Bug 194184 – Async overflow scroll is jumpy on macOS if the main thread is busy

• Component: Scrolling
• Status: RESOLVED FIXED

[Simon Fraser (smfr)]

Created attachment 360950 [details]
Testcase

Main thread and scrolling thread seem to be fighting over scroll positions.

[Simon Fraser (smfr)]

While we're scrolling on the scrolling thread, we post back to the main thread via AsyncScrollingCoordinator::updateScrollPositionAfterAsyncScroll(), which triggers RendderLayer::scrollTo() and then does a compositing update, and that update slams a stale scroll position onto the graphics layer.

[Radar WebKit Bug Importer]

<rdar://problem/47758655>

[Simon Fraser (smfr)]

iOS avoids this with:

        if (m_owningLayer.isInUserScroll()) {
            // If scrolling is happening externally, we don't want to touch the layer bounds origin here because that will cause jitter.
            m_scrollingLayer->syncBoundsOrigin(scrollOffset);
            m_owningLayer.setRequiresScrollBoundsOriginUpdate(true);
        } else {

[Simon Fraser (smfr)]

Created attachment 360963 [details]
Patch

[Antti Koivisto]

Comment on attachment 360963 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=360963&action=review

> Source/WebCore/ChangeLog:3
> +        Async overflow scroll is jumpy if the main thread is busy

"on Mac"

> Source/WebCore/rendering/RenderLayerBacking.cpp:1759
> -        m_scrollingContentsLayer->setPaintingPhase(paintPhase);
> -        m_scrollingLayer->addChild(*m_scrollingContentsLayer);
> +        m_scrolledContentsLayer->setPaintingPhase(paintPhase);
> +        m_scrollingLayer->addChild(*m_scrolledContentsLayer);

Could have also renamed m_scrollingLayer -> m_scrollContainerLayer

[Simon Fraser (smfr)]

I can do that in a followup if rs=you.

[Antti Koivisto]

(In reply to Simon Fraser (smfr) from comment #6)
> I can do that in a followup if rs=you.

rs=me

[Simon Fraser (smfr)]

https://trac.webkit.org/changeset/240897/webkit