diff --git a/src/host.rs b/src/host.rs
new file mode 100644
index 0000000..eda927c
--- /dev/null
+++ b/src/host.rs
@@ -0,0 +1,470 @@
+//! The preview host: one process that owns every thumbnail, started on
+//! demand by a capture and gone once its last thumbnail is.
+
+use std::collections::BTreeSet;
+use std::io::{ErrorKind, Read, Write};
+use std::os::unix::net::{UnixListener, UnixStream};
+use std::path::Path;
+use std::time::{Duration, Instant};
+
+use anyhow::{Context, Result};
+use smithay_client_toolkit::reexports::calloop::{
+    EventLoop, Interest, LoopHandle, Mode, PostAction,
+    generic::Generic,
+    timer::{TimeoutAction, Timer},
+};
+use smithay_client_toolkit::reexports::calloop_wayland_source::WaylandSource;
+use smithay_client_toolkit::seat::pointer::{BTN_LEFT, PointerEvent, PointerEventKind};
+use smithay_client_toolkit::shell::wlr_layer::LayerSurface;
+use wayland_client::{Connection, QueueHandle, protocol::wl_surface::WlSurface};
+use wayland_protocols::wp::cursor_shape::v1::client::wp_cursor_shape_device_v1::{
+    Shape, WpCursorShapeDeviceV1,
+};
+
+use crate::ipc::{self, Reply, Request};
+use crate::lock::Lock;
+use crate::stack;
+use crate::thumbnail::{Action, Parts, Thumbnail};
+use crate::wayland::{State, Wayland};
+
+/// A host nobody connects to within this time gives up.
+const STARTUP_GRACE: Duration = Duration::from_secs(2);
+
+/// Connection bookkeeping and the exit rule, without any Wayland.
+#[derive(Debug)]
+pub struct Core {
+    clients: BTreeSet<u64>,
+    hiding: BTreeSet<u64>,
+    seen_client: bool,
+    started: Instant,
+}
+
+impl Core {
+    pub fn new(now: Instant) -> Core {
+        Core {
+            clients: BTreeSet::new(),
+            hiding: BTreeSet::new(),
+            seen_client: false,
+            started: now,
+        }
+    }
+
+    pub fn connect(&mut self, id: u64) {
+        self.clients.insert(id);
+        self.seen_client = true;
+    }
+
+    /// Returns true if this request is what hides the thumbnails.
+    pub fn hide(&mut self, id: u64) -> bool {
+        let was_hidden = self.hidden();
+        self.hiding.insert(id);
+        !was_hidden
+    }
+
+    /// Returns true if this disconnect is what shows the thumbnails again.
+    pub fn disconnect(&mut self, id: u64) -> bool {
+        self.clients.remove(&id);
+        self.hiding.remove(&id) && self.hiding.is_empty()
+    }
+
+    pub fn hidden(&self) -> bool {
+        !self.hiding.is_empty()
+    }
+
+    /// Nothing to show and nobody talking to us. A fresh host waits a little
+    /// for the capture that started it to connect.
+    pub fn should_exit(&self, thumbnails: usize, now: Instant) -> bool {
+        thumbnails == 0
+            && self.clients.is_empty()
+            && (self.seen_client || now.duration_since(self.started) > STARTUP_GRACE)
+    }
+}
+
+pub struct Host {
+    core: Core,
+    /// Oldest first.
+    thumbs: Vec<Thumbnail>,
+    next_id: u64,
+    timeout: Duration,
+    conn: Connection,
+    handle: LoopHandle<'static, State>,
+    qh: QueueHandle<State>,
+}
+
+/// Runs the host until its last thumbnail closes. A second host exits
+/// right away: the first one owns the socket.
+pub fn run() -> Result<()> {
+    let Ok(_lock) = Lock::acquire(&ipc::host_lock_path()) else {
+        tracing::info!("another preview host is running");
+        return Ok(());
+    };
+    let socket = ipc::socket_path();
+    // Safe under the lock: whatever is there is left over from a crash.
