position: sticky element is not always re-painted on DOM update

Created attachment 461737 [details]
Video showing the bug

Distilled Reproduction Case:
https://codepen.io/patrick-mcdougle/pen/QWmYMNz

[Simon Fraser (smfr)]

Thank you for the report and the clear reproduction. I can reproduce the bug.

[Simon Fraser (smfr)]

My attempt to build a testcase without using React does not reproduce the bug.

[Simon Fraser (smfr)]

Created attachment 461742 [details]
Non-working reduction

A colleague of mine created a non-react repro case...I'm working on getting it from him. Might have to wait until Monday.

[malcolm]

I was able to reproduce this with vanilla JS
https://replit.com/@kshncodes/fixed-sticky-repaint-bug#index.html

[malcolm]

Created attachment 461745 [details]
Working reduction

I've attached a working reduction of the issue with vanilla js

[Simon Fraser (smfr)]

We're computing a bad repaint rect, because the layoutDelta is moving the rect to a negative Y offset:

(lldb) p view().frameView().layoutContext().layoutDelta()
(WebCore::LayoutSize) $0 = { width = 0px (0), height = -57.6875px (-3692) }

(lldb) p dirtyRect
(WebCore::LayoutRect) $1 = { x = 0px (0), y = -47.6875px (-3052), width = 192px (12288), height = 28px (1792) }

[Radar WebKit Bug Importer]

<rdar://problem/99201177>