wayland/xdg-toplevel-drag: Support non-grabbing mode in MetaWindowDrag

- This adds a new ClutterActor parameter to meta_compositor_drag_window
  and meta_window_drag_begin such that it's possible to inject an
  external grab actor, which is then used to implement a "non-grabbing"
  mode that allow a MetaWindowDrag to be used in conjunction, for
  example, with a Wayland drag-and-drop session;
- This new API is going to be used to implement the toplevel drag
  protocol in a subsequent commit.

Signed-off-by: Nick Diego Yamane <nickdiego@igalia.com>
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4107>

src/compositor/compositor-private.h

@@ -79,7 +79,8 @@ gboolean meta_compositor_drag_window (MetaCompositor       *compositor,
                                       ClutterInputDevice   *device,
                                       ClutterEventSequence *sequence,
                                       uint32_t              timestamp,
-                                      graphene_point_t     *pos_hint);
+                                      graphene_point_t     *pos_hint,
+                                      ClutterActor         *grab_actor);
 
 MetaWindowDrag * meta_compositor_get_current_window_drag (MetaCompositor *compositor);
 

src/compositor/compositor.c

@@ -1587,7 +1587,8 @@ meta_compositor_drag_window (MetaCompositor       *compositor,
                              ClutterInputDevice   *device,
                              ClutterEventSequence *sequence,
                              uint32_t              timestamp,
-                             graphene_point_t     *pos_hint)
+                             graphene_point_t     *pos_hint,
+                             ClutterActor         *grab_actor)
 {
   MetaCompositorPrivate *priv =
     meta_compositor_get_instance_private (compositor);
@@ -1603,7 +1604,8 @@ meta_compositor_drag_window (MetaCompositor       *compositor,
 
   priv->current_drag = g_steal_pointer (&window_drag);
 
-  if (!meta_window_drag_begin (priv->current_drag, device, sequence, timestamp))
+  if (!meta_window_drag_begin (priv->current_drag, device,
+                               sequence, timestamp, grab_actor))
     {
       g_clear_object (&priv->current_drag);
       return FALSE;

src/compositor/meta-window-drag.c

@@ -79,6 +79,7 @@ struct _MetaWindowDrag {
 
   gulong unmanaged_id;
   gulong size_changed_id;
+  gulong event_handler_id;
 
   guint tile_preview_timeout_id;
   guint preview_tile_mode : 2;
@@ -175,14 +176,24 @@ hide_tile_preview (MetaWindowDrag *window_drag)
     meta_compositor_hide_tile_preview (window->display->compositor);
 }
 
+static gboolean
+owns_grab_actor (MetaWindowDrag *window_drag)
+{
+  g_assert (META_IS_WINDOW_DRAG (window_drag));
+  return window_drag->grab != NULL;
+}
+
 static void
 meta_window_drag_finalize (GObject *object)
 {
   MetaWindowDrag *window_drag = META_WINDOW_DRAG (object);
 
   hide_tile_preview (window_drag);
-  g_clear_pointer (&window_drag->handler, clutter_actor_destroy);
+  if (owns_grab_actor (window_drag))
-  g_clear_object (&window_drag->grab);
+    {
+      g_clear_pointer (&window_drag->handler, clutter_actor_destroy);
+      g_clear_object (&window_drag->grab);
+    }
   g_clear_object (&window_drag->effective_grab_window);
 
   G_OBJECT_CLASS (meta_window_drag_parent_class)->finalize (object);
@@ -387,7 +398,11 @@ meta_window_drag_end (MetaWindowDrag *window_drag)
 
   meta_window_grab_op_ended (grab_window, grab_op);
 