+    let _ = std::fs::remove_file(&socket);
+    let listener = UnixListener::bind(&socket)
+        .with_context(|| format!("could not listen on {}", socket.display()))?;
+    listener.set_nonblocking(true)?;
+
+    let config = crate::config::load(&crate::config::default_path())?;
+    let Wayland {
+        conn,
+        queue,
+        mut state,
+        ..
+    } = Wayland::connect()?;
+    let mut event_loop: EventLoop<'static, State> =
+        EventLoop::try_new().context("could not create the event loop")?;
+    let handle = event_loop.handle();
+    let qh = queue.handle();
+    WaylandSource::new(conn.clone(), queue)
+        .insert(handle.clone())
+        .map_err(|e| anyhow::anyhow!("could not watch the Wayland socket: {e}"))?;
+
+    state.preview = Some(Host {
+        core: Core::new(Instant::now()),
+        thumbs: Vec::new(),
+        next_id: 0,
+        timeout: Duration::from_secs(config.preview.timeout_secs),
+        conn,
+        handle: handle.clone(),
+        qh,
+    });
+
+    let mut next_client = 0u64;
+    handle
+        .insert_source(
+            Generic::new(listener, Interest::READ, Mode::Level),
+            move |_, listener, state| {
+                while let Ok((stream, _)) = listener.accept() {
+                    next_client += 1;
+                    if let Some(host) = &mut state.preview {
+                        host.add_client(stream, next_client);
+                    }
+                }
+                Ok(PostAction::Continue)
+            },
+        )
+        .map_err(|e| anyhow::anyhow!("could not watch the socket: {e}"))?;
+
+    tracing::info!("preview host listening on {}", socket.display());
+    let signal = event_loop.get_signal();
+    event_loop
+        .run(Duration::from_millis(500), &mut state, |state| {
+            let host = state.preview.as_mut().expect("host state");
+            host.tidy();
+            if host.core.should_exit(host.thumbs.len(), Instant::now()) {
+                signal.stop();
+            }
+        })
+        .context("preview host event loop failed")?;
+
+    let _ = std::fs::remove_file(&socket);
+    tracing::info!("preview host done");
+    Ok(())
+}
+
+impl Host {
+    fn add_client(&mut self, stream: UnixStream, id: u64) {
+        if stream.set_nonblocking(true).is_err() {
+            return;
+        }
+        self.core.connect(id);
+        let mut buf = Vec::new();
+        let inserted = self.handle.insert_source(
+            Generic::new(stream, Interest::READ, Mode::Level),
+            move |_, stream, state| {
+                let mut chunk = [0u8; 4096];
+                loop {
+                    let mut reader: &UnixStream = stream;
+                    match reader.read(&mut chunk) {
+                        Ok(0) => {
+                            disconnect(state, id);
+                            return Ok(PostAction::Remove);
+                        }
+                        Ok(n) => buf.extend_from_slice(&chunk[..n]),
+                        Err(e) if e.kind() == ErrorKind::WouldBlock => break,
+                        Err(_) => {
+                            disconnect(state, id);
+                            return Ok(PostAction::Remove);
+                        }
+                    }
+                }
+                while let Some(end) = buf.iter().position(|&b| b == b'\n') {
+                    let line: Vec<u8> = buf.drain(..=end).collect();
+                    let reply = match ipc::parse_request(&String::from_utf8_lossy(&line)) {
+                        Ok(request) => handle(state, id, request),
+                        Err(e) => Reply::error(e),
+                    };
+                    let mut writer: &UnixStream = stream;
+                    let _ = writer.write_all(ipc::encode(&reply).as_bytes());
+                }
+                Ok(PostAction::Continue)
+            },
+        );
+        if inserted.is_err() {
+            self.core.disconnect(id);
+        }
+    }
+
+    /// Drops finished thumbnails and stacks the rest per output.
