Extracted from bug 171667. Handling transforms might be a little bit complicated, so we can start by falling back to the slow path.
Created attachment 312871 [details]
(In reply to Frédéric Wang (:fredw) from comment #30)
> Created attachment 311503 [details]
> Patch (do not use async scrolling when there are transformed nodes)
> This is an attempt to avoid async scrolling when there are transformed
> nodes, but it does not work.
I debugged this again and my attempt was actually correct... When there is a transform on the scrolling tree, ScrollingTree::shouldHandleWheelEventSynchronously will return true, tryToHandleWheelEvent will return SendToMainThread and EventDispatcher will thus pass the event to the main thread instead of calling ScrollingTree::handleWheelEvent. However, at the end the the following call stack is executed:
ScrollingTree::handleWheelEvent (scrolling thread)
ThreadedScrollingTree::handleWheelEvent (scrolling thread)
ScrollingCoordinatorMac::handleWheelEvent (main thread)
FrameView::wheelEvent (main thread)
EventHandlerMac's EventHandler::platformCompleteWheelEvent (main thread)
EventHandler::handleWheelEvent (main thread)
so the event is still handled by the scrolling thread!
@Simon: I commented the guilty part out in FrameView::wheelEvent, so that ScrollableArea::handleWheelEvent is called instead and things work as I expected, but I wonder what would be the proper way to fix that?