[Webkit-unassigned] [Bug 231108] New: Certain shaders do not render correctly in Safari 15 on Older iOS Devices

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=231108

            Bug ID: 231108
           Summary: Certain shaders do not render correctly in Safari 15
                    on Older iOS Devices
           Product: WebKit
           Version: Safari 15
          Hardware: iPhone / iPad
                OS: Other
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebGL
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: darraghburke at microsoft.com
                CC: dino at apple.com, kbr at google.com, kkinnunen at apple.com

Created attachment 439922

  --> https://bugs.webkit.org/attachment.cgi?id=439922&action=review

broken rendering on iPhone 6

Overview:
The GridMaterial shader from Babylon.js does not render correctly in Safari 15 on older iOS devices. 

Steps to Reproduce:
1) Open the following webpage on an iOS device with Safari 15: https://playground.babylonjs.com/#INI19S#6

Actual Results:
Both the background plane and the ground plane are black.

Expected Results:
The background plane and the ground plane should have the same grid material as the sphere. This is the behavior in earlier versions of Safari and on other platforms (MacOS and Windows).

Hardware:
iPhone 6 running iOS 15

Additional Builds and Platforms: 
According to users, repros on (all iOS 15):
iPhone SE
iPad (6th generation)
iPhone 7
iPad Pro

On some devices it may render as fully white rather than fully black.

Issue doesn't occur on MacOS, doesn't seem to occur on newer iPhones (but have not confirmed this personally).

Additional Information:
The post on our forum with users reporting the issue: https://forum.babylonjs.com/t/gridmaterial-not-shown-in-safari-on-ipad-after-update-to-ios-15-0/24210
The source for the fragment shader can be found here: https://github.com/BabylonJS/Babylon.js/blob/master/materialsLibrary/src/grid/grid.fragment.fx

The issue appears in both WebGL 1 and 2.

I have not yet been able to find the specific cause, but other shaders seem to work okay.

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