+    fn tidy(&mut self) {
+        let before = self.thumbs.len();
+        self.thumbs.retain(|t| !t.is_finished());
+        if self.thumbs.len() != before {
+            self.restack();
+            let _ = self.conn.flush();
+        }
+    }
+
+    fn restack(&mut self) {
+        let outputs: BTreeSet<String> = self.thumbs.iter().map(|t| t.output.clone()).collect();
+        for output in outputs {
+            let newest_first: Vec<usize> = (0..self.thumbs.len())
+                .rev()
+                .filter(|&i| self.thumbs[i].output == output)
+                .collect();
+            let heights: Vec<u32> = newest_first
+                .iter()
+                .map(|&i| self.thumbs[i].size.1)
+                .collect();
+            for (&i, margin) in newest_first.iter().zip(stack::bottom_margins(&heights)) {
+                self.thumbs[i].set_margin(margin);
+            }
+        }
+    }
+
+    fn thumb(&mut self, surface: &WlSurface) -> Option<&mut Thumbnail> {
+        self.thumbs.iter_mut().find(|t| t.is(surface))
+    }
+
+    pub fn configure(&mut self, layer: &LayerSurface, qh: &QueueHandle<State>) {
+        if let Some(t) = self.thumbs.iter_mut().find(|t| t.is_layer(layer)) {
+            t.configure(qh);
+        }
+    }
+
+    pub fn frame_done(&mut self, surface: &WlSurface, qh: &QueueHandle<State>) {
+        if let Some(t) = self.thumb(surface) {
+            t.frame_done(qh);
+        }
+    }
+
+    pub fn closed(&mut self, layer: &LayerSurface) {
+        self.thumbs.retain(|t| !t.is_layer(layer));
+    }
+
+    pub fn pointer(
+        &mut self,
+        events: &[PointerEvent],
+        cursor: Option<&WpCursorShapeDeviceV1>,
+        qh: &QueueHandle<State>,
+    ) {
+        for event in events {
+            let Some(t) = self.thumb(&event.surface) else {
+                continue;
+            };
+            let (x, y) = event.position;
+            match event.kind {
+                PointerEventKind::Enter { serial } => {
+                    if let Some(cursor) = cursor {
+                        cursor.set_shape(serial, Shape::Pointer);
+                    }
+                }
+                PointerEventKind::Press {
+                    button: BTN_LEFT, ..
+                } => t.press(x, y),
+                PointerEventKind::Motion { .. } => t.motion(x, qh),
+                PointerEventKind::Release {
+                    button: BTN_LEFT, ..
+                } => {
+                    let action = t.release(x, y, qh);
+                    if action == Some(Action::OpenEditor) {
+                        open_editor(&t.path);
+                    }
+                }
+                _ => {}
+            }
+        }
+    }
+
+    fn expire(&mut self, id: u64) {
+        let qh = self.qh.clone();
+        if let Some(t) = self.thumbs.iter_mut().find(|t| t.id == id) {
+            t.expire(&qh);
+        }
+    }
+
+    fn set_hidden(&mut self, hidden: bool) {
+        let qh = self.qh.clone();
+        for t in &mut self.thumbs {
+            if hidden {
+                t.hide(&qh);
+            } else {
+                t.show(&qh);
+            }
+        }
+    }
+}
+
+fn disconnect(state: &mut State, id: u64) {
+    let Some(host) = &mut state.preview else {
+        return;
+    };
+    if host.core.disconnect(id) {
+        host.set_hidden(false);
+        let _ = host.conn.flush();
+    }
+}
+
+fn handle(state: &mut State, id: u64, request: Request) -> Reply {
+    match request {
+        Request::Hide => {
+            let host = state.preview.as_mut().expect("host state");
+            if host.core.hide(id) {
+                host.set_hidden(true);
+                // The capture must not start until the compositor has
+                // actually taken the thumbnails off the screen.
+                if let Err(e) = host.conn.roundtrip() {
+                    return Reply::error(format!("Wayland round trip failed: {e}"));
+                }
+            }
+            Reply::ok()
+        }
+        Request::Add { path, output } => match add(state, &path, &output) {
+            Ok(()) => Reply::ok(),
+            Err(e) => {
+                tracing::warn!("could not show a preview: {e:#}");
+                Reply::error(format!("{e:#}"))
+            }
+        },
+    }
+}
+
+fn add(state: &mut State, path: &Path, output_name: &str) -> Result<()> {
+    let image = image::open(path)
+        .with_context(|| format!("could not read {}", path.display()))?
