[Webkit-unassigned] [Bug 170280] Dynamically added position:fixed element is in the wrong place

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=170280

--- Comment #5 from Simon Fraser (smfr) <simon.fraser at apple.com> ---
This only happens with UI-side compositing, and reproduces on Mac too in that configuration.

What's happening is this:

The <header> initially is the only fixed element. Its GraphicsLayer gets a position of 0,10. When you scroll, the scrolling tree calls GraphicsLayerCA::syncPosition() which sets various new positions to make fixed position work. However, these positions don't get into PlatformCALayerRemote (they don't need to).

When the body becomes fixed, <header> is now parented in another fixed element, so we stop tracking it in the scrolling tree. When we do layout for this change, <header>'s GraphicsLayer gets a setPosition({0, 10}) which is pushed to PlatformCALayerRemote during the flush, but PlatformCALayerRemote doesn't see this as a change (it still has {0,10}) so this change doesn't make it to the UI process, and the UI layer remains with its old scroll-influenced offset.

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