Bug 254575 – [css-lists] `content: counter()` does not update visually if `position: absolute` is set

• Status: RESOLVED
• Resolution: FIXED
• Component: CSS

[Antoine Quint]

This twitter thread reports an issue with animating an <integer> custom property and codepen that reduces it somewhat: https://codepen.io/jh3y/pen/vYyxoPr. We have testing coverage for <integer> animation so I expect it's not strictly an animation issue, it could be something related to `counter-reset` or `content: counter(ms)` which are also used in the demo.

[Antoine Quint]

Inspecting the page, I think there's no trouble actually animating the --milliseconds value since I see it changing every time I call getComputedStyle($0).getPropertyValue("--milliseconds"). Additionally, the first time I open Web Inspector after launching Safari and loading the demo, I see the counter value change to a different value each time. I expect this is indeed an issue with `counter()`.

[Antoine Quint]

Created attachment 465629 [details]
Test

[Antoine Quint]

When we hit RenderTreeUpdater::GeneratedContent::updatePseudoElement() the call to Style::determineChange() returns Style::Change::None and we fail to call updateStyleForContentRenderers() as required. This is not related to animations at all, this is also an issue when changing the style through other means.

[Antoine Quint]

Actually, the issue goes away if we remove the `position: absolute` style in `.counter:after`.

[Antoine Quint]

Created attachment 465630 [details]
Reduced test

[Antoine Quint]

Using the reduction, I can notice a difference under RenderBlockFlow::layoutModernLines() where in the good case (no position style set) the first call to modernLineLayout() returns false and where in the bad case (`position: absolute` is set) it returns true. Cc'd Alan who might know more as a layout expert.

[alan]

Created attachment 465649 [details]
[fast-cq]Patch

[EWS]

Committed 262269@main (1b539330d2ad): <https://commits.webkit.org/262269@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 465649 [details].

[Radar WebKit Bug Importer]

<rdar://problem/107363679>