compositor: Focus the no focus window during modal plugin operations

Modal operations are usually far from transient, so it makes sense to
unfocus the currently focused window while the operation is going on.
If no other window was focused and the window is still alive when the
modal operation is finished, focus will be restored back.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/452

src/compositor/compositor-private.h

@@ -40,6 +40,8 @@ struct _MetaCompositor
 
   MetaPluginManager *plugin_mgr;
 
+  MetaWindow *restore_focus_window;
+
   gboolean frame_has_updated_xsurfaces;
   gboolean have_x11_sync_object;
 };

src/compositor/compositor.c

@@ -375,6 +375,11 @@ meta_begin_modal_for_plugin (MetaCompositor   *compositor,
   if (!grab_devices (options, timestamp))
     return FALSE;
 
+  g_set_object (&compositor->restore_focus_window,
+                meta_display_get_focus_window (display));
+  meta_x11_display_focus_the_no_focus_window (display->x11_display,
+                                              timestamp);
+
   display->grab_op = META_GRAB_OP_COMPOSITOR;
   display->event_route = META_EVENT_ROUTE_COMPOSITOR_GRAB;
   display->grab_window = NULL;
@@ -418,6 +423,19 @@ meta_end_modal_for_plugin (MetaCompositor *compositor,
   display->grab_have_pointer = FALSE;
   display->grab_have_keyboard = FALSE;
 
+  if (compositor->restore_focus_window)
+    {
+      if (!compositor->restore_focus_window->unmanaging &&
+          !meta_display_get_focus_window (display))
+        {
+          meta_x11_display_set_input_focus_window (display->x11_display,
+                                                   compositor->restore_focus_window,
+                                                   FALSE, timestamp);
+        }
+
+      g_clear_object (&compositor->restore_focus_window);
+    }
+
   meta_backend_ungrab_device (backend, META_VIRTUAL_CORE_POINTER_ID, timestamp);
   meta_backend_ungrab_device (backend, META_VIRTUAL_CORE_KEYBOARD_ID, timestamp);