The following Shadertoy demo: https://www.shadertoy.com/view/Xt3cWS renders garbage in the top and bottom strips of the target surface, which appears to be uninitialized VRAM being copied to the display surface. It does not appear to matter which WebGL capable browser I use to run the demo, they all replicate the issue on my RX480. I cannot get apitrace to run on Firefox without that particular page crashing on load, and I cannot get RenderDoc to attach to the tab's process and actually capture anything.
Adding else { fragColor = vec4(0); } at the bottom of mainImage appears to "fix" it. fragColor is otherwise ending up with random values (effectively whatever happened to be in the registers, probably left over from the division check). I don't think this is a bug in mesa.
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