Keep expanded if mouse moves off of scrollbar while dragging

ui/events/blink/input_handler_proxy.cc

Index: ui/events/blink/input_handler_proxy.cc
diff --git a/ui/events/blink/input_handler_proxy.cc b/ui/events/blink/input_handler_proxy.cc
index f52553411664137dee6048642429b128deaec6d0..e2a2d3dbf525f63ef1234af00717d4fd81c0c7be 100644
--- a/ui/events/blink/input_handler_proxy.cc
+++ b/ui/events/blink/input_handler_proxy.cc
@@ -374,10 +374,31 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleInputEvent(
     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_);