[Webkit-unassigned] [Bug 226330] New: Shadow DOM unnecessarily invalidated on pseudo element change, causing CPU spikes

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=226330

            Bug ID: 226330
           Summary: Shadow DOM unnecessarily invalidated on pseudo element
                    change, causing CPU spikes
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: ldebeasi at gmail.com

Created attachment 429874

  --> https://bugs.webkit.org/attachment.cgi?id=429874&action=review

Code reproduction

This is a continuation of https://bugs.webkit.org/show_bug.cgi?id=222187 which was partially resolved in STP 125. When using Web Components inside of the Shadow DOM that have CSS Variables set on the host, there is a significant delay in response to clicks/touches. There were fixes in STP 125 that improved this issue, but I am still able to reproduce this issue in other ways.

Steps to reproduce:

1. Open code reproduction in STP 125.
2. Quickly click multiple times on the "Click Me" label (not the checkbox) and you should see Safari freeze up.
3. If you repeat step 2 while taking a Timeline sample, you should see the CPU usage spike significantly. In my tests I clicked about 4-5 times and saw the CPU spike to over 80% on average. 

Expected Behavior:

I would expect Safari to not freeze up when quickly clicking the label multiple times.

Actual Behavior:

Safari freezes up when quickly clicking the label multiple times.

Other Info:

- As noted in https://bugs.webkit.org/show_bug.cgi?id=222187, the example I used is inherently heavy in shadow trees to emphasize the issue. Please see https://github.com/ionic-team/ionic-framework/issues/22951 for more "real world" examples of this impacting applications.

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