fix: handle dynamic output changes while session is locked

When monitors are powered off (e.g. via niri power-off-monitors or
physical power switches), niri destroys and re-advertises the Wayland
outputs as they come back. Previously all three OutputHandler callbacks
were no-ops, causing two bugs:

- new_output: no lock surface was created for outputs that appeared
  after the initial lock, so the slowest monitor to wake up would show
  the compositor's red fallback instead of the lock screen.
- output_destroyed: stale LockedSurface, SessionLockSurface, output,
  and captured_background entries accumulated for gone outputs.

Fix new_output to create a lock surface (and register it with the lock
manager) whenever a new output appears while the session is locked.

Fix output_destroyed to remove the corresponding entries from
lock_surfaces, lock_manager.surfaces, outputs, and
captured_backgrounds, keeping all parallel vecs in sync. Add
LockManager::remove_surface_by_output to support this, returning the
removal index so lock_surfaces can be updated with the same index.

The all-monitors-off scenario (all outputs destroyed simultaneously)
is handled naturally: the vecs are emptied and repopulated as each
monitor fires new_output on wake.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main.rs

@@ -296,9 +296,61 @@ impl OutputHandler for WaylandLock {
     fn output_state(&mut self) -> &mut OutputState {
         &mut self.output_state
     }
-    fn new_output(&mut self, _conn: &Connection, _qh: &QueueHandle<Self>, _output: WlOutput) {}
-    fn update_output(&mut self, _conn: &Connection, _qh: &QueueHandle<Self>, _output: WlOutput) {}
-    fn output_destroyed(&mut self, _conn: &Connection, _qh: &QueueHandle<Self>, _output: WlOutput) {
+    fn new_output(&mut self, _conn: &Connection, qh: &QueueHandle<Self>, output: WlOutput) {
+        // If we are already locked, we must create a lock surface for this newly
+        // available output. This happens e.g. when a monitor powers back on after
+        // "niri msg action power-off-monitors" — niri re-advertises the output and
+        // the compositor requires a lock surface on every output or it shows a
+        // compositor-defined fallback (typically a solid red/black screen).
+        if let Some(session_lock) = &self.session_lock {
+            let surface = self.compositor_state.create_surface(qh);
+            let (width, height) = self.get_output_dimensions(&output);
+            let lock_surface = session_lock.create_lock_surface(surface.clone(), &output, qh);
+            self.lock_surfaces.push(lock_surface);
+            if !self.outputs.contains(&output) {
+                self.outputs.push(output.clone());
+            }
+            if let Ok(mut lm) = self.lock_manager.lock() {
+                lm.add_surface(width, height, output);
+                let count = lm.surface_count();
+                if let Some(ls) = lm.get_surface_mut(count - 1) {
+                    ls.set_wayland_surface(surface);
+                }
+            }
+            log::info!("Created lock surface for newly available output");
+        }
+    }
+    fn update_output(&mut self, _conn: &Connection, _qh: &QueueHandle<Self>, _output: WlOutput) {
+        // Dimension changes while locked are handled by the compositor sending a configure
+        // event on the lock surface, which the SessionLockHandler::configure callback
+        // already processes via locked_surface.resize().
+    }
+    fn output_destroyed(&mut self, _conn: &Connection, _qh: &QueueHandle<Self>, output: WlOutput) {
+        // Clean up the lock surface and associated state for this output.
+        // This happens e.g. when a monitor is powered off with its physical power switch.
+        // When the monitor comes back on, new_output() will fire and recreate everything.
+        //
+        // outputs and captured_backgrounds are kept at the same indices, so we remove
+        // from both using the same position.
+        let output_id = Proxy::id(&output);
+        if let Some(idx) = self.outputs.iter().position(|o| Proxy::id(o) == output_id) {
+            self.outputs.remove(idx);
+            if idx < self.captured_backgrounds.len() {
+                self.captured_backgrounds.remove(idx);
+            }
+        }
+
+        // lock_manager.surfaces and lock_surfaces are built in tandem and share indices,
+        // so the index returned from the lock_manager removal applies to lock_surfaces too.
+        if let Ok(mut lm) = self.lock_manager.lock() {
+            if let Some(idx) = lm.remove_surface_by_output(&output) {
+                if idx < self.lock_surfaces.len() {
+                    drop(self.lock_surfaces.remove(idx));
+                }
+            }
+        }
+
+        log::info!("Removed lock surface for destroyed output");
     }
 }