compositor: Add an API to focus the stage X window

gnome-shell has traditionally just called XSetInputFocus when wanting to
set the input focus to the stage window, but this might cause strange,
hard-to-reproduce bugs because of an interference with mutter's focus
prediction. Add API to allow gnome-shell to focus the stage window that
also updates mutter's internal focus prediction state.

https://bugzilla.gnome.org/show_bug.cgi?id=700735

src/compositor/compositor.c

@@ -369,6 +369,28 @@ meta_empty_stage_input_region (MetaScreen *screen)
   meta_set_stage_input_region (screen, region);
 }
 
+void
+meta_focus_stage_window (MetaScreen *screen,
+                         guint32     timestamp)
+{
+  ClutterStage *stage;
+  Window window;
+
+  stage = CLUTTER_STAGE (meta_get_stage_for_screen (screen));
+  if (!stage)
+    return;
+
+  window = clutter_x11_get_stage_window (stage);
+
+  if (window == None)
+    return;
+
+  meta_display_set_input_focus_xwindow (screen->display,
+                                        screen,
+                                        window,
+                                        timestamp);
+}
+
 gboolean
 meta_begin_modal_for_plugin (MetaScreen       *screen,
                              MetaPlugin       *plugin,

src/core/display-private.h

@@ -465,4 +465,9 @@ gboolean meta_display_process_barrier_event (MetaDisplay    *display,
                                              XIBarrierEvent *event);
 #endif /* HAVE_XI23 */
 
+void meta_display_set_input_focus_xwindow (MetaDisplay *display,
+                                           MetaScreen  *screen,
+                                           Window       window,
+                                           guint32      timestamp);
+
 #endif

src/core/display.c

@@ -5806,6 +5806,18 @@ meta_display_request_take_focus (MetaDisplay *display,
                                   timestamp);
 }
 
+void
+meta_display_set_input_focus_xwindow (MetaDisplay *display,
+                                      MetaScreen  *screen,
+                                      Window       window,
+                                      guint32      timestamp)
+{
+  request_xserver_input_focus_change (display,
+                                      screen,
+                                      window,
+                                      timestamp);
+}
+
 void
 meta_display_focus_the_no_focus_window (MetaDisplay *display,
                                         MetaScreen  *screen,

src/meta/compositor-mutter.h

@@ -46,5 +46,7 @@ void        meta_enable_unredirect_for_screen   (MetaScreen *screen);
 void meta_set_stage_input_region     (MetaScreen    *screen,
                                       XserverRegion  region);
 void meta_empty_stage_input_region   (MetaScreen    *screen);
+void meta_focus_stage_window         (MetaScreen    *screen,
+                                      guint32        timestamp);
 
 #endif