[sashakosti]

Summary

Fixes a crash (SIGSEGV in window_last_done_commit_index during paint_all) when an XDG-type external overlay client (such as a zwlr_layer_surface_v1 overlay, HUD, or background client) disconnects or is abruptly terminated without an orderly protocol teardown.

Problem

In src/steamcompmgr.cpp:

1. Commit 038afdc previously added cleanup for pFocus->externalOverlayWindow in destroy_win(), but that function only handles X11/XWayland windows.
2. For native Wayland XDG/layer-shell surfaces, destruction is processed in steamcompmgr_check_xdg(). When wlserver_xdg_dirty() is true, steamcompmgr_check_xdg() clears stale window references across g_VirtualConnectorFocuses, but omitted externalOverlayWindow.
3. Additionally, steamcompmgr_xdg_determine_and_apply_focus() never resets g_steamcompmgr_xdg_focus.overlayWindow or externalOverlayWindow to nullptr before iterating active windows, causing stale references to persist if the disconnected window was the only overlay.
4. During subsequent frames in paint_all() -> paint_window(), externalOverlay is dereferenced after the window has already been deallocated, triggering a crash:

#0  0x00000000004706c7 in window_last_done_commit_index (w=0x2a8cd620) at ../src/steamcompmgr.cpp:1882
1882			if ( w->commit_queue[ i ]->done )
#1  0x000000000047073d in get_window_last_done_commit (w=0x2a8cd620, commit=...) at ../src/steamcompmgr.cpp:1900
#2  0x0000000000473a31 in paint_window (pFocus=..., w=0x2a8cd620, ...) at ../src/steamcompmgr.cpp:2757
#3  0x0000000000476914 in paint_all (pFocus=..., async=false) at ../src/steamcompmgr.cpp:3304

Solution

• In steamcompmgr_check_xdg():
  • Clear pFocus->externalOverlayWindow if type == steamcompmgr_win_type_t::XDG.
  • Clear g_steamcompmgr_xdg_focus.focusWindow, overlayWindow, and externalOverlayWindow if type == steamcompmgr_win_type_t::XDG.
• In steamcompmgr_xdg_determine_and_apply_focus():
  • Reset g_steamcompmgr_xdg_focus.overlayWindow and externalOverlayWindow to nullptr prior to scanning active windows.

Tested by spawning a layer-shell overlay client (swaybg) into a running gamescope session presenting vkcube, sending SIGKILL to the overlay client, and verifying that gamescope remains fully stable with no crashes.

Closes #2434