[Webkit-unassigned] [Bug 260925] New: PERFORMANCE: CSS Custom Properties used within Custom Element Style triggers significant CPU usage and a rise in process size

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=260925

            Bug ID: 260925
           Summary: PERFORMANCE: CSS Custom Properties used within Custom
                    Element Style triggers significant CPU usage and a
                    rise in process size
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Mac (Apple Silicon)
                OS: macOS 13
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: jblas at adobe.com

Created attachment 467500

  --> https://bugs.webkit.org/attachment.cgi?id=467500&action=review

HTML file that illustrates the performance degradation and rise in memory. Press the Test button and watch the process size grow.

In our web application we've noticed a significant rise in process memory after showing and hiding DOM UI subtrees made up of a large number of custom elements that utilize style tags that refer to CSS custom properties.

In Safari 16.5 as well as WebKit 267464 at main, the attached test case can start out consuming about 44 MB but after a few iterations, the process size can jump up to 1.6 GB or more. A GC or something will eventually kick in and the process size will drop back down to the 100 MB range, but then the process size will slowly climb back up again to the 1 GB range. As memory grows, performance also degrades significantly to the point where rendering stops happening, and you'll see occasional flashes of the content being shown.

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