ANGLE and Dawn lean on backend safeguards to make higher-level guarantees: the front-end OOB index validation only fires when the backend advertises createContextRobustness; a uniform buffer is safe to over-read only if it was padded; a framebuffer read is safe only if robust resource init marked its attachments MayNeedInit. Each safeguard is set in one place and assumed everywhere else.

The bug is that the safeguard is quietly not in force for some case: the Metal backend never advertises the robustness extension (so front-end index validation is inactive on Apple GPUs), the padding is gated on the buffer’s current bind target rather than on any buffer that could later become a uniform buffer, or an invalidate/discard path drops attachment contents but forgets to reset InitState. The rest of the pipeline still assumes the guarantee holds, so an out-of-bounds index, an over-read past a buffer, or a read of never-initialized attachment memory reaches the GPU process — all from ordinary WebGL/WebGPU content.

Make the safeguard unconditional for every case the higher layer covers: advertise the robustness extension so the front-end validation path always runs, pad every buffer that could ever be bound as a uniform buffer rather than only the currently-bound ones, and re-arm the resource-init flag (markAttachmentsUninitialized) on every invalidate/discard/reconfigure path.