+        .to_rgba8();
+    let outputs = state.output_list();
+    let output = outputs
+        .iter()
+        .find(|o| o.geom.name == output_name)
+        .or_else(|| outputs.first())
+        .context("the compositor reported no outputs")?;
+    let parts = Parts {
+        compositor: &state.compositor,
+        layer_shell: state
+            .layer_shell
+            .as_ref()
+            .context("compositor has no wlr-layer-shell")?,
+        viewporter: state
+            .viewporter
+            .as_ref()
+            .context("compositor has no wp-viewporter")?,
+        shm: &state.shm,
+    };
+    let host = state.preview.as_mut().expect("host state");
+    host.next_id += 1;
+    let id = host.next_id;
+    let mut thumb = Thumbnail::new(id, path.to_path_buf(), &image, output, &parts, &host.qh)?;
+    if host.core.hidden() {
+        thumb.hide(&host.qh);
+    }
+    host.thumbs.push(thumb);
+
+    // Beyond MAX on this output, the oldest slide away.
+    let qh = host.qh.clone();
+    let open: Vec<usize> = (0..host.thumbs.len())
+        .filter(|&i| host.thumbs[i].output == output.geom.name && !host.thumbs[i].is_closing())
+        .collect();
+    for &i in open.iter().take(stack::excess(open.len())) {
+        host.thumbs[i].slide_out(&qh);
+    }
+    host.restack();
+
+    host.handle
+        .insert_source(Timer::from_duration(host.timeout), move |_, _, state| {
+            if let Some(host) = &mut state.preview {
+                host.expire(id);
+            }
+            TimeoutAction::Drop
+        })
+        .map_err(|e| anyhow::anyhow!("could not start the preview timer: {e}"))?;
+    let _ = host.conn.flush();
+    Ok(())
+}
+
+/// Opens the screenshot in Satty; saving there overwrites the file.
+fn open_editor(path: &Path) {
+    use std::os::unix::process::CommandExt;
+    use std::process::{Command, Stdio};
+
+    let mut command = Command::new("satty");
+    command
+        .arg("--filename")
+        .arg(path)
+        .arg("--output-filename")
+        .arg(path)
+        .stdin(Stdio::null())
+        .stdout(Stdio::null())
+        .stderr(Stdio::null());
+    // SAFETY: setsid is async-signal-safe.
+    unsafe {
+        command.pre_exec(|| rustix::process::setsid().map(drop).map_err(Into::into));
+    }
+    if let Err(e) = command.spawn() {
+        tracing::warn!("could not start satty: {e}");
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn hidden_until_every_hider_leaves() {
+        let mut core = Core::new(Instant::now());
+        core.connect(1);
+        core.connect(2);
+        assert!(core.hide(1), "first hide hides");
+        assert!(!core.hide(2), "second hide changes nothing");
+        assert!(!core.disconnect(1), "still hidden by 2");
+        assert!(core.hidden());
+        assert!(core.disconnect(2), "last hider leaving shows");
+        assert!(!core.hidden());
+    }
+
+    #[test]
+    fn a_client_that_never_hid_does_not_show() {
+        let mut core = Core::new(Instant::now());
+        core.connect(1);
+        core.connect(2);
+        core.hide(1);
+        assert!(!core.disconnect(2));
+        assert!(core.hidden());
+    }
+
+    #[test]
+    fn exits_only_when_empty_and_alone() {
+        let now = Instant::now();
+        let mut core = Core::new(now);
+        assert!(!core.should_exit(0, now), "waits for its first client");
+        assert!(
+            core.should_exit(0, now + Duration::from_secs(3)),
+            "gives up after the grace period"
+        );
+
+        core.connect(1);
+        assert!(!core.should_exit(0, now), "a client is connected");
+        core.disconnect(1);
+        assert!(!core.should_exit(1, now), "a thumbnail is up");
+        assert!(core.should_exit(0, now));
+    }
+
+    #[test]
+    fn a_second_host_gives_up_quietly() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("valw-preview.lock");
+        let _first = Lock::acquire(&path).unwrap();
+        assert!(Lock::acquire(&path).is_err());
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index 6afeb0b..28c0eb2 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -7,6 +7,7 @@ mod detach;
 mod doctor;
 mod error;
 mod frame;
+mod host;
 mod ipc;
 mod lock;
 mod log;
@@ -59,6 +60,9 @@ enum Command {
     },
     /// Report what the compositor and system support.
