[Webkit-unassigned] [Bug 228952] New: "Hole punch" rendering glitch in Safari for iOS 15

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=228952

            Bug ID: 228952
           Summary: "Hole punch" rendering glitch in Safari for iOS 15
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: iPhone / iPad
                OS: Other
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Compositing
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: j at josephpearson.org
                CC: simon.fraser at apple.com

Created attachment 435255

  --> https://bugs.webkit.org/attachment.cgi?id=435255&action=review

The hole-punch effect in Safari for iOS 15

When semi-opaque elements are transformed over other content, they can punch a hole in the box behind them, through to the underlying content.

I have a minimal, consistent reproduction here: https://josephpearson.org/experiments/saf-15-holepunch/

See the comments in the HTML source for the CSS properties that seem to be significant to this reduction.

Note that any GPU activity (like the navigation bar shrinking) can "correct" the holepunch. If it doesn't happen on your first try, tap the ghost emoji a few times to retry. Opacity *may* be a red herring, but it has been common to all the instances I've seen. In this case the semi-opacity seems to be the alpha channel in the glyph.

IMPORTANT: I have only observed this in iOS 15 developer betas (from the first one in June through the most recent one). I have not observed it in macOS Safari TP yet.

I have also filed a radar in Apple's Feedback Assistant for this problem: FB9482087

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