.foreground(true);
    let prepared = options
        .prepare_copy(
            Source::Bytes(png.into()),
            MimeType::Specific("image/png".into()),
        )
        .context("could not copy to the clipboard")
        .hint("the compositor needs ext-data-control or wlr-data-control; see `valw doctor`")?;
    crate::detach::spawn(lock, move || {
        let _ = prepared.serve();
    })
}

/// Where a capture goes.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Target {
    /// Save under the config's directory and filename template.
    Default,
    /// `--output <path>`.
    Path(PathBuf),
    /// `--output -`.
    Stdout,
    /// `--clipboard-only`.
    ClipboardOnly,
}

impl Target {
    /// clap rejects `--output` together with `--clipboard-only`.
    pub fn from_args(output: Option<PathBuf>, clipboard_only: bool) -> Target {
        match output {
            _ if clipboard_only => Target::ClipboardOnly,
            Some(p) if p.as_os_str() == "-" => Target::Stdout,
            Some(p) => Target::Path(p),
            None => Target::Default,
        }
    }
}
```

- [ ] **Step 4: Run the tests and watch them pass**

Run: `cargo nextest run output::`
Expected: all 11 tests in `output::tests` pass.

- [ ] **Step 5: Lint**

Run: `cargo fmt && cargo clippy --all-targets -- -D warnings && cargo nextest run`
Expected: no warnings; every test passes.

- [ ] **Step 6: Commit**

```sh
jj commit -m "feat: file names, atomic PNG writes and clipboard"
```

---


### Task 9: Wayland connection, niri IPC and `valw doctor` (Phase 0 discovery)

**Files:**
- Create: `src/niri.rs`, `src/wayland.rs`, `src/doctor.rs`
- Replace: `src/main.rs`

**Interfaces:**
- Consumes: `config::{load, default_path, Config::save_dir}` (Task 3); `log::{init, default_dir, install_panic_hook}` (Task 4); `error::{render, Cancelled, HintExt}` (Task 2); `frame::OutputGeom` (Task 6).
- Produces:
  - `niri`: `pub const IPC_VERSION: (u32, u32)`; `pub fn focused_output() -> Result<Option<String>>`; `pub fn version() -> Result<String>`; `pub fn parse_version(v: &str) -> Option<(u32, u32)>`.
  - `wayland`: `pub struct Output { pub wl: WlOutput, pub geom: OutputGeom, pub transform: Transform }`; `pub struct Wayland { pub conn, pub queue: EventQueue<State>, pub globals: GlobalList, pub state: State }` with `connect() -> Result<Wayland>`, `outputs(&self) -> Vec<Output>`, `global_list(&self) -> Vec<(String, u32)>`, `dispatch_until(&mut self, deadline: Instant, done: impl FnMut(&State) -> bool) -> Result<()>`, `dispatch_blocking(&mut self, done) -> Result<()>`; `pub struct State` (SCTK state plus `layer_shell`, `screencopy`, `viewporter`, `cursor_shape`, `cursor_device`, `keyboard`, `pointer`).
  - `doctor`: `pub enum Level { Ok, Warn, Fail }`, `pub struct Check { level, text }` (Display: `"ok    text"`), `check_globals`, `check_niri`, `check_config`, `check_save_dir`, `pub fn run() -> Result<()>`.
  - CLI: `valw doctor`; exit codes 0/1/3 wired in `main`.

Background for the Wayland code: SCTK 0.21 dispatches through `delegate_dispatch2!`. Protocol objects that SCTK doesn't wrap (screencopy manager, viewporter) use `wayland_client::delegate_noop!`. The handlers that do nothing yet become meaningful in Tasks 10 and 11.

- [ ] **Step 1: Write the failing tests for `niri`**

Create `src/niri.rs` with only the test module:

```rust
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn parses_versions() {
        assert_eq!(parse_version("26.04 (Nixpkgs)"), Some((26, 4)));
        assert_eq!(parse_version("25.11"), Some((25, 11)));
        assert_eq!(parse_version("26.04.1-dev"), Some((26, 4)));
        assert_eq!(parse_version("unknown"), None);
    }

    #[test]
    fn ipc_version_matches_cargo_toml() {
        let (y, m) = IPC_VERSION;
        let pin = format!("niri-ipc = \"={y}.{m}.0\"");
        assert!(
            include_str!("../Cargo.toml").contains(&pin),
            "Cargo.toml should pin {pin}"
        );
    }
}
```

Add `mod niri;` to the `mod` list at the top of `src/main.rs` (keep it sorted).

- [ ] **Step 2: Run the tests and watch them fail**

Run: `cargo nextest run niri::`
Expected: compilation fails, e.g. `` cannot find function `parse_version` in this scope ``.

- [ ] **Step 3: Implement `niri`**

Put this above the test module in `src/niri.rs`:

```rust
use anyhow::{Context, Result, anyhow};
use niri_ipc::{Request, Response, socket::Socket};

/// The niri release the pinned `niri-ipc` crate belongs to. Keep in sync
/// with the `niri-ipc = "=X.Y.0"` line in Cargo.toml (a test checks this).
pub const IPC_VERSION: (u32, u32) = (26, 4);

fn request(request: Request) -> Result<Response> {
    let mut socket =
        Socket::connect().context("could not connect to niri (is NIRI_SOCKET set?)")?;
    socket
        .send(request)
        .context("niri IPC request failed")?
        .map_err(|e| anyhow!("niri replied with an error: {e}"))
}

/// Name of the output niri has focused, if any.
pub fn focused_output() -> Result<Option<String>> {
    match request(Request::FocusedOutput)? {
        Response::FocusedOutput(output) => Ok(output.map(|o| o.name)),
        other => Err(anyhow!("unexpected niri reply: {other:?}")),
    }
}

/// niri's version string, e.g. "26.04 (Nixpkgs)".
pub fn version() -> Result<String> {
    match request(Request::Version)? {
        Response::Version(v) => Ok(v),
        other => Err(anyhow!("unexpected niri reply: {other:?}")),
    }
}

/// `(year, month)` from a niri version string like "26.04 (Nixpkgs)".
pub fn parse_version(v: &str) -> Option<(u32, u32)> {
    let (year, rest) = v.split_once('.')?;
    let month: String = rest.chars().take_while(char::is_ascii_digit).collect();
    Some((year.parse().ok()?, month.parse().ok()?))
}
```

- [ ] **Step 4: Run the tests and watch them pass**

Run: `cargo nextest run niri::`
Expected: all 2 tests in `niri::tests` pass.

- [ ] **Step 5: Write `src/wayland.rs`**

This is glue with no unit tests; `valw doctor` exercises it in Step 11. Add `mod wayland;` to the `mod` list in `src/main.rs`.

```rust
use std::time::Instant;

use anyhow::{Context, Result, bail};
use rustix::event::{PollFd, PollFlags, Timespec, poll};
use smithay_client_toolkit::{
    compositor::{CompositorHandler, CompositorState},
    delegate_registry,
    output::{OutputHandler, OutputState},
    registry::{ProvidesRegistryState, RegistryState},
    registry_handlers,
    seat::{
        Capability, SeatHandler, SeatState,
        keyboard::{KeyEvent, KeyboardHandler, Keysym, Modifiers, RawModifiers},
        pointer::{PointerEvent, PointerHandler, cursor_shape::CursorShapeManager},
    },
    shell::wlr_layer::{LayerShell, LayerShellHandler, LayerSurface, LayerSurfaceConfigure},
    shm::{Shm, ShmHandler},
};
use wayland_client::{
    Connection, EventQueue, QueueHandle,
    globals::{GlobalList, registry_queue_init},
    protocol::{wl_keyboard, wl_output, wl_pointer, wl_seat, wl_surface},
};
use wayland_protocols::wp::cursor_shape::v1::client::wp_cursor_shape_device_v1::WpCursorShapeDeviceV1;
use wayland_protocols::wp::viewporter::client::{
    wp_viewport::WpViewport, wp_viewporter::WpViewporter,
};
use wayland_protocols_wlr::screencopy::v1::client::zwlr_screencopy_manager_v1::ZwlrScreencopyManagerV1;

use crate::error::HintExt;
use crate::frame::OutputGeom;

/// An output as valw sees it.
#[derive(Debug, Clone)]
pub struct Output {
    pub wl: wl_output::WlOutput,
    pub geom: OutputGeom,
    pub transform: wl_output::Transform,
}

pub struct Wayland {
    pub conn: Connection,
    pub queue: EventQueue<State>,
    pub globals: GlobalList,
    pub state: State,
}

pub struct State {
    registry: RegistryState,
    pub outputs: OutputState,
    pub seat: SeatState,
    pub shm: Shm,
    pub compositor: CompositorState,
    pub layer_shell: Option<LayerShell>,
    pub screencopy: Option<ZwlrScreencopyManagerV1>,
    pub viewporter: Option<WpViewporter>,
    pub cursor_shape: Option<CursorShapeManager>,
    pub cursor_device: Option<WpCursorShapeDeviceV1>,
    pub keyboard: Option<wl_keyboard::WlKeyboard>,
    pub pointer: Option<wl_pointer::WlPointer>,
}

impl Wayland {
    pub fn connect() -> Result<Wayland> {
        let conn = Connection::connect_to_env()
            .context("could not connect to the Wayland compositor")
            .hint("valw needs a Wayland session; is WAYLAND_DISPLAY set?")?;
        let (globals, mut queue) = registry_queue_init::<State>(&conn)
            .context("could not read the compositor's globals")?;
        let qh = queue.handle();

        let mut state = State {
            registry: RegistryState::new(&globals),
            outputs: OutputState::new(&globals, &qh),
            seat: SeatState::new(&globals, &qh),
            shm: Shm::bind(&globals, &qh).context("compositor has no wl_shm")?,
            compositor: CompositorState::bind(&globals, &qh)
                .context("compositor has no wl_compositor")?,
            layer_shell: LayerShell::bind(&globals, &qh).ok(),
            screencopy: globals.bind(&qh, 1..=3, ()).ok(),
            viewporter: globals.bind(&qh, 1..=1, ()).ok(),
            cursor_shape: CursorShapeManager::bind(&globals, &qh).ok(),
            cursor_device: None,
            keyboard: None,
            pointer: None,
        };
        // Two round trips: one for wl_output, one for the xdg-output details.
        queue
            .roundtrip(&mut state)
            .context("Wayland round trip failed")?;
        queue
            .roundtrip(&mut state)
            .context("Wayland round trip failed")?;

        let wl = Wayland {
            conn,
            queue,
            globals,
            state,
        };
        wl.log_environment();
        Ok(wl)
    }

