desktop: point the mutter touch patch at upstream !5375

The crash is mutter issue #4871, a regression from !5057, and
!5375 ("wayland/touch: Do not crash on empty focus surface") carries
the same NULL check as our patch. Say so in the overlay and the
patch, so it is clear when the patch can go.

Assisted-by: Claude:opus-5

modules/desktop.nix

@@ -17,8 +17,11 @@
 
   # gnome-shell segfaults in touch_can_grab_surface() when a client asks for
   # a move/resize grab with the serial of a cancelled touch (NULL
-  # focus_surface). Not fixed upstream as of mutter 50.2 / main 2026-10-13.
+  # focus_surface); a mutter 50 regression from !5057. Upstream issue
-  # Drop once nixpkgs' mutter carries a fix.
+  # https://gitlab.gnome.org/GNOME/mutter/-/work_items/4871, fix in
+  # https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/5375 ("wayland/touch:
+  # Do not crash on empty focus surface"), the same NULL check as this patch.
+  # Drop once nixpkgs' mutter carries !5375.
   nixpkgs.overlays = [
     (final: prev: {
       mutter = prev.mutter.overrideAttrs (old: {

patches/mutter-touch-cancelled-grab.patch

@@ -14,6 +14,9 @@ touch_can_grab_surface(), which dereferences the NULL focus_surface:
   #3 xdg_toplevel_resize
 
 A cancelled touch cannot grab anything; treat it as such.
+
+Upstream: https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/5375
+(closes https://gitlab.gnome.org/GNOME/mutter/-/work_items/4871)
 ---
 --- a/src/wayland/meta-wayland-touch.c
 +++ b/src/wayland/meta-wayland-touch.c