[Webkit-unassigned] [Bug 198292] [async scrolling] Fixed positioning inside stacking context overflow scroll is jumpy

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=198292

--- Comment #8 from Simon Fraser (smfr) <simon.fraser at apple.com> ---
Comment on attachment 370738
  --> https://bugs.webkit.org/attachment.cgi?id=370738
patch

View in context: https://bugs.webkit.org/attachment.cgi?id=370738&action=review

> Source/WebCore/page/scrolling/cocoa/ScrollingTreeFixedNode.mm:90
> +        FloatSize overflowScrollDelta;
> +        // FIXME: This code is wrong in complex cases where the fixed element is inside a positioned node as
> +        //        the scroll container order does not match the scrolling tree ancestor order.
> +        for (auto* node = parent(); node; node = node->parent()) {
> +            if (is<ScrollingTreeFrameScrollingNode>(*node)) {
> +                // Fixed nodes are positioned relative to the containing frame scrolling node.
> +                // We bail out after finding one.
> +                auto layoutViewport = downcast<ScrollingTreeFrameScrollingNode>(*node).layoutViewport();
> +                return m_constraints.layerPositionForViewportRect(layoutViewport) - overflowScrollDelta;
> +            }
> +
> +            if (is<ScrollingTreeOverflowScrollingNode>(*node)) {
> +                // To keep the layer still during async scrolling we adjust by how much the position has changed since layout.
> +                auto& overflowNode = downcast<ScrollingTreeOverflowScrollingNode>(*node);
> +                auto localDelta = overflowNode.lastCommittedScrollPosition() - overflowNode.currentScrollPosition();
> +                overflowScrollDelta += localDelta;
> +            }
> +        }
> +        ASSERT_NOT_REACHED();
> +        return FloatPoint();

Won't Sticky nodes need similar logic?

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