[Webkit-unassigned] [Bug 237230] New: REGRESSION (iOS 15.4 beta) - WebGL race condition between clearing depth buffer and switching framebuffers

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=237230

            Bug ID: 237230
           Summary: REGRESSION (iOS 15.4 beta) - WebGL race condition
                    between clearing depth buffer and switching
                    framebuffers
           Product: WebKit
           Version: Safari 15
          Hardware: iPhone / iPad
                OS: iOS 15
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebGL
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: christoph at 8thwall.com
                CC: dino at apple.com, kbr at google.com, kkinnunen at apple.com

This is a regression between iOS 15.3.1 and 15.4.

As can be see in this demo: https://codepen.io/cmb8/pen/GROXmKp

We should see two gears spinning normally, but instead we get visual artifacts that obscure the objects from getting properly rendered.

This behavior is caused by calling:
context.clear(context.DEPTH_BUFFER_BIT)

then immediately calling:
context.bindFramebuffer(...)   

If we add a finish() call between the two calls, the issue does not occur.

This problem affects many 8th Wall projects that use three.js, like

our sample project here: https://8thwall.8thwall.app/unitcube-threejs/
or more recently, this Super Bowl promotional tie-in from Kia: https://www.kiadogmentedreality.com

Disabling "WebGL via Metal" in "Experimental WebKit Features" and relaunching Safari fixes the behavior.

Video of expected behavior: https://www.youtube.com/watch?v=SAp_RXDD7_0

Video of unexpected behavior: https://www.youtube.com/watch?v=d8cr-LP40hs

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