-  clutter_grab_dismiss (window_drag->grab);
+  if (owns_grab_actor (window_drag))
+    clutter_grab_dismiss (window_drag->grab);
+  else
+    g_clear_signal_handler (&window_drag->event_handler_id,
+                            window_drag->handler);
 
   g_clear_signal_handler (&window_drag->unmanaged_id, grab_window);
   g_clear_signal_handler (&window_drag->size_changed_id, grab_window);
@@ -1780,7 +1795,8 @@ gboolean
 meta_window_drag_begin (MetaWindowDrag       *window_drag,
                         ClutterInputDevice   *device,
                         ClutterEventSequence *sequence,
-                        uint32_t              timestamp)
+                        uint32_t              timestamp,
+                        ClutterActor         *grab_actor)
 {
   MetaWindow *window = window_drag->window, *grab_window = NULL;
   MetaDisplay *display = meta_window_get_display (window);
@@ -1845,26 +1861,39 @@ meta_window_drag_begin (MetaWindowDrag       *window_drag,
 
   stage = meta_backend_get_stage (backend);
 
-  window_drag->handler = clutter_actor_new ();
+  if (grab_actor)
-  clutter_actor_hide (window_drag->handler);
-  clutter_actor_set_name (window_drag->handler,
-                          "Window drag helper");
-  clutter_actor_set_accessible_name (window_drag->handler,
-                                     "Window drag helper");
-  g_signal_connect_swapped (window_drag->handler, "event",
-                            G_CALLBACK (on_window_drag_event), window_drag);
-  clutter_actor_add_child (stage, window_drag->handler);
-
-  window_drag->grab = clutter_stage_grab (CLUTTER_STAGE (stage),
-                                          window_drag->handler);
-
-  if ((clutter_grab_get_seat_state (window_drag->grab) &
-       CLUTTER_GRAB_STATE_POINTER) == 0 &&
-      !meta_grab_op_is_keyboard (grab_op))
     {
-      meta_topic (META_DEBUG_WINDOW_OPS,
+      meta_topic (META_DEBUG_WINDOW_OPS, "Reusing grab actor %p.", grab_actor);
-                  "Pointer grab failed on a pointer grab op");
+      window_drag->handler = grab_actor;
-      return FALSE;
+      window_drag->event_handler_id =
+        g_signal_connect_swapped (window_drag->handler, "event",
+                                  G_CALLBACK (on_window_drag_event), window_drag);
+    }
+  else
+    {
+      meta_topic (META_DEBUG_WINDOW_OPS, "Creating a new grab.");
+      window_drag->handler = clutter_actor_new ();
+      clutter_actor_hide (window_drag->handler);
+      clutter_actor_set_name (window_drag->handler,
+                              "Window drag helper");
+      clutter_actor_set_accessible_name (window_drag->handler,
+                                         "Window drag helper");
+      window_drag->event_handler_id =
+        g_signal_connect_swapped (window_drag->handler, "event",
+                                  G_CALLBACK (on_window_drag_event), window_drag);
+      clutter_actor_add_child (stage, window_drag->handler);
+
+      window_drag->grab = clutter_stage_grab (CLUTTER_STAGE (stage),
+                                              window_drag->handler);
+
+      if ((clutter_grab_get_seat_state (window_drag->grab) &
+           CLUTTER_GRAB_STATE_POINTER) == 0 &&
+          !meta_grab_op_is_keyboard (grab_op))
+        {
+          meta_topic (META_DEBUG_WINDOW_OPS,
+                      "Pointer grab failed on a pointer grab op");
+          return FALSE;
+        }
     }
 
   /* Temporarily release the passive key grabs on the window */

src/compositor/meta-window-drag.h

@@ -32,7 +32,8 @@ MetaWindowDrag * meta_window_drag_new (MetaWindow *window,
 gboolean meta_window_drag_begin (MetaWindowDrag       *drag,
                                  ClutterInputDevice   *device,
                                  ClutterEventSequence *sequence,
-                                 uint32_t              timestamp);
+                                 uint32_t              timestamp,
+                                 ClutterActor         *grab_actor);
 
 void meta_window_drag_end (MetaWindowDrag *drag);
 

src/core/window.c

@@ -6149,7 +6149,7 @@ meta_window_begin_grab_op (MetaWindow           *window,
                                       window, op,
                                       device, sequence,
                                       timestamp,
-                                      pos_hint);
+                                      pos_hint, NULL);
 }
 
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