ash: Stop propagation of gesture events when drag drop is in progress.

ash/drag_drop/drag_drop_controller.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/drag_drop/drag_drop_controller.h"
 
 #include "ash/drag_drop/drag_drop_tracker.h"
 #include "ash/drag_drop/drag_image_view.h"
 #include "ash/shell.h"
 #include "ash/wm/coordinate_conversion.h"
@@ skipping 388 matching lines @@
     return;
 
   if (event->type() == ui::ET_TOUCH_CANCELLED)
     DragCancel();
 }
 
 void DragDropController::OnGestureEvent(ui::GestureEvent* event) {
   if (!IsDragDropInProgress())
     return;
 
-  // If current drag session was not started by touch, dont process this touch
-  // event, but consume it so it does not interfere with current drag session.
-  if (current_drag_event_source_ !=
-      ui::DragDropTypes::DRAG_EVENT_SOURCE_TOUCH) {
-    event->StopPropagation();
+  // No one else should handle gesture events when in drag drop. Note that it is
+  // not enough to just set ER_HANDLED because the dispatcher only stops
+  // dispatching when the event has ER_CONSUMED. If we just set ER_HANDLED, the
+  // event will still be dispatched to other handlers and we depend on
+  // individual handlers' kindness to not touch events marked ER_HANDLED (not
+  // all handlers are so kind and may cause bugs like crbug.com/236493).
+  event->StopPropagation();
+
+  // If current drag session was not started by touch, dont process this event.
+  if (current_drag_event_source_ != ui::DragDropTypes::DRAG_EVENT_SOURCE_TOUCH)
     return;
-  }
 
   // Apply kTouchDragImageVerticalOffset to the location.
   ui::GestureEvent touch_offset_event(*event,
                                       static_cast<aura::Window*>(NULL),
                                       static_cast<aura::Window*>(NULL));
   gfx::Point touch_offset_location = touch_offset_event.location();
   gfx::Point touch_offset_root_location = touch_offset_event.root_location();
   touch_offset_location.Offset(0, kTouchDragImageVerticalOffset);
   touch_offset_root_location.Offset(0, kTouchDragImageVerticalOffset);
   touch_offset_event.set_location(touch_offset_location);
@@ skipping 135 matching lines @@
     drag_window_->RemoveObserver(this);
   drag_window_ = NULL;
   drag_data_ = NULL;
   // Cleanup can be called again while deleting DragDropTracker, so use Pass
   // instead of reset to avoid double free.
   drag_drop_tracker_.Pass();
 }
 
 }  // namespace internal
 }  // namespace ash