    /// All outputs with a known name and logical geometry.
    pub fn outputs(&self) -> Vec<Output> {
        self.state
            .outputs
            .outputs()
            .filter_map(|wl| {
                let info = self.state.outputs.info(&wl)?;
                let (x, y) = info.logical_position?;
                let (width, height) = info.logical_size?;
                Some(Output {
                    geom: OutputGeom {
                        name: info
                            .name
                            .clone()
                            .unwrap_or_else(|| format!("output-{}", info.id)),
                        x,
                        y,
                        width,
                        height,
                    },
                    transform: info.transform,
                    wl,
                })
            })
            .collect()
    }

    /// `(interface, version)` of every global, sorted.
    pub fn global_list(&self) -> Vec<(String, u32)> {
        let mut list: Vec<_> = self
            .globals
            .contents()
            .clone_list()
            .into_iter()
            .map(|g| (g.interface, g.version))
            .collect();
        list.sort();
        list.dedup();
        list
    }

    fn log_environment(&self) {
        let globals: Vec<String> = self
            .global_list()
            .into_iter()
            .map(|(interface, version)| format!("{interface} v{version}"))
            .collect();
        tracing::info!("globals: {}", globals.join(", "));
        for o in self.outputs() {
            let g = &o.geom;
            tracing::info!(
                "output {} at {},{} size {}x{} (logical) transform {:?}",
                g.name,
                g.x,
                g.y,
                g.width,
                g.height,
                o.transform
            );
        }
    }

    /// Dispatches events until `done` returns true or `deadline` passes.
    pub fn dispatch_until(
        &mut self,
        deadline: Instant,
        mut done: impl FnMut(&State) -> bool,
    ) -> Result<()> {
        loop {
            self.queue
                .dispatch_pending(&mut self.state)
                .context("Wayland dispatch failed")?;
            if done(&self.state) {
                return Ok(());
            }
            self.queue.flush().context("Wayland flush failed")?;
            let Some(guard) = self.queue.prepare_read() else {
                continue;
            };
            let left = deadline.saturating_duration_since(Instant::now());
            if left.is_zero() {
                bail!("the compositor did not answer in time");
            }
            let timeout = Timespec::try_from(left).expect("duration fits");
            let fd = guard.connection_fd();
            let mut fds = [PollFd::new(&fd, PollFlags::IN)];
            match poll(&mut fds, Some(&timeout)) {
                Ok(0) => continue,
                Ok(_) => {
                    guard.read().context("Wayland read failed")?;
                }
                Err(rustix::io::Errno::INTR) => continue,
                Err(e) => return Err(e).context("poll failed"),
            }
        }
    }

    /// Dispatches events until `done` returns true. Waits as long as it takes.
    pub fn dispatch_blocking(&mut self, mut done: impl FnMut(&State) -> bool) -> Result<()> {
        while !done(&self.state) {
            self.queue
                .blocking_dispatch(&mut self.state)
                .context("Wayland dispatch failed")?;
        }
        Ok(())
    }
}

impl CompositorHandler for State {
    fn scale_factor_changed(
        &mut self,
        _: &Connection,
        _: &QueueHandle<Self>,
        _: &wl_surface::WlSurface,
        _: i32,
    ) {
    }

    fn transform_changed(
        &mut self,
        _: &Connection,
        _: &QueueHandle<Self>,
        _: &wl_surface::WlSurface,
        _: wl_output::Transform,
    ) {
    }

    fn frame(&mut self, _: &Connection, _: &QueueHandle<Self>, _: &wl_surface::WlSurface, _: u32) {}

    fn surface_enter(
        &mut self,
        _: &Connection,
        _: &QueueHandle<Self>,
        _: &wl_surface::WlSurface,
        _: &wl_output::WlOutput,
    ) {
    }

    fn surface_leave(
        &mut self,
        _: &Connection,
        _: &QueueHandle<Self>,
        _: &wl_surface::WlSurface,
        _: &wl_output::WlOutput,
    ) {
    }
}

impl OutputHandler for State {
    fn output_state(&mut self) -> &mut OutputState {
        &mut self.outputs
    }

    fn new_output(&mut self, _: &Connection, _: &QueueHandle<Self>, _: wl_output::WlOutput) {}

    fn update_output(&mut self, _: &Connection, _: &QueueHandle<Self>, _: wl_output::WlOutput) {}

    fn output_destroyed(&mut self, _: &Connection, _: &QueueHandle<Self>, _: wl_output::WlOutput) {}
}

impl LayerShellHandler for State {
    fn closed(&mut self, _: &Connection, _: &QueueHandle<Self>, _: &LayerSurface) {}

    fn configure(
        &mut self,
        _: &Connection,
        _: &QueueHandle<Self>,
        _: &LayerSurface,
        _: LayerSurfaceConfigure,
        _: u32,
    ) {
    }
}

impl SeatHandler for State {
    fn seat_state(&mut self) -> &mut SeatState {
        &mut self.seat
    }

    fn new_seat(&mut self, _: &Connection, _: &QueueHandle<Self>, _: wl_seat::WlSeat) {}

    fn new_capability(
        &mut self,
        _: &Connection,
        qh: &QueueHandle<Self>,
        seat: wl_seat::WlSeat,
        capability: Capability,
    ) {
        if capability == Capability::Keyboard && self.keyboard.is_none() {
            self.keyboard = self.seat.get_keyboard(qh, &seat, None).ok();
        }
        if capability == Capability::Pointer
            && self.pointer.is_none()
            && let Ok(pointer) = self.seat.get_pointer(qh, &seat)
        {
            self.cursor_device = self
                .cursor_shape
                .as_ref()
                .map(|m| m.get_shape_device(&pointer, qh));
            self.pointer = Some(pointer);
        }
    }

    fn remove_capability(
        &mut self,
        _: &Connection,
        _: &QueueHandle<Self>,
        _: wl_seat::WlSeat,
        capability: Capability,
    ) {
        if capability == Capability::Keyboard
            && let Some(k) = self.keyboard.take()
        {
            k.release();
        }
        if capability == Capability::Pointer {
            if let Some(d) = self.cursor_device.take() {
                d.destroy();
            }
            if let Some(p) = self.pointer.take() {
                p.release();
            }
        }
    }

    fn remove_seat(&mut self, _: &Connection, _: &QueueHandle<Self>, _: wl_seat::WlSeat) {}
}

impl KeyboardHandler for State {
    fn enter(
        &mut self,
        _: &Connection,
        _: &QueueHandle<Self>,
        _: &wl_keyboard::WlKeyboard,
        _: &wl_surface::WlSurface,
        _: u32,
        _: &[u32],
        _: &[Keysym],
    ) {
    }

    fn leave(
        &mut self,
        _: &Connection,
        _: &QueueHandle<Self>,
        _: &wl_keyboard::WlKeyboard,
        _: &wl_surface::WlSurface,
        _: u32,
    ) {
    }

    fn press_key(
        &mut self,
        _: &Connection,
        _: &QueueHandle<Self>,
        _: &wl_keyboard::WlKeyboard,
        _: u32,
        _: KeyEvent,
    ) {
    }

    fn repeat_key(
        &mut self,
        _: &Connection,
        _: &QueueHandle<Self>,
        _: &wl_keyboard::WlKeyboard,
        _: u32,
        _: KeyEvent,
    ) {
    }

    fn release_key(
        &mut self,
        _: &Connection,
        _: &QueueHandle<Self>,
        _: &wl_keyboard::WlKeyboard,
        _: u32,
        _: KeyEvent,
    ) {
    }

    fn update_modifiers(
        &mut self,
        _: &Connection,
        _: &QueueHandle<Self>,
        _: &wl_keyboard::WlKeyboard,
        _: u32,
        _: Modifiers,
        _: RawModifiers,
        _: u32,
    ) {
    }
}

impl PointerHandler for State {
    fn pointer_frame(
        &mut self,
        _: &Connection,
        _: &QueueHandle<Self>,
        _: &wl_pointer::WlPointer,
        _: &[PointerEvent],
    ) {
    }
}

impl ShmHandler for State {
    fn shm_state(&mut self) -> &mut Shm {
        &mut self.shm
    }
}

impl ProvidesRegistryState for State {
    fn registry(&mut self) -> &mut RegistryState {
        &mut self.registry
    }
    registry_handlers![OutputState, SeatState];
}

delegate_registry!(State);
smithay_client_toolkit::delegate_dispatch2!(State);
wayland_client::delegate_noop!(State: ZwlrScreencopyManagerV1);
wayland_client::delegate_noop!(State: WpViewporter);
wayland_client::delegate_noop!(State: WpViewport);
```

- [ ] **Step 6: Write the failing tests for `doctor`**

Create `src/doctor.rs` with only the test module:

