[Webkit-unassigned] [Bug 246048] Incorrect scroll position reported

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=246048

--- Comment #6 from Simon Fraser (smfr) <simon.fraser at apple.com> ---
There are a number of data races that cause the web process scroll position to be stale relative to the UI process.

In the following, {} represents some asynchronous behavior, and newlines represent thread or process hop.

A page render is triggered via :

CADisplayLink -> RemoteLayerTreeDrawingAreaProxy::didRefreshDisplay ->
                IPC -> RemoteLayerTreeDrawingArea::displayDidRefresh -> { zero delay timer } -> Page::updateRendering()

A scroll update getting to the web process goes via:

UIScrollView -> { Core Animation pre-commit hook } -> -[WKContentView didUpdateVisibleRect:] ->
                IPC -> ViewUpdateDispatcher work queue -> 
                             -> ViewUpdateDispatcher::visibleContentRectUpdate

There are two main races here. The RemoteLayerTreeDrawingArea::displayDidRefresh message may reach the web process before the ViewUpdateDispatcher::visibleContentRectUpdate, and the ViewUpdateDispatcher work queue can delay the visibleContentRectUpdate getting to the main thread.

For everything to work correctly, the new scroll position from UIScrollView needs to get into a visibleContentRectUpdate before Page::updateRendering() runs. That rarely happens.

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