docs: end-of-project test checklist

docs/test-checklist.md

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+# valw: end-of-project test checklist
+
+Manual checks on niri that each phase leaves for one test pass once every
+phase is done. Each phase appends its own section; nothing here blocks a
+phase from landing. Automated checks (`cargo clippy`, `cargo nextest`,
+`nix flake check`) still run per phase.
+
+Before starting: update copland's lock (`nix flake update valw` in
+`~/copland`), rebuild, and check that `valw doctor` is all `ok`.
+
+## Preview drag-out (2026-10-02)
+
+Already seen working while building it: a drop into Telegram delivered the
+image, a cancel brought the thumbnail back, and a swipe still dismissed it.
+
+- [ ] Drop into Dolphin → the file is copied (the default name has spaces, so this also checks `%20`).
+- [ ] Drop into a browser upload field → it uploads.
+- [ ] Drop into a Discord message → it attaches.
+- [ ] Drop into an image editor that takes `image/png` → the pixels arrive.
+- [ ] Release on nothing, or Esc mid-drag → the thumbnail comes back.
+- [ ] With `timeout_secs = 3`, hold a drag past 3 s, then release on nothing → it slides out.
+- [ ] While dragging, press Print → after the capture the other thumbnails come back but the dragged one doesn't; finishing the drag then behaves normally.
+- [ ] Same, with 5 thumbnails on that output and the oldest one being dragged → the dragged one is never evicted.
+- [ ] A fast flick to the left and an immediate release → the thumbnail comes back (or the drag runs normally); it never stays invisible.
+- [ ] Drag onto the other monitor and drop there → it works.
+- [ ] Swipe right still dismisses; a click still opens Satty (installed build).
+
+### Known minors (deferred from the drag-out review)
+
+- A drag-out press that never became a drag (no data device, or a second drag refused) ignores an expired timer; it should slide out on release.
+- A receiver that opens its pipe O_NONBLOCK can get a truncated large PNG; clear O_NONBLOCK in the writer thread.
+- A press during the slide-in gives a shifted grab point; use (x - offset, y).
+- A swipe released back near its start opens Satty (Phase 2 behaviour); a locked swipe should never become a click.
+- The (hidden, dragging) visibility/input decision has no unit test; extract it as a pure function.