```rust
#[cfg(test)]
mod tests {
    use super::*;

    fn globals(list: &[(&str, u32)]) -> Vec<(String, u32)> {
        list.iter().map(|&(n, v)| (n.to_string(), v)).collect()
    }

    fn niri_26_04() -> Vec<(String, u32)> {
        globals(&[
            ("wl_compositor", 6),
            ("wl_shm", 2),
            ("wl_seat", 9),
            ("zxdg_output_manager_v1", 3),
            ("zwlr_layer_shell_v1", 5),
            ("zwlr_screencopy_manager_v1", 3),
            ("wp_viewporter", 1),
            ("wp_cursor_shape_manager_v1", 2),
            ("zwlr_data_control_manager_v1", 2),
            ("ext_data_control_manager_v1", 1),
        ])
    }

    #[test]
    fn all_present_is_all_ok() {
        let checks = check_globals(&niri_26_04());
        assert!(checks.iter().all(|c| c.level == Level::Ok), "{checks:#?}");
        assert!(
            checks
                .iter()
                .any(|c| c.text == "ext_data_control_manager_v1 v1 (clipboard copy)")
        );
    }

    #[test]
    fn missing_required_fails_missing_optional_warns() {
        let list: Vec<_> = niri_26_04()
            .into_iter()
            .filter(|(n, _)| n != "zwlr_screencopy_manager_v1" && n != "wp_cursor_shape_manager_v1")
            .collect();
        let checks = check_globals(&list);
        assert!(checks.contains(&check(
            Level::Fail,
            "zwlr_screencopy_manager_v1 missing (screen capture)"
        )));
        assert!(checks.contains(&check(
            Level::Warn,
            "wp_cursor_shape_manager_v1 missing (crosshair cursor in region mode disabled)"
        )));
    }

    #[test]
    fn old_version_fails() {
        let mut list = niri_26_04();
        list.retain(|(n, _)| n != "zxdg_output_manager_v1");
        list.push(("zxdg_output_manager_v1".into(), 1));
        assert!(check_globals(&list).contains(&check(
            Level::Fail,
            "zxdg_output_manager_v1 v1 is too old, need v2 (output names and positions)"
        )));
    }

    #[test]
    fn niri_versions() {
        assert_eq!(check_niri(Ok("26.04 (Nixpkgs)".into())).level, Level::Ok);
        assert_eq!(check_niri(Ok("25.11".into())).level, Level::Warn);
        assert_eq!(
            check_niri(Err(anyhow::anyhow!("no socket"))).level,
            Level::Warn
        );
    }

    #[test]
    fn save_dir_checks() {
        let dir = tempfile::tempdir().unwrap();
        assert_eq!(check_save_dir(&dir.path().join("new")).level, Level::Ok);
        let file = dir.path().join("file");
        std::fs::write(&file, b"").unwrap();
        assert_eq!(check_save_dir(&file).level, Level::Fail);
    }

    #[test]
    fn line_format() {
        assert_eq!(check(Level::Warn, "x").to_string(), "warn  x");
        assert_eq!(check(Level::Ok, "x").to_string(), "ok    x");
    }
}
```

Add `mod doctor;` to the `mod` list at the top of `src/main.rs` (keep it sorted).

- [ ] **Step 7: Run the tests and watch them fail**

Run: `cargo nextest run doctor::`
Expected: compilation fails, e.g. `` cannot find function `check_globals` in this scope ``.

- [ ] **Step 8: Implement `doctor`**

Put this above the test module in `src/doctor.rs`:

```rust
use std::fmt;
use std::path::Path;

use anyhow::{Result, bail};

use crate::config;
use crate::niri;
use crate::wayland::Wayland;

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Level {
    Ok,
    Warn,
    Fail,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Check {
    pub level: Level,
    pub text: String,
}

impl fmt::Display for Check {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let tag = match self.level {
            Level::Ok => "ok",
            Level::Warn => "warn",
            Level::Fail => "fail",
        };
        write!(f, "{tag:<5} {}", self.text)
    }
}

fn check(level: Level, text: impl Into<String>) -> Check {
    Check {
        level,
        text: text.into(),
    }
}

/// Globals valw can't work without: (interface, minimum version, used for).
const REQUIRED: [(&str, u32, &str); 7] = [
    ("wl_compositor", 1, "surfaces"),
    ("wl_shm", 1, "pixel buffers"),
    ("wl_seat", 1, "mouse and keyboard"),
    ("zxdg_output_manager_v1", 2, "output names and positions"),
    ("zwlr_layer_shell_v1", 1, "the selection overlay"),
    ("zwlr_screencopy_manager_v1", 1, "screen capture"),
    ("wp_viewporter", 1, "sharp overlays at fractional scale"),
];

/// Globals that enable a feature: (any of these interfaces, feature).
const OPTIONAL: [(&[&str], &str); 2] = [
    (
        &["wp_cursor_shape_manager_v1"],
        "crosshair cursor in region mode",
    ),
    (
        &[
            "ext_data_control_manager_v1",
            "zwlr_data_control_manager_v1",
        ],
        "clipboard copy",
    ),
];

pub fn check_globals(globals: &[(String, u32)]) -> Vec<Check> {
    let version = |name: &str| globals.iter().find(|(i, _)| i == name).map(|&(_, v)| v);
    let mut checks = Vec::new();
    for (name, min, purpose) in REQUIRED {
        checks.push(match version(name) {
            Some(v) if v >= min => check(Level::Ok, format!("{name} v{v} ({purpose})")),
            Some(v) => check(
                Level::Fail,
                format!("{name} v{v} is too old, need v{min} ({purpose})"),
            ),
            None => check(Level::Fail, format!("{name} missing ({purpose})")),
        });
    }
    for (names, feature) in OPTIONAL {
        checks.push(
            match names.iter().find_map(|&n| version(n).map(|v| (n, v))) {
                Some((n, v)) => check(Level::Ok, format!("{n} v{v} ({feature})")),
                None => check(
                    Level::Warn,
                    format!("{} missing ({feature} disabled)", names.join(" / ")),
                ),
            },
        );
    }
    checks
}

pub fn check_niri(version: Result<String>) -> Check {
    let (y, m) = niri::IPC_VERSION;
    match version {
        Err(e) => check(
            Level::Warn,
            format!("niri IPC unavailable ({e:#}); using the first output for `screen`"),
        ),
        Ok(v) => match niri::parse_version(&v) {
            Some(found) if found == niri::IPC_VERSION => check(Level::Ok, format!("niri {v}")),
            _ => check(
                Level::Warn,
                format!(
                    "niri {v}, but valw was built against niri-ipc {y}.{m}; IPC replies may not parse"
                ),
            ),
        },
    }
}

pub fn check_config(path: &Path) -> Check {
    match config::load(path) {
        Ok(_) if path.exists() => check(Level::Ok, format!("config {}", path.display())),
        Ok(_) => check(
            Level::Ok,
            format!("config {} not found, using defaults", path.display()),
        ),
        Err(e) => check(Level::Fail, format!("config: {e:#}")),
    }
}

pub fn check_save_dir(dir: &Path) -> Check {
    let probe = dir.join(".valw-doctor");
    let result = std::fs::create_dir_all(dir)
        .and_then(|()| std::fs::write(&probe, b""))
        .and_then(|()| std::fs::remove_file(&probe));
    match result {
        Ok(()) => check(
            Level::Ok,
            format!("save directory {} is writable", dir.display()),
        ),
        Err(e) => check(
            Level::Fail,
            format!("save directory {}: {e}", dir.display()),
        ),
    }
}

/// Runs every check and prints one line each. Fails if any check failed.
pub fn run() -> Result<()> {
    let mut checks = Vec::new();
    match Wayland::connect() {
        Ok(wl) => {
            checks.extend(check_globals(&wl.global_list()));
            for o in wl.outputs() {
                let g = &o.geom;
                checks.push(check(
                    Level::Ok,
                    format!(
                        "output {}: {}x{} logical at {},{}, transform {:?}",
                        g.name, g.width, g.height, g.x, g.y, o.transform
                    ),
                ));
            }
        }
        Err(e) => checks.push(check(Level::Fail, format!("wayland: {e:#}"))),
    }
    checks.push(check_niri(niri::version()));
    let config_path = config::default_path();
    checks.push(check_config(&config_path));
    if let Ok(config) = config::load(&config_path) {
        checks.push(check_save_dir(&config.save_dir()));
    }

    for c in &checks {
        println!("{c}");
    }
    let failed = checks.iter().filter(|c| c.level == Level::Fail).count();
    if failed > 0 {
        bail!("{failed} check(s) failed");
    }
    Ok(())
}
```

- [ ] **Step 9: Run the tests and watch them pass**

Run: `cargo nextest run doctor::`
Expected: all 6 tests in `doctor::tests` pass.

- [ ] **Step 10: Replace `src/main.rs`**

```rust
// Modules land before the CLI uses them; Task 11 removes this.
#![allow(dead_code)]

mod config;
mod detach;
mod doctor;
mod error;
mod frame;
mod lock;
mod log;
mod niri;
mod output;
mod render;
mod selection;
mod wayland;

use std::process::ExitCode;

use anyhow::Result;
use chrono::Local;
use clap::{Parser, Subcommand};

use crate::error::Cancelled;

#[derive(Parser)]
#[command(version, about = "macOS-style screenshots for niri")]
struct Cli {
    #[command(subcommand)]
    command: Command,
}

