WebGL 1 Driver Renderer	X.Org -- AMD HAWAII (DRM 2.50.0, 4.20.6-arch1-1-ARCH, LLVM 7.0.1)
WebGL 1 Driver Version	4.5 (Compatibility Profile) Mesa 18.3.2

This seems to be sporadic and heisen-buggy, and fortunately conformance submission is not blocked by this.

My feeling is that this is a driver bug that we only hit sometimes, after the driver gets in a bad state. When this happens, it does look like we're inexplicably getting output from a different resource, possibly the window or system framebuffer. However, this happens fairly rarely, and I can't reliably reproduce this issue, nor have I had much luck isolating or debugging it.