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).
-  virtual void TransferEventsTo(GestureConsumer* current_consumer,
+  // Transfer the gesture stream from the drag source (current_consumer) to the
+  // consumer used for dragging (new_consumer). This dispatches cancel events to
+  // |current_consumer| to ensure that its touch stream remains valid.
+  virtual void BeginDragAndDrop(GestureConsumer* current_consumer,
                                 GestureConsumer* new_consumer) = 0;
 
+  // Transfer the gesture stream from the drag source (current_consumer) to the
+  // consumer used for dragging (new_consumer). This doesn't dispatch cancel
+  // events to |current_consumer|.
+  virtual void BeginTabDrag(GestureConsumer* current_consumer,

[sky, 2016/05/12 15:43:34]

This code sits lower in the stack than the tab dragging code. So using 'tab' in
the name feels wrong. Did you consider combining BeginDragAndDrop and
BeginTabDrag and making the combined function take a parameter indicating if the
events should be canceled?

+                            GestureConsumer* new_consumer) = 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_