gdkdnd: Add private means to commit the drag status

The way gdk_drag_status() may be called multiple times during the
processing of drag and drop events throughout the widget hierarchy
brings some superfluous messaging going in, esp. when it's the last
request the one we want to honor, yet we emit messaging requests on
all.

This is barely appreciable in the X11 backend, but due to the design
of the wayland protocol, quick series of changes like this it have
some self-amplificating consequences which may end up flooding the
connection.

We can delegate this to a late "commit" call, performed within GDK
event management. This way gdk_drag_status() calls may be cached
and only result in windowing messaging once per ::drag-motion or
::drag-data-received event. Emitting the final status will also
avoid spurious action changes on the compositor and the other peer.

https://bugzilla.gnome.org/show_bug.cgi?id=763298

gdk/gdkdnd.c

@@ -764,3 +764,53 @@ gdk_drag_get_cursor (GdkDragAction action)
                                                        drag_cursors[i].name);
   return drag_cursors[i].cursor;
 }
+
+static void
+gdk_drag_context_commit_drag_status (GdkDragContext *context)
+{
+  GdkDragContextClass *context_class;
+
+  g_return_if_fail (GDK_IS_DRAG_CONTEXT (context));
+  g_return_if_fail (!context->is_source);
+
+  context_class = GDK_DRAG_CONTEXT_GET_CLASS (context);
+
+  if (context_class->commit_drag_status)
+    context_class->commit_drag_status (context);
+}
+
+gboolean
+gdk_drag_context_handle_dest_event (GdkEvent *event)
+{
+  GdkDragContext *context = NULL;
+  GList *l;
+
+  switch (event->type)
+    {
+    case GDK_DRAG_MOTION:
+    case GDK_DROP_START:
+      context = event->dnd.context;
+      break;
+    case GDK_SELECTION_NOTIFY:
+      for (l = contexts; l; l = l->next)
+        {
+          GdkDragContext *c = l->data;
+
+          if (!c->is_source &&
+              event->selection.selection == gdk_drag_get_selection (c))
+            {
+              context = c;
+              break;
+            }
+        }
+      break;
+    default:
+      return FALSE;
+    }
+
+  if (!context)
+    return FALSE;
+
+  gdk_drag_context_commit_drag_status (context);
+  return TRUE;;
+}

gdk/gdkdndprivate.h

@@ -84,6 +84,8 @@ struct _GdkDragContextClass {
                                  const GdkEvent  *event);
   void        (*action_changed) (GdkDragContext  *context,
                                  GdkDragAction    action);
+
+  void        (*commit_drag_status) (GdkDragContext  *context);
 };
 
 struct _GdkDragContext {
@@ -116,6 +118,7 @@ void     gdk_drag_context_set_cursor          (GdkDragContext *context,
 void     gdk_drag_context_cancel              (GdkDragContext      *context,
                                                GdkDragCancelReason  reason);
 gboolean gdk_drag_context_handle_source_event (GdkEvent *event);
+gboolean gdk_drag_context_handle_dest_event   (GdkEvent *event);
 GdkCursor * gdk_drag_get_cursor (GdkDragAction action);
 
 G_END_DECLS

gdk/gdkevents.c

@@ -71,6 +71,9 @@ _gdk_event_emit (GdkEvent *event)
 
   if (_gdk_event_func)
     (*_gdk_event_func) (event, _gdk_event_data);
+
+  if (gdk_drag_context_handle_dest_event (event))
+    return;
 }
 
 /*********************************************

gdk/wayland/gdkdnd-wayland.c

@@ -287,8 +287,6 @@ gdk_wayland_drag_context_drag_status (GdkDragContext *context,
 
   wayland_context = GDK_WAYLAND_DRAG_CONTEXT (context);
   wayland_context->selected_action = action;
-
-  gdk_wayland_drag_context_commit_status (context);
 }
 
 static void
@@ -471,6 +469,7 @@ gdk_wayland_drag_context_class_init (GdkWaylandDragContextClass *klass)
   context_class->action_changed = gdk_wayland_drag_context_action_changed;
   context_class->drop_performed = gdk_wayland_drag_context_drop_performed;
   context_class->cancel = gdk_wayland_drag_context_cancel;
+  context_class->commit_drag_status = gdk_wayland_drag_context_commit_status;
 }
 
 GdkDragProtocol