Release mouse pointer capture when it starts drag

third_party/WebKit/Source/core/input/MouseEventManager.cpp

Index: third_party/WebKit/Source/core/input/MouseEventManager.cpp
diff --git a/third_party/WebKit/Source/core/input/MouseEventManager.cpp b/third_party/WebKit/Source/core/input/MouseEventManager.cpp
index e4a1ec7df78bfad490fc49ea7b359e7fa48cdcce..ffe57a1eaf1008680cf7a9db724222ecec4083c7 100644
--- a/third_party/WebKit/Source/core/input/MouseEventManager.cpp
+++ b/third_party/WebKit/Source/core/input/MouseEventManager.cpp
@@ -873,6 +873,16 @@ bool MouseEventManager::HandleDrag(const MouseEventWithHitTestResults& event,
     // Something failed to start the drag, clean up.
     ClearDragDataTransfer();
     ResetDragState();
+  } else {
+    // Since drag operation started we need to send a pointercancel for the
+    // corresponding pointer.
+    if (initiator == DragInitiator::kMouse) {
+      frame_->GetEventHandler().HandlePointerEvent(
+          WebPointerEvent(WebInputEvent::Type::kPointerCancel, event.Event()),
+          event.InnerNode());
+    }
+    // TODO(crbug.com/708278): If the drag starts with touch the touch cancel
+    // should trigger the release of pointer capture.
   }
 
   mouse_down_may_start_drag_ = false;