#[derive(Subcommand)]
enum Command {
    /// Report what the compositor and system support.
    Doctor,
}

fn main() -> ExitCode {
    let cli = Cli::parse();
    let log = log::init(&log::default_dir(), Local::now());
    log::install_panic_hook(log.clone());
    tracing::info!(
        "valw {} {:?}",
        env!("CARGO_PKG_VERSION"),
        std::env::args().collect::<Vec<_>>()
    );

    match run(cli) {
        Ok(()) => ExitCode::SUCCESS,
        Err(e) if e.downcast_ref::<Cancelled>().is_some() => {
            tracing::info!("cancelled by the user");
            ExitCode::from(3)
        }
        Err(e) => {
            tracing::error!("{e:#}");
            eprint!("{}", error::render(&e, log.as_deref()));
            ExitCode::FAILURE
        }
    }
}

fn run(cli: Cli) -> Result<()> {
    match cli.command {
        Command::Doctor => doctor::run(),
    }
}
```

Run: `cargo fmt && cargo clippy --all-targets -- -D warnings && cargo nextest run`
Expected: no warnings; every test passes.

- [ ] **Step 11: Run `valw doctor` on niri (Phase 0 discovery report)**

Run: `cargo run --release -- doctor; echo "exit=$?"`
Expected, on the user's machine, is something like the following (the spec's section 2 table lists the same globals):

```
ok    wl_compositor v6 (surfaces)
ok    wl_shm v2 (pixel buffers)
ok    wl_seat v9 (mouse and keyboard)
ok    zxdg_output_manager_v1 v3 (output names and positions)
ok    zwlr_layer_shell_v1 v5 (the selection overlay)
ok    zwlr_screencopy_manager_v1 v3 (screen capture)
ok    wp_viewporter v1 (sharp overlays at fractional scale)
ok    wp_cursor_shape_manager_v1 v2 (crosshair cursor in region mode)
ok    ext_data_control_manager_v1 v1 (clipboard copy)
ok    output LVDS-1: 1366x768 logical at 0,0, transform Normal
ok    output HDMI-A-1: 1920x1080 logical at -277,-1080, transform Normal
ok    niri 26.04 (Nixpkgs)
ok    config /home/elars/.config/valw/config.toml not found, using defaults
ok    save directory /home/elars/Pictures/Screenshots is writable
exit=0
```

Output positions depend on the current monitor layout. Any `fail` line is a blocker; report it rather than working around it. Also check that `~/.local/state/valw/logs/` now holds a log file whose first lines include the valw version and the `globals:` line.

- [ ] **Step 12: Check the error path**

Run: `WAYLAND_DISPLAY=nope cargo run --release -- doctor; echo "exit=$?"`
Expected: a `fail  wayland: could not connect to the Wayland compositor: ...` line and exit 1. This is the only path where doctor itself fails.

- [ ] **Step 13: Commit**

```sh
jj commit -m "feat: wayland connection, niri IPC and valw doctor"
```

---


### Task 10: Screen capture and `valw screen`

**Files:**
- Create: `src/capture.rs`
- Modify: `src/wayland.rs` (screencopy bookkeeping)
- Replace: `src/main.rs`

**Interfaces:**
- Consumes: `wayland::{Wayland, State, Output}` and `Wayland::dispatch_until` (Task 9); `frame::{Frame, SUPPORTED, to_bgrx, apply_transform, Frame::to_rgba, Frame::full}` (Task 6); `output::*` (Task 8); `lock::Lock` (Task 5); `niri::{focused_output, version}` (Task 9); `config` (Task 3).
- Produces: `pub trait Capturer { fn capture(&mut self, outputs: &[Output], cursor: bool) -> Result<Vec<Frame>>; }` implemented for `Wayland`; `pub struct Pending` (one in-flight screencopy, stored in `State::captures`); CLI `valw screen [--all] [--clipboard-only] [-o PATH|-] [--delay SECS] [--cursor]`.

Two facts the code depends on, both verified on niri 26.04:
- **niri rejects a screencopy shm buffer unless its `wl_shm_pool` is exactly the buffer's size** (`src/protocols/screencopy.rs`: `shm_len == stride * height`). Every capture therefore gets its own `RawPool`. A shared, larger pool fails with `invalid buffer`.
- **wl-clipboard-rs never forks.** Its background mode is a thread, which dies with the process. `output::copy_to_clipboard` (Task 8) forks through `detach::spawn` instead.

- [ ] **Step 1: Write `src/capture.rs`**

Add `mod capture;` to the `mod` list in `src/main.rs`. The pixel logic here was unit-tested in Task 6; this file is protocol plumbing.

```rust
use std::sync::Arc;
use std::time::{Duration, Instant};

use anyhow::{Context, Result, anyhow};
use smithay_client_toolkit::shm::raw::RawPool;
use wayland_client::{
    Connection, Dispatch, Proxy, QueueHandle, WEnum,
    protocol::{wl_buffer::WlBuffer, wl_shm::Format},
};
use wayland_protocols_wlr::screencopy::v1::client::zwlr_screencopy_frame_v1::{
    self, ZwlrScreencopyFrameV1,
};

use crate::error::HintExt;
use crate::frame::{self, Frame, SUPPORTED};
use crate::wayland::{Output, State, Wayland};

/// How long the compositor gets to deliver all frames.
const TIMEOUT: Duration = Duration::from_secs(5);

/// Something that captures outputs. Only wlr-screencopy exists today;
/// ext-image-copy-capture would be a second implementation.
pub trait Capturer {
    fn capture(&mut self, outputs: &[Output], cursor: bool) -> Result<Vec<Frame>>;
}

/// A buffer layout the compositor offered.
#[derive(Debug, Clone, Copy)]
struct Layout {
    format: Format,
    width: u32,
    height: u32,
    stride: u32,
}

/// One screencopy request in flight.
pub struct Pending {
    output: Output,
    offered: Vec<Format>,
    layout: Option<Layout>,
    // niri rejects a screencopy buffer unless its pool is exactly the
    // buffer's size, so every capture gets a pool of its own.
    pool: Option<RawPool>,
    buffer: Option<WlBuffer>,
    y_invert: bool,
    result: Option<Result<Frame>>,
}

impl Capturer for Wayland {
    fn capture(&mut self, outputs: &[Output], cursor: bool) -> Result<Vec<Frame>> {
        let manager = self
            .state
            .screencopy
            .clone()
            .context("compositor does not support wlr-screencopy")
            .hint("run `valw doctor` to see what the compositor supports")?;
        let qh = self.queue.handle();

        self.state.captures = outputs
            .iter()
            .map(|o| Pending {
                output: o.clone(),
                offered: Vec::new(),
                layout: None,
                pool: None,
                buffer: None,
                y_invert: false,
                result: None,
            })
            .collect();
        for (i, o) in outputs.iter().enumerate() {
            manager.capture_output(cursor as i32, &o.wl, &qh, i);
        }

        let started = Instant::now();
        self.dispatch_until(started + TIMEOUT, |s| {
            s.captures.iter().all(|c| c.result.is_some())
        })
        .context("screen capture failed")?;
        tracing::info!(
            "captured {} output(s) in {:?} ({:?} after start)",
            outputs.len(),
            started.elapsed(),
            crate::log::since_start()
        );

        std::mem::take(&mut self.state.captures)
            .into_iter()
            .map(|c| {
                let name = c.output.geom.name.clone();
                c.result
                    .expect("checked above")
                    .with_context(|| format!("could not capture output {name}"))
            })
            .collect()
    }
}

impl Dispatch<ZwlrScreencopyFrameV1, usize> for State {
    fn event(
        state: &mut Self,
        frame: &ZwlrScreencopyFrameV1,
        event: zwlr_screencopy_frame_v1::Event,
        &i: &usize,
        _: &Connection,
        qh: &QueueHandle<Self>,
    ) {
        use zwlr_screencopy_frame_v1::Event;
        match event {
            Event::Buffer {
                format: WEnum::Value(format),
                width,
                height,
                stride,
            } => {
                let c = &mut state.captures[i];
                c.offered.push(format);
                let better = c.layout.is_none_or(|l| rank(format) < rank(l.format));
                if SUPPORTED.contains(&format) && better {
                    c.layout = Some(Layout {
                        format,
                        width,
                        height,
                        stride,
                    });
                }
                // Before v3 there is no buffer_done; the first buffer event is all we get.
                if frame.version() < 3 {
                    start_copy(state, frame, i, qh);
                }
            }
            Event::BufferDone => start_copy(state, frame, i, qh),
            Event::Flags { flags } => {
                state.captures[i].y_invert = flags
                    .into_result()
                    .is_ok_and(|f| f.contains(zwlr_screencopy_frame_v1::Flags::YInvert));
            }
            Event::Ready { .. } => {
                let c = &mut state.captures[i];
                c.result = Some(finish(c));
                if let Some(buffer) = c.buffer.take() {
                    buffer.destroy();
                }
                frame.destroy();
            }
            Event::Failed => {
                state.captures[i].result = Some(Err(anyhow!("the compositor refused the capture")));
                frame.destroy();
            }
            _ => {}
        }
    }
}

fn rank(format: Format) -> usize {
    SUPPORTED
        .iter()
        .position(|&f| f == format)
        .unwrap_or(usize::MAX)
}

fn start_copy(state: &mut State, frame: &ZwlrScreencopyFrameV1, i: usize, qh: &QueueHandle<State>) {
    let c = &mut state.captures[i];
    if c.buffer.is_some() || c.result.is_some() {
        return;
    }
    let Some(l) = c.layout else {
        c.result = Some(
            Err(anyhow!(
                "compositor offered only {:?}, which valw can't convert yet",
                c.offered
            ))
            .hint("run `valw doctor` and include its output in a bug report"),
        );
        frame.destroy();
        return;
    };
    tracing::info!(
        "{}: offered {:?}, using {:?} {}x{}",
        c.output.geom.name,
        c.offered,
        l.format,
        l.width,
        l.height
    );
    let len = (l.stride * l.height) as usize;
    match RawPool::new(len, &state.shm) {
        Ok(mut pool) => {
            let buffer = pool.create_buffer(
                0,
                l.width as i32,
                l.height as i32,
                l.stride as i32,
                l.format,
                (),
                qh,
            );
            frame.copy(&buffer);
            c.pool = Some(pool);
            c.buffer = Some(buffer);
        }
        Err(e) => {
            c.result = Some(Err(e).context("could not allocate a capture buffer"));
            frame.destroy();
        }
    }
}

fn finish(c: &mut Pending) -> Result<Frame> {
    let l = c.layout.context("ready without a buffer")?;
    let pool = c.pool.as_mut().context("ready without a buffer")?;
    let data = &pool.mmap()[..];
    let pixels = frame::to_bgrx(l.format, l.width, l.height, l.stride, data, c.y_invert)?;
    Ok(Frame {
        pixels: Arc::new(frame::apply_transform(pixels, c.output.transform)),
        output: c.output.geom.clone(),
    })
}
```

- [ ] **Step 2: Track captures in `State`**

Apply this change to `src/wayland.rs`:

