Drag and drop cleans up touch sequences from the source window.

ui/events/gestures/gesture_recognizer.h

 // 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.
 
 #ifndef UI_EVENTS_GESTURES_GESTURE_RECOGNIZER_H_
 #define UI_EVENTS_GESTURES_GESTURE_RECOGNIZER_H_
 
 #include <stdint.h>
 
 #include <vector>
@@ skipping 44 matching lines @@
   // |source_device_id|, within
   // GestureConfiguration::max_separation_for_gesture_touches_in_pixels of
   // |location|, or NULL if no such point exists.
   virtual GestureConsumer* GetTargetForLocation(
       const gfx::PointF& location, int source_device_id) = 0;
 
   // Cancels all touches except those targeted to |not_cancelled|. If
   // |not_cancelled| == nullptr, cancels all touches.
   virtual void CancelActiveTouchesExcept(GestureConsumer* not_cancelled) = 0;
 
-  // Makes |new_consumer| the target for events previously targeting
-  // |current_consumer|. Touches targeting all other targets are
-  // canceled. The caller is responsible for updating the state of the
-  // consumers to be aware of this transfer of control (there are no
-  // ENTERED/EXITED events).
+  enum class ShouldCancelTouches { Cancel, DontCancel };
+
+  // Transfer the gesture stream from the drag source (current_consumer) to the
+  // consumer used for dragging (new_consumer). If |should_cancel_touches| is
+  // Cancel, dispatches cancel events to |current_consumer| to ensure that its
+  // touch stream remains valid.
   virtual void TransferEventsTo(GestureConsumer* current_consumer,
-                                GestureConsumer* new_consumer) = 0;
+                                GestureConsumer* new_consumer,
+                                ShouldCancelTouches should_cancel_touches) = 0;
 
   // If a gesture is underway for |consumer| |point| is set to the last touch
   // point and true is returned. If no touch events have been processed for
   // |consumer| false is returned and |point| is untouched.
   virtual bool GetLastTouchPointForTarget(GestureConsumer* consumer,
                                           gfx::PointF* point) = 0;
 
   // Sends a touch cancel event for every active touch. Returns true iff any
   // touch cancels were sent.
   virtual bool CancelActiveTouches(GestureConsumer* consumer) = 0;
 
   // Subscribes |helper| for dispatching async gestures such as long press.
   // The Gesture Recognizer does NOT take ownership of |helper| and it is the
   // responsibility of the |helper| to call |RemoveGestureEventHelper()| on
   // destruction.
   virtual void AddGestureEventHelper(GestureEventHelper* helper) = 0;
 
   // Unsubscribes |helper| from async gesture dispatch.
   // Since the GestureRecognizer does not own the |helper|, it is not deleted
   // and must be cleaned up appropriately by the caller.
   virtual void RemoveGestureEventHelper(GestureEventHelper* helper) = 0;
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_GESTURES_GESTURE_RECOGNIZER_H_