[Webkit-unassigned] [Bug 178348] New: WebGL clear not actually clearing

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=178348

            Bug ID: 178348
           Summary: WebGL clear not actually clearing
           Product: WebKit
           Version: Safari 11
          Hardware: Macintosh
                OS: macOS 10.13
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebGL
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: ashley at scirra.com
                CC: dino at apple.com

We have a WebGL-powered HTML5 game engine. We've started noticing in some cases our calls to gl.clear() are not working. I've tested a MacBook Pro mid-2015 (with Intel Iris Pro graphics). In Chrome and Firefox on the same device, as well as Edge 16 on a Windows 10 PC, it works fine. It's only Safari that seems to have a problem.

Test URL: https://www.scirra.com/labs/bugs/safariclear/
Expected result (using Chrome): https://www.scirra.com/labs/bugs/safariclear/expected.png
Observed result (in Safari 11): https://www.scirra.com/labs/bugs/safariclear/observed.png
Safari TP 41 also reproduces the problem.

Our engine uses gl.clearColor(1, 1, 1, 1) followed by gl.clear(gl.COLOR_BUFFER_BIT) to clear the background opaque white. In all other browsers this makes the background white as expected. In Safari though it appears to have no effect, leaving the background black. The background does not appear to be written to at all. The previous framebuffer contents is left behind, so as the blue box moves, it leaves behind a trail. Strangely any drawing done with an alpha channel does appear to correctly blend with the white background - the demo uses a blue texture with 1px of transparency around the edges, and that renders as white.

I suspect there's a driver bug here, but we can't figure out how to work around it. Preferably Safari would internally work around it, but it would be useful to know what we could do about it too.

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