```diff
--- a/src/wayland.rs
+++ b/src/wayland.rs
@@ -19,7 +19,7 @@
 use wayland_client::{
     Connection, EventQueue, QueueHandle,
     globals::{GlobalList, registry_queue_init},
-    protocol::{wl_keyboard, wl_output, wl_pointer, wl_seat, wl_surface},
+    protocol::{wl_buffer::WlBuffer, wl_keyboard, wl_output, wl_pointer, wl_seat, wl_surface},
 };
 use wayland_protocols::wp::cursor_shape::v1::client::wp_cursor_shape_device_v1::WpCursorShapeDeviceV1;
 use wayland_protocols::wp::viewporter::client::{
@@ -27,6 +27,7 @@
 };
 use wayland_protocols_wlr::screencopy::v1::client::zwlr_screencopy_manager_v1::ZwlrScreencopyManagerV1;
 
+use crate::capture::Pending;
 use crate::error::HintExt;
 use crate::frame::OutputGeom;
 
@@ -58,6 +59,8 @@
     pub cursor_device: Option<WpCursorShapeDeviceV1>,
     pub keyboard: Option<wl_keyboard::WlKeyboard>,
     pub pointer: Option<wl_pointer::WlPointer>,
+    /// Screencopy requests in flight.
+    pub captures: Vec<Pending>,
 }
 
 impl Wayland {
@@ -83,6 +86,7 @@
             cursor_device: None,
             keyboard: None,
             pointer: None,
+            captures: Vec::new(),
         };
         // Two round trips: one for wl_output, one for the xdg-output details.
         queue
@@ -425,3 +429,4 @@
 wayland_client::delegate_noop!(State: ZwlrScreencopyManagerV1);
 wayland_client::delegate_noop!(State: WpViewporter);
 wayland_client::delegate_noop!(State: WpViewport);
+wayland_client::delegate_noop!(State: ignore WlBuffer);
```

- [ ] **Step 3: Write the failing CLI tests**

Replace `src/main.rs` with the version below. It adds the `screen` command, the capture → deliver flow, and a test module that pins down the flag conflicts and `--delay` parsing.

```rust
// Modules land before the CLI uses them; Task 11 removes this.
#![allow(dead_code)]

mod capture;
mod config;
mod detach;
mod doctor;
mod error;
mod frame;
mod lock;
mod log;
mod niri;
mod output;
mod render;
mod selection;
mod wayland;

use std::path::PathBuf;
use std::process::ExitCode;
use std::time::Duration;

use anyhow::{Context, Result};
use chrono::Local;
use clap::{Args, Parser, Subcommand};
use image::RgbaImage;

use crate::capture::Capturer;
use crate::config::Config;
use crate::error::Cancelled;
use crate::lock::Lock;
use crate::output::Target;
use crate::wayland::Wayland;

#[derive(Parser)]
#[command(version, about = "macOS-style screenshots for niri")]
struct Cli {
    #[command(subcommand)]
    command: Command,
}

#[derive(Subcommand)]
enum Command {
    /// Capture the focused output (Cmd+Shift+3).
    Screen {
        /// Capture every output, one file each.
        #[arg(long, conflicts_with_all = ["output", "clipboard_only"])]
        all: bool,
        #[command(flatten)]
        common: Common,
    },
    /// Report what the compositor and system support.
    Doctor,
}

#[derive(Args)]
struct Common {
    /// Copy to the clipboard without saving a file.
    #[arg(long, conflicts_with = "output")]
    clipboard_only: bool,
    /// Write to this path instead of the configured directory; "-" for stdout.
    #[arg(short, long, value_name = "PATH")]
    output: Option<PathBuf>,
    /// Wait this many seconds before capturing.
    #[arg(long, value_name = "SECS", value_parser = parse_delay)]
    delay: Option<Duration>,
    /// Include the mouse cursor.
    #[arg(long)]
    cursor: bool,
}

fn parse_delay(s: &str) -> Result<Duration, String> {
    let secs: f64 = s.parse().map_err(|_| format!("not a number: {s}"))?;
    Duration::try_from_secs_f64(secs)
        .map_err(|_| format!("must be a non-negative number of seconds: {s}"))
}

enum Mode {
    Screen { all: bool },
}

fn main() -> ExitCode {
    let cli = Cli::parse();
    let log = log::init(&log::default_dir(), Local::now());
    log::install_panic_hook(log.clone());
    tracing::info!(
        "valw {} {:?}",
        env!("CARGO_PKG_VERSION"),
        std::env::args().collect::<Vec<_>>()
    );

    match run(cli) {
        Ok(()) => ExitCode::SUCCESS,
        Err(e) if e.downcast_ref::<Cancelled>().is_some() => {
            tracing::info!("cancelled by the user");
            ExitCode::from(3)
        }
        Err(e) => {
            tracing::error!("{e:#}");
            eprint!("{}", error::render(&e, log.as_deref()));
            ExitCode::FAILURE
        }
    }
}

fn run(cli: Cli) -> Result<()> {
    match cli.command {
        Command::Doctor => doctor::run(),
        Command::Screen { all, common } => capture(Mode::Screen { all }, common),
    }
}

fn capture(mode: Mode, common: Common) -> Result<()> {
    let config = config::load(&config::default_path())?;
    let lock = Lock::acquire(&lock::default_path())?;
    if let Some(delay) = common.delay {
        std::thread::sleep(delay);
    }
    let cursor = common.cursor || config.capture.show_cursor;
    let mut wl = Wayland::connect()?;
    if let Ok(version) = niri::version() {
        tracing::info!("niri {version}");
    }
    let outputs = wl.outputs();
    anyhow::ensure!(!outputs.is_empty(), "the compositor reported no outputs");

    // (image, output name for the file name) and which one goes to the clipboard.
    let (shots, clip): (Vec<(RgbaImage, Option<String>)>, usize) = match mode {
        Mode::Screen { all } => {
            let focused = focused_output(&outputs);
            if all {
                let frames = wl.capture(&outputs, cursor)?;
                let shots = frames
                    .into_iter()
                    .map(|f| (f.to_rgba(f.full()), Some(f.output.name)))
                    .collect();
                (shots, focused)
            } else {
                let frame = wl.capture(&outputs[focused..=focused], cursor)?.remove(0);
                (vec![(frame.to_rgba(frame.full()), None)], 0)
            }
        }
    };
    drop(wl);

    let target = Target::from_args(common.output, common.clipboard_only);
    deliver(shots, clip, &target, &config, lock)
}

/// Index of niri's focused output, or 0 if niri can't tell us.
fn focused_output(outputs: &[wayland::Output]) -> usize {
    let name = match niri::focused_output() {
        Ok(Some(name)) => name,
        Ok(None) => return 0,
        Err(e) => {
            tracing::warn!("using the first output: {e:#}");
            return 0;
        }
    };
    outputs
        .iter()
        .position(|o| o.geom.name == name)
        .unwrap_or_else(|| {
            tracing::warn!("niri's focused output {name} is not among the Wayland outputs");
            0
        })
}

fn deliver(
    shots: Vec<(RgbaImage, Option<String>)>,
    clip: usize,
    target: &Target,
    config: &Config,
    lock: Lock,
) -> Result<()> {
    let now = Local::now();
    let pngs = shots
        .iter()
        .map(|(img, _)| output::encode_png(img))
        .collect::<Result<Vec<_>>>()?;

    match target {
        Target::Default => {
            let dir = config.save_dir();
            for (png, (_, name)) in pngs.iter().zip(&shots) {
                let file = output::file_name(&config.save.filename, &now, name.as_deref())?;
                let path = output::unique_path(&dir, &file);
                output::write_atomic(&path, png)?;
                println!("{}", path.display());
            }
        }
        Target::Path(path) => {
            output::write_atomic(path, &pngs[0])?;
            println!("{}", path.display());
        }
        Target::Stdout => output::write_stdout(&pngs[0])?,
        Target::ClipboardOnly => {}
    }

    if *target == Target::ClipboardOnly || config.save.copy_to_clipboard {
        let png = pngs
            .into_iter()
            .nth(clip)
            .context("no image for the clipboard")?;
        output::copy_to_clipboard(png, lock)?;
    }
    Ok(())
}

#[cfg(test)]
mod tests {
    use super::*;
    use clap::CommandFactory;

    fn parses(args: &[&str]) -> bool {
        Cli::try_parse_from(std::iter::once("valw").chain(args.iter().copied())).is_ok()
    }

