Strong performance issues with Safari on pull-requests

[karlcow]

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Hi,

Safari is experiencing for the last couple of weeks, strong performance issues on pull requests to the point where code review is becoming very hard to complete.

The @WebKit project is exploring solutions to resolve it, but it would be great to find ways to mitigate the issue on the GitHub project side too.

This is happening mostly on very larger PR.
For example going to WebKit/WebKit#48412
And trying to assign a label will result in a very long delay.

Screen.Recording.2025-08-25.at.09.07.04.mov

The WebKit team has also identified issues with comparing commits, commenting, etc.

It seems related to Style::ElementRuleCollector::collectMatchingRules in the WebKit project.

https://github.com/WebKit/WebKit/blob/be6b53f67441fb461d9a861593bab437b07b8a51/Source/WebCore/style/ElementRuleCollector.h#L95-L96

But a change in the GitHub UI for the CSS rules seems to have created the issue by revealing this issue.
Firefox and Chrome are not affected.

[ghugues]

Thanks for the investigation ! Can you link your report here https://github.com/orgs/community/discussions/170758 ?

[karlcow]

This is currently tracked at Apple by rdar://146582783

Some improvements being implemented

• Create separate pseudo-class invalidation RuleSets for attribute selectors WebKit/WebKit#49843
• Limit the scope of invalidation traversal for :has(> .changed) .subject WebKit/WebKit#49877

[willsmythe]

From https://github.com/orgs/community/discussions/170758#discussioncomment-14248533:

Thanks for looking into this, @karlcow (and for the fixes you made).

The team identified a change that shipped last week (to a component that exists on most GitHub.com pages but is not visible by default) involving :has() selector which appears to be the cause of the regression. The quick fix was to switch from CSS-based conditional styling to JavaScript.

Can you verify this fix has resolved the problem?

[karlcow]

See my other message on the thread. Thanks a lot again. Really appreciated.