[Webkit-unassigned] [Bug 194289] [iOS] Glitch free fixed positioning with async scrolling in frames

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=194289

Simon Fraser (smfr) <simon.fraser at apple.com> changed:

           What    |Removed                     |Added
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                 CC|                            |thorton at apple.com,
                   |                            |zalan at apple.com

--- Comment #4 from Simon Fraser (smfr) <simon.fraser at apple.com> ---
Design statement: all data which is input to the scrolling tree should be computed independent from any scroll positions or viewport rects which the scrolling tree itself manipulates.

That implies that the scrolling tree should compute layer positions simply using the data in the scrolling tree + the current state of scrolling-tree-owned scroll positions (for the root, frames and overflow) and viewport rects.

We'll need to be careful that the "viewport constraints" used for fixed and sticky are "pure" in the sense the they don't encode the current layout and viewport rects. I'm not sure what should happen when a programmatic scroll (when zoomed) affects visual and viewport rects; they need to be update synchronously in the web process for various APIs to work correctly, then I guess the "requested scroll position" is sent to the scrolling tree, and the scrolling thread/UI process basically replicate the same logic and hopefully end up with the same rects?

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