    #[test]
    fn cli_definition_is_valid() {
        Cli::command().debug_assert();
    }

    #[test]
    fn flag_conflicts() {
        assert!(parses(&["screen", "--all"]));
        assert!(parses(&["screen", "-o", "-"]));
        assert!(parses(&["screen", "--clipboard-only", "--cursor"]));
        assert!(!parses(&["screen", "--all", "-o", "-"]));
        assert!(!parses(&["screen", "--all", "--clipboard-only"]));
        assert!(!parses(&["screen", "--clipboard-only", "-o", "a.png"]));
    }

    #[test]
    fn delay_values() {
        assert_eq!(parse_delay("1.5"), Ok(Duration::from_millis(1500)));
        assert_eq!(parse_delay("0"), Ok(Duration::ZERO));
        assert!(parse_delay("-1").is_err());
        assert!(parse_delay("soon").is_err());
        assert!(parse_delay("NaN").is_err());
    }
}
```

- [ ] **Step 4: Run the tests**

Run: `cargo nextest run`
Expected: every test passes, including `tests::flag_conflicts`, `tests::delay_values` and `tests::cli_definition_is_valid`. (The CLI tests were written together with the code they test because clap's derive definitions are the implementation.)

- [ ] **Step 5: Lint**

Run: `cargo fmt && cargo clippy --all-targets -- -D warnings`
Expected: no warnings.

- [ ] **Step 6: Capture on niri without touching the real clipboard or Pictures**

```sh
t=$(mktemp -d)
mkdir -p $t/config/valw
printf '[save]\ndirectory = "%s/shots"\ncopy_to_clipboard = false\n' $t > $t/config/valw/config.toml
export XDG_CONFIG_HOME=$t/config XDG_STATE_HOME=$t/state
cargo build --release
V=$CARGO_TARGET_DIR/release/valw
$V screen -o - | file -
$V screen --all && file $t/shots/*
$V screen --delay 0.5 -o $t/one.png && file $t/one.png
```

Expected: `PNG image data, 1366 x 768` (or whichever output niri has focused). With `--all`, one file per output named `Screenshot … (LVDS-1).png` / `(HDMI-A-1).png` at each output's physical size. The log in `$t/state/valw/logs/` shows `offered [Xrgb8888], using Xrgb8888 …` and `captured N output(s) in …ms`.

- [ ] **Step 7: Clipboard and pipes**

```sh
printf '[save]\ndirectory = "%s/shots"\n' $t > $t/config/valw/config.toml
time ($V screen -o - | wc -c)
wl-paste --list-types
wl-paste --type image/png | file -
```

Expected: the pipeline returns immediately (well under a second), even though the clipboard child is still running. `wl-paste --list-types` lists `image/png`, and the pasted data is the PNG. This step overwrites the clipboard, so tell the user before running it.

- [ ] **Step 8: Hotkey spam (Review Focus 4)**

```sh
$V screen --delay 3 -o $t/a.png & sleep 0.5; $V screen -o $t/b.png; echo "exit=$?"; wait
$V screen -o $t/c.png; echo "exit=$?"
```

Expected: the second command prints `error: valw is already running` with `hint:  wait for the other capture to finish, or press Esc in it` and `exit=1`. The first one then succeeds. The third succeeds (`exit=0`) even though the first run's clipboard child is still alive.

- [ ] **Step 9: Commit**

```sh
jj commit -m "feat: wlr-screencopy capture and valw screen"
```

---


### Task 11: Region selection overlay and `valw region`

**Files:**
- Create: `src/region.rs`
- Modify: `src/wayland.rs` (route overlay events), `src/main.rs` (add `region`, drop `#![allow(dead_code)]`)

**Interfaces:**
- Consumes: `selection::{Selection, Point, clip, resolve}` (Task 7); `render::{dim, compose, DIM}` (Task 7); `frame::{Bgrx, Frame, OutputGeom, PixelRect}` (Task 6); `wayland::{Wayland, State, Output}`, `Wayland::dispatch_blocking` (Task 9); `log::since_start` (Task 4); `error::{Cancelled, HintExt}` (Task 2).
- Produces: `pub const NAMESPACE: &str = "valw-overlay"`; `pub type Picked = (usize, PixelRect)`; `pub fn select(wl: &mut Wayland, outputs: &[Output], frames: &[Frame]) -> Result<Picked>` (returns `Cancelled` on Esc or right click); `pub struct Overlay` with `cancel`, `configure`, `frame_done`, `pointer`, called from `State`'s handlers. CLI: `valw region [COMMON]`.

How it works: one fullscreen layer surface per output shows that output's frozen frame, darkened. The buffer is XRGB8888 (the frame's own byte order, so no conversion) at the frame's physical size, and `wp_viewporter` maps it onto the surface's logical size, so it is shown 1:1 at any scale. The undimmed selection is pasted straight from the shared frame (`Arc<Bgrx>`, no copy). Pointer positions are converted to global logical coordinates. The drag is drawn (and later cropped) only on the output where it started. Redraws are paced by frame callbacks.

- [ ] **Step 1: Write `src/region.rs`**

Add `mod region;` to the `mod` list in `src/main.rs`.

```rust
use std::sync::Arc;

use anyhow::{Context, Result};
use smithay_client_toolkit::{
    compositor::FrameCallbackData,
    seat::pointer::{BTN_LEFT, BTN_RIGHT, PointerEvent, PointerEventKind},
    shell::{
        WaylandSurface,
        wlr_layer::{Anchor, KeyboardInteractivity, Layer, LayerSurface},
    },
    shm::{
        Shm,
        slot::{Buffer, SlotPool},
    },
};
use wayland_client::{
    QueueHandle,
    protocol::{wl_shm, wl_surface::WlSurface},
};
use wayland_protocols::wp::cursor_shape::v1::client::wp_cursor_shape_device_v1::{
    Shape, WpCursorShapeDeviceV1,
};
use wayland_protocols::wp::viewporter::client::wp_viewport::WpViewport;

use crate::error::{Cancelled, HintExt};
use crate::frame::{Bgrx, Frame, OutputGeom, PixelRect};
use crate::render;
use crate::selection::{self, Point, Selection};
use crate::wayland::{Output, State, Wayland};

pub const NAMESPACE: &str = "valw-overlay";

/// What the user picked: an output index and a rectangle in its frame.
pub type Picked = (usize, PixelRect);

/// One full-screen layer surface per output, showing the frozen frame.
pub struct Overlay {
    surfaces: Vec<Surface>,
    selection: Selection,
    /// Output where the current drag started; the capture is clipped to it.
    anchor: Option<usize>,
    outcome: Option<Option<Picked>>,
}

struct Surface {
    layer: LayerSurface,
    viewport: WpViewport,
    geom: OutputGeom,
    width: u32,
    height: u32,
    dark: Vec<u8>,
    bright: Arc<Bgrx>,
    pool: SlotPool,
    buffer: Option<Buffer>,
    configured: bool,
    frame_pending: bool,
    dirty: bool,
}

/// Shows the overlay on every output and waits for a selection.
/// Returns `Cancelled` if the user presses Esc or right-clicks.
pub fn select(wl: &mut Wayland, outputs: &[Output], frames: &[Frame]) -> Result<Picked> {
    let qh = wl.queue.handle();
    let s = &wl.state;
    let layer_shell = s
        .layer_shell
        .as_ref()
        .context("compositor does not support wlr-layer-shell")
        .hint("run `valw doctor` to see what the compositor supports")?;
    let viewporter = s
        .viewporter
        .as_ref()
        .context("compositor does not support wp-viewporter")
        .hint("run `valw doctor` to see what the compositor supports")?;

    let mut surfaces = Vec::new();
    for (output, frame) in outputs.iter().zip(frames) {
        let surface = s.compositor.create_surface(&qh);
        let layer = layer_shell.create_layer_surface(
            &qh,
            surface,
            Layer::Overlay,
            Some(NAMESPACE),
            Some(&output.wl),
        );
        layer.set_anchor(Anchor::all());
        layer.set_exclusive_zone(-1);
        layer.set_keyboard_interactivity(KeyboardInteractivity::Exclusive);
        layer.commit();
        let viewport = viewporter.get_viewport(layer.wl_surface(), &qh, ());
        surfaces.push(Surface::new(layer, viewport, frame, &s.shm)?);
    }

    tracing::debug!(
        "overlay buffers ready {:?} after start",
        crate::log::since_start()
    );
    wl.state.overlay = Some(Overlay {
        surfaces,
        selection: Selection::default(),
        anchor: None,
        outcome: None,
    });
    let result = wl.dispatch_blocking(|s| s.overlay.as_ref().is_some_and(|o| o.outcome.is_some()));
    let outcome = wl.state.overlay.take().and_then(|o| o.outcome);
    // Dropping the overlay destroys the surfaces; make that visible right away.
    let _ = wl.conn.flush();
    result?;
    outcome.flatten().ok_or_else(|| Cancelled.into())
}

impl Surface {
    fn new(layer: LayerSurface, viewport: WpViewport, frame: &Frame, shm: &Shm) -> Result<Surface> {
        let (width, height) = frame.pixels.dimensions();
        let len = (width * height * 4) as usize;
        Ok(Surface {
            layer,
            viewport,
            geom: frame.output.clone(),
            width,
            height,
            dark: render::dim(frame.pixels.as_raw(), render::DIM),
            bright: Arc::clone(&frame.pixels),
            // Room for two buffers so one can be on screen while we draw the next.
            pool: SlotPool::new(len * 2, shm).context("could not create shm pool")?,
            buffer: None,
            configured: false,
            frame_pending: false,
            dirty: true,
        })
    }

    fn draw(&mut self, sel: Option<PixelRect>, qh: &QueueHandle<State>) {
        let stride = self.width as i32 * 4;
        let Ok((buffer, canvas)) = self.pool.create_buffer(
            self.width as i32,
            self.height as i32,
            stride,
            wl_shm::Format::Xrgb8888,
        ) else {
            tracing::warn!("could not allocate an overlay buffer");
            return;
        };
        render::compose(canvas, &self.dark, self.bright.as_raw(), self.width, sel);

        let surface = self.layer.wl_surface();
        surface.damage_buffer(0, 0, self.width as i32, self.height as i32);
        surface.frame(qh, FrameCallbackData(surface.clone()));
        if buffer.attach_to(surface).is_err() {
            tracing::warn!("could not attach the overlay buffer");
            return;
        }
        self.layer.commit();
        self.buffer = Some(buffer);
        self.frame_pending = true;
        self.dirty = false;
    }
}

impl Overlay {
    pub fn cancel(&mut self) {
        self.outcome.get_or_insert(None);
    }

    fn index_of(&self, surface: &WlSurface) -> Option<usize> {
        self.surfaces
            .iter()
            .position(|s| s.layer.wl_surface() == surface)
    }

