[Webkit-unassigned] [Bug 219023] New: Blank glReadPixels since iOS 14.2

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=219023

            Bug ID: 219023
           Summary: Blank glReadPixels since iOS 14.2
           Product: WebKit
           Version: Safari 14
          Hardware: iPhone / iPad
                OS: Other
            Status: NEW
          Severity: Major
          Priority: P2
         Component: WebGL
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: paul at sketchfab.com
                CC: dino at apple.com

Reproduction steps:
- iOS 14.2
- go to https://labs.sketchfab.com/experiments/screenshots/#model/818a31e7b4194dc688dc19be9dcafdec
- click on "export screenshot" blue button on the right panel
- get a PNG file downloaded, but blank


Sketchfab is a webGL based app, here using glReadpixels webgl command to read pixel back from GPU framebuffer.

Since ios 14.2, on safari, glReadPixels return blank image where it was working before.

- iPhone 8+ iOS 14.2 - does not work
- iPhone 11, iOS 14.2 - does not work
VS
- iphone 7+, 13.6.1 works

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