window: Add comment documenting the target widget checks during window drag

Was suggested during review of
https://bugzilla.gnome.org/show_bug.cgi?id=745562#c2

diff --git a/gtk/gtkwindow.c b/gtk/gtkwindow.c
index 92d7a03cdea78c1dfda0b19b3dd29e855635661d..c4196980db72bc30c8b454a6e6c67bac85ea9f97 100644 (file)
--- a/gtk/gtkwindow.c
+++ b/gtk/gtkwindow.c
@@ -1549,6 +1549,15 @@ drag_gesture_update_cb (GtkGestureDrag *gesture,
       event = gtk_gesture_get_last_event (GTK_GESTURE (gesture), sequence);
       event_widget = gtk_get_event_widget ((GdkEvent *) event);
 
+      /* Check whether the target widget should be left alone at handling
+       * the sequence, this is better done late to give room for gestures
+       * there to go denied.
+       *
+       * Besides claiming gestures, we must bail out too if there's gestures
+       * in the "none" state at this point, as those are still handling events
+       * and can potentially go claimed, and we don't want to stop the target
+       * widget from doing anything.
+       */
       if (event_widget != GTK_WIDGET (window) &&
           !gtk_widget_has_grab (event_widget) &&
           _gtk_widget_consumes_motion (event_widget, sequence))