    /// The selection as drawn on surface `i`: only the anchor output shows it.
    fn selection_on(&self, i: usize) -> Option<PixelRect> {
        let Selection::Dragging { start, end } = self.selection else {
            return None;
        };
        let s = &self.surfaces[i];
        (self.anchor == Some(i))
            .then(|| selection::clip(start, end, &s.geom, s.width, s.height))
            .flatten()
    }

    fn redraw(&mut self, i: usize, qh: &QueueHandle<State>) {
        let sel = self.selection_on(i);
        let s = &mut self.surfaces[i];
        s.dirty = true;
        if s.configured && !s.frame_pending {
            s.draw(sel, qh);
        }
    }

    pub fn configure(&mut self, layer: &LayerSurface, (w, h): (u32, u32), qh: &QueueHandle<State>) {
        let Some(i) = self.index_of(layer.wl_surface()) else {
            return;
        };
        let s = &mut self.surfaces[i];
        // Show the physical-size buffer 1:1 on the logical-size surface.
        s.viewport.set_destination(w as i32, h as i32);
        if !s.configured {
            s.configured = true;
            let name = s.geom.name.clone();
            self.redraw(i, qh);
            tracing::info!(
                "overlay on {name} committed {:?} after start",
                crate::log::since_start()
            );
        }
    }

    pub fn frame_done(&mut self, surface: &WlSurface, qh: &QueueHandle<State>) {
        let Some(i) = self.index_of(surface) else {
            return;
        };
        self.surfaces[i].frame_pending = false;
        if self.surfaces[i].dirty {
            self.redraw(i, qh);
        }
    }

    pub fn pointer(
        &mut self,
        events: &[PointerEvent],
        cursor: Option<&WpCursorShapeDeviceV1>,
        qh: &QueueHandle<State>,
    ) {
        for event in events {
            let Some(i) = self.index_of(&event.surface) else {
                continue;
            };
            let g = &self.surfaces[i].geom;
            let p = Point {
                x: g.x as f64 + event.position.0,
                y: g.y as f64 + event.position.1,
            };
            match event.kind {
                PointerEventKind::Enter { serial } => {
                    if let Some(cursor) = cursor {
                        cursor.set_shape(serial, Shape::Crosshair);
                    }
                }
                PointerEventKind::Press {
                    button: BTN_LEFT, ..
                } => {
                    self.selection.press(p);
                    self.anchor = Some(i);
                }
                PointerEventKind::Press {
                    button: BTN_RIGHT, ..
                } => self.cancel(),
                PointerEventKind::Motion { .. } => {
                    self.selection.motion(p);
                    if let Some(a) = self.anchor {
                        self.redraw(a, qh);
                    }
                }
                PointerEventKind::Release {
                    button: BTN_LEFT, ..
                } => {
                    let (Some((start, end)), Some(a)) = (self.selection.release(p), self.anchor)
                    else {
                        continue;
                    };
                    let s = &self.surfaces[a];
                    match selection::resolve(start, end, &s.geom, s.width, s.height) {
                        Some(rect) => {
                            tracing::info!(
                                "selected {rect:?} on {} (logical {start:?} to {end:?})",
                                s.geom.name
                            );
                            self.outcome = Some(Some((a, rect)));
                        }
                        // A click, not a drag: clear it and keep waiting.
                        None => self.redraw(a, qh),
                    }
                }
                _ => {}
            }
        }
    }
}
```

- [ ] **Step 2: Route overlay events from `State`**

Apply this change to `src/wayland.rs`:

```diff
--- a/src/wayland.rs
+++ b/src/wayland.rs
@@ -30,6 +30,7 @@
 use crate::capture::Pending;
 use crate::error::HintExt;
 use crate::frame::OutputGeom;
+use crate::region::Overlay;
 
 /// An output as valw sees it.
 #[derive(Debug, Clone)]
@@ -61,6 +62,8 @@
     pub pointer: Option<wl_pointer::WlPointer>,
     /// Screencopy requests in flight.
     pub captures: Vec<Pending>,
+    /// The region selection overlay, while it is open.
+    pub overlay: Option<Overlay>,
 }
 
 impl Wayland {
@@ -87,6 +90,7 @@
             keyboard: None,
             pointer: None,
             captures: Vec::new(),
+            overlay: None,
         };
         // Two round trips: one for wl_output, one for the xdg-output details.
         queue
@@ -233,7 +237,17 @@
     ) {
     }
 
-    fn frame(&mut self, _: &Connection, _: &QueueHandle<Self>, _: &wl_surface::WlSurface, _: u32) {}
+    fn frame(
+        &mut self,
+        _: &Connection,
+        qh: &QueueHandle<Self>,
+        surface: &wl_surface::WlSurface,
+        _: u32,
+    ) {
+        if let Some(overlay) = &mut self.overlay {
+            overlay.frame_done(surface, qh);
+        }
+    }
 
     fn surface_enter(
         &mut self,
@@ -267,16 +281,23 @@
 }
 
 impl LayerShellHandler for State {
-    fn closed(&mut self, _: &Connection, _: &QueueHandle<Self>, _: &LayerSurface) {}
+    fn closed(&mut self, _: &Connection, _: &QueueHandle<Self>, _: &LayerSurface) {
+        if let Some(overlay) = &mut self.overlay {
+            overlay.cancel();
+        }
+    }
 
     fn configure(
         &mut self,
         _: &Connection,
-        _: &QueueHandle<Self>,
-        _: &LayerSurface,
-        _: LayerSurfaceConfigure,
-        _: u32,
-    ) {
+        qh: &QueueHandle<Self>,
+        layer: &LayerSurface,
+        configure: LayerSurfaceConfigure,
+        _: u32,
+    ) {
+        if let Some(overlay) = &mut self.overlay {
+            overlay.configure(layer, configure.new_size, qh);
+        }
     }
 }
 
@@ -363,8 +384,13 @@
         _: &QueueHandle<Self>,
         _: &wl_keyboard::WlKeyboard,
         _: u32,
-        _: KeyEvent,
-    ) {
+        event: KeyEvent,
+    ) {
+        if event.keysym == Keysym::Escape
+            && let Some(overlay) = &mut self.overlay
+        {
+            overlay.cancel();
+        }
     }
 
     fn repeat_key(
@@ -404,10 +430,13 @@
     fn pointer_frame(
         &mut self,
         _: &Connection,
-        _: &QueueHandle<Self>,
+        qh: &QueueHandle<Self>,
         _: &wl_pointer::WlPointer,
-        _: &[PointerEvent],
-    ) {
+        events: &[PointerEvent],
+    ) {
+        if let Some(overlay) = &mut self.overlay {
+            overlay.pointer(events, self.cursor_device.as_ref(), qh);
+        }
     }
 }
 
```

- [ ] **Step 3: Add the `region` command and remove the dead-code allowance**

Apply this change to `src/main.rs`:

```diff
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,6 +1,3 @@
-// Modules land before the CLI uses them; Task 11 removes this.
-#![allow(dead_code)]
-
 mod capture;
 mod config;
 mod detach;
@@ -11,6 +8,7 @@
 mod log;
 mod niri;
 mod output;
+mod region;
 mod render;
 mod selection;
 mod wayland;
@@ -45,6 +43,11 @@
         /// Capture every output, one file each.
         #[arg(long, conflicts_with_all = ["output", "clipboard_only"])]
         all: bool,
+        #[command(flatten)]
+        common: Common,
+    },
+    /// Drag to select a region (Cmd+Shift+4).
+    Region {
         #[command(flatten)]
         common: Common,
     },
@@ -76,6 +79,7 @@
 
 enum Mode {
     Screen { all: bool },
+    Region,
 }
 
 fn main() -> ExitCode {
@@ -106,6 +110,7 @@
     match cli.command {
         Command::Doctor => doctor::run(),
         Command::Screen { all, common } => capture(Mode::Screen { all }, common),
+        Command::Region { common } => capture(Mode::Region, common),
     }
 }
 
@@ -139,6 +144,11 @@
                 (vec![(frame.to_rgba(frame.full()), None)], 0)
             }
         }
+        Mode::Region => {
+            let frames = wl.capture(&outputs, cursor)?;
+            let (i, r) = region::select(&mut wl, &outputs, &frames)?;
+            (vec![(frames[i].to_rgba(r), None)], 0)
+        }
     };
     drop(wl);
 
