Mainframe scrollbars should get events even if document prevent-defaults the mouse events.

Source/core/page/EventHandler.cpp

Index: Source/core/page/EventHandler.cpp
diff --git a/Source/core/page/EventHandler.cpp b/Source/core/page/EventHandler.cpp
index 55a07108fe384eddf4e7d7407552417884a1d2ed..ab6f49b6c125f4034241fd3a31b1db9589056706 100644
--- a/Source/core/page/EventHandler.cpp
+++ b/Source/core/page/EventHandler.cpp
@@ -1288,6 +1288,10 @@ bool EventHandler::handleMousePressEvent(const PlatformMouseEvent& mouseEvent)
     if (swallowEvent) {
         // scrollbars should get events anyway, even disabled controls might be scrollable
         Scrollbar* scrollbar = mev.scrollbar();
+        if (!scrollbar) {
+            FrameView* view = m_frame->view();
+            scrollbar = view ? view->scrollbarAtPoint(mouseEvent.position()) : 0;
+        }
 
         updateLastScrollbarUnderMouse(scrollbar, true);