     Doctor,
+    /// Internal: the process that shows preview thumbnails.
+    #[command(name = "__preview-host", hide = true)]
+    PreviewHost,
 }
 
 #[derive(Args)]
@@ -115,6 +119,7 @@ fn main() -> ExitCode {
 fn run(cli: Cli) -> Result<()> {
     match cli.command {
         Command::Doctor => doctor::run(),
+        Command::PreviewHost => host::run(),
         Command::Screen { all, common } => capture(Mode::Screen { all }, common),
         Command::Region { common } => capture(Mode::Region, common),
     }
@@ -246,6 +251,13 @@ mod tests {
         assert!(!parses(&["region", "--clipboard-only", "-o", "a.png"]));
     }
 
+    #[test]
+    fn preview_host_is_hidden_from_help() {
+        assert!(parses(&["__preview-host"]));
+        let help = Cli::command().render_help().to_string();
+        assert!(!help.contains("preview-host"), "{help}");
+    }
+
     #[test]
     fn delay_values() {
         assert_eq!(parse_delay("1.5"), Ok(Duration::from_millis(1500)));
diff --git a/src/wayland.rs b/src/wayland.rs
index 750f399..ac93707 100644
--- a/src/wayland.rs
+++ b/src/wayland.rs
@@ -30,6 +30,7 @@ use wayland_protocols_wlr::screencopy::v1::client::zwlr_screencopy_manager_v1::Z
 use crate::capture::Pending;
 use crate::error::HintExt;
 use crate::frame::OutputGeom;
+use crate::host::Host;
 use crate::region::Overlay;
 
 /// An output as valw sees it.
@@ -66,6 +67,8 @@ pub struct State {
     pub captures: Vec<Pending>,
     /// The region selection overlay, while it is open.
     pub overlay: Option<Overlay>,
+    /// The preview thumbnails, in the preview host process.
+    pub preview: Option<Host>,
 }
 
 impl Wayland {
@@ -93,6 +96,7 @@ impl Wayland {
             pointer: None,
             captures: Vec::new(),
             overlay: None,
+            preview: None,
         };
         // Two round trips: one for wl_output, one for the xdg-output details.
         queue
@@ -278,6 +282,9 @@ impl CompositorHandler for State {
         if let Some(overlay) = &mut self.overlay {
             overlay.frame_done(surface, qh);
         }
+        if let Some(preview) = &mut self.preview {
+            preview.frame_done(surface, qh);
+        }
     }
 
     fn surface_enter(
@@ -312,10 +319,13 @@ impl OutputHandler for State {
 }
 
 impl LayerShellHandler for State {
-    fn closed(&mut self, _: &Connection, _: &QueueHandle<Self>, _: &LayerSurface) {
+    fn closed(&mut self, _: &Connection, _: &QueueHandle<Self>, layer: &LayerSurface) {
         if let Some(overlay) = &mut self.overlay {
             overlay.cancel();
         }
+        if let Some(preview) = &mut self.preview {
+            preview.closed(layer);
+        }
     }
 
     fn configure(
@@ -329,6 +339,9 @@ impl LayerShellHandler for State {
         if let Some(overlay) = &mut self.overlay {
             overlay.configure(layer, configure.new_size, qh);
         }
+        if let Some(preview) = &mut self.preview {
+            preview.configure(layer, qh);
+        }
     }
 }
 
@@ -468,6 +481,9 @@ impl PointerHandler for State {
         if let Some(overlay) = &mut self.overlay {
             overlay.pointer(events, self.cursor_device.as_ref(), qh);
         }
+        if let Some(preview) = &mut self.preview {
+            preview.pointer(events, self.cursor_device.as_ref(), qh);
+        }
     }
 }