@@ -224,10 +234,10 @@
     fn flag_conflicts() {
         assert!(parses(&["screen", "--all"]));
         assert!(parses(&["screen", "-o", "-"]));
-        assert!(parses(&["screen", "--clipboard-only", "--cursor"]));
+        assert!(parses(&["region", "--clipboard-only", "--cursor"]));
         assert!(!parses(&["screen", "--all", "-o", "-"]));
         assert!(!parses(&["screen", "--all", "--clipboard-only"]));
-        assert!(!parses(&["screen", "--clipboard-only", "-o", "a.png"]));
+        assert!(!parses(&["region", "--clipboard-only", "-o", "a.png"]));
     }
 
     #[test]
```

- [ ] **Step 4: Run tests and lint without the allowance**

Run: `cargo fmt && cargo clippy --all-targets -- -D warnings && cargo nextest run`
Expected: no warnings (in particular, no `dead_code`); every test passes.

- [ ] **Step 5: Select a region on niri**

Use the throwaway config from Task 10 (`copy_to_clipboard = false`):

```sh
cargo build --release
$V region -o $t/r.png; echo "exit=$?"; file $t/r.png
grep -h "committed\|selected" $t/state/valw/logs/*.log | tail -3
```

Ask the user to drag over a static area. Expected: every output freezes and dims, the cursor becomes a crosshair, the selection shows at full brightness with a white 1 px outline, and the overlay disappears on release. The command prints `exit=0`, and `r.png` has the logged pixel size. The log shows `overlay on … committed …ms after start` and `selected PixelRect { … } on …`.

- [ ] **Step 6: Cancel paths**

Ask the user to run `$V region -o $t/x.png; echo "exit=$?"` three times: press Esc, then right-click, then click once without dragging and press Esc afterwards.
Expected: `exit=3` each time, and no `x.png`. A plain click does **not** end the selection; the overlay stays until Esc.

- [ ] **Step 7: Drag across monitors (Review Focus 3)**

Ask the user to start a drag on one monitor and release it on the other, and once more to release outside every monitor's area (for example past a screen edge).
Expected: no panic. The result is clipped to the monitor where the drag started, and the log's `selected` line shows a rectangle inside that monitor.

- [ ] **Step 8: No stuck overlay**

```sh
$V region -o $t/k.png & sleep 1; kill -9 $!
$V screen -o /dev/null; echo "exit=$?"
```

Expected: the overlay vanishes when the process is killed, and the next command gets the lock (`exit=0`).

- [ ] **Step 9: Commit**

```sh
jj commit -m "feat: frozen-frame region selection overlay"
```

---


### Task 12: Nix checks (clippy, fmt, nextest, headless sway)

**Files:**
- Create: `nix/checks.ulu.nix`

**Interfaces:**
- Consumes: `packages.default` (Task 1); `valw doctor` and `valw screen -o -` (Tasks 9–10).
- Produces: `checks.<system>.{clippy,fmt,nextest,headless}`, run by `nix flake check`.

Notes:
- `cargoCheck` reuses the package derivation, so dependencies are vendored once. Setting `buildPhase` and `installPhase` replaces cargo's build and install hooks.
- `fmt` calls `rustfmt` directly because `cargo fmt` would try to resolve dependencies offline.
- The headless check must use `sway-unwrapped`: nixpkgs' `sway` wrapper starts through `dbus-run-session`, which does not exist in the sandbox.
- `valw screen -o - | cat` runs under `timeout 10`, so a clipboard child that kept the pipe open would fail the check.

- [ ] **Step 1: Write `nix/checks.ulu.nix`**

```nix
{ ... }:
{
  perSystem =
    { pkgs, self', ... }:
    let
      valw = self'.packages.default;

      # Reuses the package's vendored dependencies, runs `cmd` instead of the build.
      cargoCheck =
        name: tools: cmd:
        valw.overrideAttrs (old: {
          pname = "valw-${name}";
          nativeBuildInputs = old.nativeBuildInputs ++ tools;
          buildPhase = cmd;
          installPhase = "touch $out";
        });
    in
    {
      checks = {
        clippy = cargoCheck "clippy" [ pkgs.clippy ] "cargo clippy --all-targets --offline -- -D warnings";
        nextest = cargoCheck "nextest" [ pkgs.cargo-nextest ] "cargo nextest run --offline";

        fmt = pkgs.runCommand "valw-fmt" { nativeBuildInputs = [ pkgs.rustfmt ]; } ''
          rustfmt --check --edition 2024 ${valw.src}/src/main.rs
          touch $out
        '';

        # Runs the real screencopy path against a headless sway. This is
        # wlroots, not niri, but it catches capture and format bugs on every build.
        headless =
          pkgs.runCommand "valw-headless"
            {
              nativeBuildInputs = [
                pkgs.file
                pkgs.sway-unwrapped
                valw
              ];
            }
            ''
              export HOME=$TMPDIR/home XDG_RUNTIME_DIR=$TMPDIR/run
              mkdir -p $HOME
              mkdir -m 700 $XDG_RUNTIME_DIR
              export WLR_BACKENDS=headless WLR_RENDERER=pixman WLR_LIBINPUT_NO_DEVICES=1

              echo 'output HEADLESS-1 resolution 1280x720' > sway.conf
              sway -c sway.conf > sway.log 2>&1 &

              for _ in $(seq 100); do
                socket=$(ls $XDG_RUNTIME_DIR | grep -m1 '^wayland-[0-9]*$' || true)
                [ -n "$socket" ] && break
                sleep 0.1
              done
              [ -n "$socket" ] || { cat sway.log; exit 1; }
              export WAYLAND_DISPLAY=$socket

              valw doctor | tee doctor.txt
              ! grep -q '^fail' doctor.txt

              # Through a pipe: the clipboard child must not keep it open.
              timeout 10 valw screen -o - | cat > shot.png
              file shot.png | tee file.txt
              grep -q 'PNG image data, 1280 x 720' file.txt

              touch $out
            '';
      };
    };
}
```

- [ ] **Step 2: Run every check**

Run: `nix flake check -L --keep-going`
Expected: all four checks build. The `valw-headless` log shows the doctor lines, all `ok` except `warn  niri IPC unavailable …` (sway is not niri), plus `shot.png: PNG image data, 1280 x 720`.

- [ ] **Step 3: Format the Nix files**

Run: `nix fmt`
Expected: no changes beyond whitespace. If anything changed, rerun Step 2.

- [ ] **Step 4: Commit**

```sh
jj commit -m "test: nix flake checks with headless sway capture"
```

---


### Task 13: Acceptance on niri

**Files:** none (verification only). If a check fails, fix the owning task's code, add a unit test that reproduces the failure where possible, and commit the fix separately.

**Interfaces:**
- Consumes: the release binary from `nix build`.
- Produces: confirmation that Phase 0 + 1 meets the spec's sections 10.3 and 11.

Before starting, tell the user this task temporarily changes the HDMI output's scale and rotation through `niri msg`. That is runtime only and doesn't touch the niri config, and every step restores it. Use the throwaway config from Task 10.

- [ ] **Step 1: Build the real package**

```sh
nix build && V=$PWD/result/bin/valw
```

- [ ] **Step 2: Pixel exactness at scale 1 and 1.25 (Review Focus 2)**

For each output, and then again after `niri msg output HDMI-A-1 scale 1.25`:

```sh
$V region -o $t/a.png                         # user drags over a static area
grep -h selected $t/state/valw/logs/*.log | tail -1
# -> selected PixelRect { x: X, y: Y, width: W, height: H } on NAME ...
grim -o NAME $t/full.png
magick $t/full.png -crop WxH+X+Y +repage $t/b.png
magick compare -metric AE $t/a.png $t/b.png null:; echo
```

Expected: `0` every time. At 1.25 the overlay must also look sharp, with text in it as crisp as on the live screen. Restore with `niri msg output HDMI-A-1 scale 1`.

- [ ] **Step 3: Latency**

Run `$V region -o /dev/null` and press Esc; repeat three times.
Expected: every `overlay on … committed …ms after start` log line is under 150 ms. (Prototype measurements: 38–42 ms warm and 90 ms cold on a headless 1080p output; 270 ms before frames were kept in B, G, R, X order.) If it's over, the `captured … (… after start)` and `overlay buffers ready …` lines (`VALW_LOG=debug`) show which phase is slow.

- [ ] **Step 4: Rotated output (Review Focus 1)**

```sh
niri msg output HDMI-A-1 transform 90
$V screen --all && grim -o HDMI-A-1 $t/g.png
magick compare -metric AE "$(ls $t/shots/*HDMI-A-1*.png | tail -1)" $t/g.png null:; echo
niri msg output HDMI-A-1 transform normal
```

Expected: `0`. If the images are rotated relative to each other, the mapping in `frame::apply_transform` is wrong for niri. Fix the table in `transforms_match_grim` and the match arms together.

- [ ] **Step 5: Noctalia**

Run `$V region -o /dev/null` and have the user confirm the dimmed overlay covers the Noctalia bar, then press Esc.

- [ ] **Step 6: Unwritable log directory (Review Focus 5)**

```sh
XDG_STATE_HOME=/proc $V screen -o $t/nolog.png; echo "exit=$?"; file $t/nolog.png
$V screen -o /proc/nope.png; echo "exit=$?"
```

Expected: the first prints a `WARN logging to stderr only: …` line, `exit=0`, and a valid PNG. The second prints `error: could not write /proc/nope.png`-style output with `cause:` lines and the `log:` path, and `exit=1`.

- [ ] **Step 7: Full check suite**

Run: `nix flake check -L --keep-going`
Expected: every check passes.

- [ ] **Step 8: Report**

Summarise the results of Steps 2–6 to the user, including exact numbers (AE values and latencies). There is nothing to commit unless a fix was needed.