[Webkit-unassigned] [Bug 236713] New: Inconsistent render to screen behavior

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=236713

            Bug ID: 236713
           Summary: Inconsistent render to screen behavior
           Product: WebKit
           Version: Safari 15
          Hardware: iPhone / iPad
                OS: iOS 15
            Status: NEW
          Severity: Major
          Priority: P2
         Component: WebGL
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: fgn2.3ds at gmail.com
                CC: dino at apple.com, kbr at google.com, kkinnunen at apple.com

We have noticed some odd behavior on iOS 15.4 beta 2 on the selection shader of our app. It's supposed to draw blue lines around the selected model but doesn't behave that way in this new version.
The rendering steps are:

1. Render things to the screen ( null framebuffer )
2. Render data that will be used in step 3 to a specific render target
3. Render the blue lines to the screen ( null framebuffer ) by reading the render target filled in step 2. This step discards the pixels that should not be drawn in blue.

For some reason, on iOS 15.4 beta 2 the screen seems to be cleared ( or at least not persisted ) between step 1 and step 3. So the pixels that should be blue are still blue but everything else is garbage. ( I've seen it be the clear color or full black )

I don't have a minimal repro case but our app HomeByMe exhibits it. If you follow this link with an Ipad ( probably won't work on Iphone ), select the asset in the middle and move the camera around you will see the problem. This same version of the app works perfectly on Safari 15.3, Chrome and Firefox.

https://platform.by.me/?projectId=9358156&VP={%22Pos%22:[5888.697504997253,-3914.4912362098694,8771.067690849304],%22Sight%22:[-0.5888697504997253,0.39144912362098694,-0.7071067690849304],%22Up%22:[0,0,1],%22FOV%22:0.7853981633974483,%22Mode%22:1,%22Zoom%22:1,%22AroundCenter%22:[0,0,1700],%22AroundDistance%22:9999}&levelIndex=0

Let me know if you need more information

François

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