[Webkit-unassigned] [Bug 237113] REGRESSION (iOS 15.4 beta) - WebGL yields incorrect results when using preserveDrawingBuffer on iOS 15.4 Beta

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=237113

--- Comment #8 from Kenneth Russell <kbr at google.com> ---
This bug reproduces in Chromium, too, when using the ANGLE/Metal backend on M1 hardware. I used the tools to run a bisect there:

./src/tools/bisect-builds.py -a mac-arm -g 874000 -b 974000 --use-local-cache --verify-range -- --no-first-run --use-cmd-decoder=passthrough --use-angle=metal https://codepen.io/cmb8/pen/bGYKPrP

which turned up:

You are probably looking for a change made after 901356 (known good), but no later than 901367 (first known bad).
CHANGELOG URL:
  https://chromium.googlesource.com/chromium/src/+log/047f67dec3ae1322e17a408954b12bf0e000a433..5eca15b4748370af38b90b98314d84523af8c6ba

containing the ANGLE roll in which Apple's direct-to-Metal translator first began upstreaming last July:

https://chromium.googlesource.com/chromium/src/+/3fc5546c60cfc7db7b3cf8c0d0c0ccf16927c850
https://chromium.googlesource.com/angle/angle.git/+log/b7cfbac4f8c5..2a668d9219dd

so unfortunately this bug crept in a while ago and there's no fine-grained way to track it down. It probably occurred in this large patch:

https://chromium-review.googlesource.com/c/angle/angle/+/2950067

and the reason this is showing up in 15.4 is that contains the ANGLE roll into WebKit from last December, Bug 220896.

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