--- a/widget/PuppetWidget.cpp
+++ b/widget/PuppetWidget.cpp
@@ -1521,17 +1521,16 @@ PuppetWidget::HasPendingInputEvent()
   bool ret = false;
 
   mTabChild->GetIPCChannel()->PeekMessages(
     [&ret](const IPC::Message& aMsg) -> bool {
       if ((aMsg.type() & mozilla::dom::PBrowser::PBrowserStart)
           == mozilla::dom::PBrowser::PBrowserStart) {
         switch (aMsg.type()) {
           case mozilla::dom::PBrowser::Msg_RealMouseMoveEvent__ID:
-          case mozilla::dom::PBrowser::Msg_SynthMouseMoveEvent__ID:
           case mozilla::dom::PBrowser::Msg_RealMouseButtonEvent__ID:
           case mozilla::dom::PBrowser::Msg_RealKeyEvent__ID:
           case mozilla::dom::PBrowser::Msg_MouseWheelEvent__ID:
           case mozilla::dom::PBrowser::Msg_RealTouchEvent__ID:
           case mozilla::dom::PBrowser::Msg_RealTouchMoveEvent__ID:
           case mozilla::dom::PBrowser::Msg_RealDragEvent__ID:
           case mozilla::dom::PBrowser::Msg_UpdateDimensions__ID:
           case mozilla::dom::PBrowser::Msg_MouseEvent__ID: