niri-scratchpad: fix close-while-stashed eviction + add live acceptance test

Refresh the patch with the destroy-path fix (a client dying while stashed is
now evicted instead of leaked/zombied), and add pkg/niri-scratchpad/tests/
acceptance.py: a headless IPC-driven regression test that drives a nested
patched niri and asserts on `niri msg -j windows`. It found the above bug
(unit tests miss it — the harness calls remove_window directly). Run it after
rebasing the patch onto a new niri release.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

niri-patches/0001-scratchpad.patch

@@ -56,6 +56,26 @@ index 0aa3cb4f..a5b712e2 100644
      /// Toggle (open/close) the Overview.
      ToggleOverview {},
      /// Open the Overview.
+diff --git a/src/handlers/xdg_shell.rs b/src/handlers/xdg_shell.rs
+index 38440c90..a174921e 100644
+--- a/src/handlers/xdg_shell.rs
++++ b/src/handlers/xdg_shell.rs
+@@ -834,6 +834,15 @@ impl XdgShellHandler for State {
+             .find_window_and_output(surface.wl_surface());
+ 
+         let Some((mapped, output)) = win_out else {
++            // A window whose client died while stashed in the scratchpad is in no
++            // workspace, so find_window_and_output can't see it; evict it here.
++            if self
++                .niri
++                .layout
++                .remove_from_scratchpad(surface.wl_surface())
++            {
++                return;
++            }
+             // I have no idea how this can happen, but I saw it happen once, in a weird interaction
+             // involving laptop going to sleep and resuming.
+             error!("toplevel missing from both unmapped_windows and layout");
 diff --git a/src/input/mod.rs b/src/input/mod.rs
 index f1ad4932..91bc8942 100644
 --- a/src/input/mod.rs
@@ -86,7 +106,7 @@ index f1ad4932..91bc8942 100644
                  self.niri.layout.toggle_overview();
                  self.niri.queue_redraw_all();
 diff --git a/src/layout/mod.rs b/src/layout/mod.rs
-index 5c4dd639..077a0570 100644
+index 5c4dd639..a90d9e6e 100644
 --- a/src/layout/mod.rs
 +++ b/src/layout/mod.rs
 @@ -32,6 +32,7 @@
@@ -149,7 +169,7 @@ index 5c4dd639..077a0570 100644
          if let Some(state) = &self.interactive_move {
              match state {
                  InteractiveMoveState::Starting { window_id, .. } => {
-@@ -1204,6 +1222,93 @@ impl<W: LayoutElement> Layout<W> {
+@@ -1204,6 +1222,106 @@ impl<W: LayoutElement> Layout<W> {
          None
      }
  
@@ -164,6 +184,19 @@ index 5c4dd639..077a0570 100644
 +        // (Task 4) if this window was the one shown.
 +    }
 +
++    /// Evict a stashed window from the scratchpad by its surface. Returns true
++    /// if one was removed. Needed on the destroy/unmap paths: a window whose
++    /// client dies *while stashed* is in no workspace, so `find_window_and_output`
++    /// (and therefore `remove_window`) never sees it — this is the only thing
++    /// that drops it. A shown scratchpad window lives in a workspace and is
++    /// handled by `remove_window` instead.
++    pub fn remove_from_scratchpad(&mut self, surface: &WlSurface) -> bool {
++        let before = self.scratchpad.len();
++        self.scratchpad
++            .retain(|rt| !rt.tile.window().is_wl_surface(surface));
++        self.scratchpad.len() != before
++    }
++
 +    /// Show / toggle / cycle the scratchpad. See the state table in the plan.
 +    pub fn scratchpad_show(&mut self) {
 +        // Drop a stale "shown" id (window was closed or moved away).
@@ -243,7 +276,7 @@ index 5c4dd639..077a0570 100644
      pub fn descendants_added(&mut self, id: &W::Id) -> bool {
          for ws in self.workspaces_mut() {
              if ws.descendants_added(id) {
-@@ -2444,6 +2549,24 @@ impl<W: LayoutElement> Layout<W> {
+@@ -2444,6 +2562,24 @@ impl<W: LayoutElement> Layout<W> {
              }
          }