[mutter] compositor: Ignore offscreen windows when unredirecting

[Jonas Ådahl <jadahl src gnome org>]

commit 2e99963087a306460749ffbabce64069ab9738c4
Author: Jonas Ådahl <jadahl gmail com>
Date:   Fri Oct 13 16:29:20 2017 +0800

    compositor: Ignore offscreen windows when unredirecting
    
    When determining whether we should unredirect a window or not, ignore
    offscreen windows, and just check the top most visible window.
    
    Previously this was not an issue, but since 'stack-tracker: Keep
    override redirect windows on top' we started sorting the UI frames
    window, which is an offscreen override redirect window, on top, causing
    the unredirect checking code to always check whether to unredirect the
    UI frames window. This effectively disabled the compositor bypass
    functionality.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=788493

 src/compositor/compositor-private.h |    2 +
 src/compositor/compositor.c         |   47 ++++++++++++++++++++++++++++++----
 2 files changed, 43 insertions(+), 6 deletions(-)
---
diff --git a/src/compositor/compositor-private.h b/src/compositor/compositor-private.h
index ba58bf1..40c0344 100644
--- a/src/compositor/compositor-private.h
+++ b/src/compositor/compositor-private.h
@@ -31,6 +31,8 @@ struct _MetaCompositor
 
   CoglContext           *context;
 
+  MetaWindowActor       *top_window_actor;
+
   /* Used for unredirecting fullscreen windows */
   guint                  disable_unredirect_count;
   MetaWindow            *unredirected_window;
diff --git a/src/compositor/compositor.c b/src/compositor/compositor.c
index 17e8a55..a75ac12 100644
--- a/src/compositor/compositor.c
+++ b/src/compositor/compositor.c
@@ -923,6 +923,32 @@ sync_actor_stacking (MetaCompositor *compositor)
   g_list_free (backgrounds);
 }
 
+/*
+ * Find the top most window that is visible on the screen. The intention of
+ * this is to avoid offscreen windows that isn't actually part of the visible
+ * desktop (such as the UI frames override redirect window).
+ */
+static MetaWindowActor *
+get_top_visible_window_actor (MetaCompositor *compositor)
+{
+  GList *l;
+
+  for (l = g_list_last (compositor->windows); l; l = l->prev)
+    {
+      MetaWindowActor *window_actor = l->data;
+      MetaWindow *window = meta_window_actor_get_meta_window (window_actor);
+      MetaRectangle buffer_rect;
+
+      meta_window_get_buffer_rect (window, &buffer_rect);
+
+      if (meta_rectangle_overlap (&compositor->display->screen->rect,
+                                  &buffer_rect))
+        return window_actor;
+    }
+
+  return NULL;
+}
+
 void
 meta_compositor_sync_stack (MetaCompositor  *compositor,
                            GList           *stack)
@@ -1009,6 +1035,8 @@ meta_compositor_sync_stack (MetaCompositor  *compositor,
     }
 
   sync_actor_stacking (compositor);
+
+  compositor->top_window_actor = get_top_visible_window_actor (compositor);
 }
 
 void
@@ -1065,19 +1093,26 @@ static gboolean
 meta_pre_paint_func (gpointer data)
 {
   GList *l;
-  MetaWindowActor *top_window;
+  MetaWindowActor *top_window_actor;
   MetaCompositor *compositor = data;
 
   if (compositor->windows == NULL)
     return TRUE;
 
-  top_window = g_list_last (compositor->windows)->data;
-
-  if (meta_window_actor_should_unredirect (top_window) &&
+  top_window_actor = compositor->top_window_actor;
+  if (top_window_actor &&
+      meta_window_actor_should_unredirect (top_window_actor) &&
       compositor->disable_unredirect_count == 0)
-    set_unredirected_window (compositor, meta_window_actor_get_meta_window (top_window));
+    {
+      MetaWindow *top_window;
+
+      top_window = meta_window_actor_get_meta_window (top_window_actor);
+      set_unredirected_window (compositor, top_window);
+    }
   else
-    set_unredirected_window (compositor, NULL);
+    {
+      set_unredirected_window (compositor, NULL);
+    }
 
   for (l = compositor->windows; l; l = l->next)
     meta_window_actor_pre_paint (l->data);