Significant performance hit when :has selectors enabled in Firefox

[brawaru]

Describe the bug
After Firefox shipped :has selector in the recent version, Phanpy performance has worsened severely, which is visible by page frequently freezing and overall feeling unresponsive especially with many elements on the screen.

It seems that Phanpy either uses :has selectors in a manner that is very expensive or does something else that causes Firefox to invalidate a huge amount of styles. Perhaps it's the same in Chromium browsers too.

There is no Styles events like that when :has support is disabled, most of them are very short and have a completely different stack trace.

• Which site: phanpy.social
• Which instance: mstdn.social

To Reproduce
Steps to reproduce the behavior:

1. Ensure :has selector is enabled in about:config (layout.css.has-selector.enabled must be true)
2. Interact with Phanpy for some time and notice the bad performance
3. Disable :has selector support via about:config by setting layout.css.has-selector.enabled to false.
4. Close all Phanpy tabs and re-open it (if you are unsure how to know that preference took effect, just restart Firefox)
5. Open and interact with Phanpy again. Notice how performant it is.

Expected behavior
Phanpy is performant even when :has support is enabled.

Screenshots
N/A

Desktop (please complete the following information):

• OS: Windows
• Browser: Firefox
• Version: 121.0

Smartphone (please complete the following information):
N/A

Additional context
I'm not sure how to get anything useful from the Firefox profiler.

Chromium blames forced reflows for styles recalculation blaming various scroll accessors.

[cheeaun]

Hmm, this will probably take some time; either I remove all usage of :has() on my side or wait for Firefox to fix their performance issues.

I've been using :has() on Phanpy since beginning of the year, which works on Chrome, so the base performance evaluation follows Chrome's. They're also coded with progressive enhancement in mind, which means they're optional and shouldn't affect base functionality of the app (It used to, but I fixed them again and again)

Nevertheless I'll try to take some time to test this on my side, and hopefully can report some issues to Firefox too.

[cheeaun]

Possibly related: https://bugzilla.mozilla.org/show_bug.cgi?id=1874066