[Webkit-unassigned] [Bug 232673] New: CSS outlines in contenteditable elements causes performance issues

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=232673

            Bug ID: 232673
           Summary: CSS outlines in contenteditable elements causes
                    performance issues
           Product: WebKit
           Version: Safari 15
          Hardware: Mac (Apple Silicon)
                OS: macOS 11
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: HTML Editing
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: briankimpossible at gmail.com
                CC: wenson_hsieh at apple.com

Created attachment 443215

  --> https://bugs.webkit.org/attachment.cgi?id=443215&action=review

Safari Devtools timeline dump

Typing in contenteditable elements is slow when using the default focused outline CSS rules.

Steps to reproduce:
1. Go to https://www.slatejs.org/examples/plaintext
2. Inspect the contenteditable element and disable the `outline: none` style.
3. Try inserting many newlines using the Enter key.
4. When there is a visible outline, you should see noticeable lag and frame drops as the document size increases.
5. This lag does not exist when the `outline: none` style is re-enabled.

I’ve attached a timeline export from Safari Devtools. The important detail is that in the frames view, Devtools reports that time is spent in Other (not Layout, Painting, Scripting). I do not have the ability to investigate further at this time.

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