Keep expanded if mouse moves off of scrollbar while dragging

ui/events/blink/input_handler_proxy.cc

 // Copyright 2013 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 "ui/events/blink/input_handler_proxy.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 356 matching lines @@
 
     case WebInputEvent::TouchStart:
       return HandleTouchStart(static_cast<const WebTouchEvent&>(event));
 
     case WebInputEvent::TouchMove:
       return HandleTouchMove(static_cast<const WebTouchEvent&>(event));
 
     case WebInputEvent::TouchEnd:
       return HandleTouchEnd(static_cast<const WebTouchEvent&>(event));
 
+    case WebInputEvent::MouseDown: {
+      // Only for check scrollbar captured
+      const WebMouseEvent& mouse_event =
+          static_cast<const WebMouseEvent&>(event);
+
+      if (mouse_event.button == blink::WebMouseEvent::Button::Left) {
+        CHECK(input_handler_);
+        input_handler_->MouseDown();
+      }
+      return DID_NOT_HANDLE;
+    }
+    case WebInputEvent::MouseUp: {
+      // Only for release scrollbar captured
+      const WebMouseEvent& mouse_event =
+          static_cast<const WebMouseEvent&>(event);
+
+      if (mouse_event.button == blink::WebMouseEvent::Button::Left) {
+        CHECK(input_handler_);
+        input_handler_->MouseUp();
+      }
+      return DID_NOT_HANDLE;
+    }
     case WebInputEvent::MouseMove: {
       const WebMouseEvent& mouse_event =
           static_cast<const WebMouseEvent&>(event);
-      // TODO(tony): Ignore when mouse buttons are down?
       // TODO(davemoore): This should never happen, but bug #326635 showed some
       // surprising crashes.
       CHECK(input_handler_);
       input_handler_->MouseMoveAt(gfx::Point(mouse_event.x, mouse_event.y));
       return DID_NOT_HANDLE;
     }
 
     default:
       if (WebInputEvent::isKeyboardEventType(event.type)) {
         // Only call |CancelCurrentFling()| if a fling was active, as it will
@@ skipping 979 matching lines @@
   // is made asynchronously, to minimize divergence between main thread and
   // impl thread event handling paths.
   base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE,
       base::Bind(&InputScrollElasticityController::ObserveGestureEventAndResult,
                  scroll_elasticity_controller_->GetWeakPtr(), gesture_event,
                  scroll_result));
 }
 
 }  // namespace ui