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GL_PATCH_VERTICES decides how many vertices one tessellation patch consumes, and glPatchParameteri was a stub - so the value stayed at the driver's default of 3 no matter what the application asked for. KHR-GL40.texture_gather.gather-tesselation-shader sets it to 1 and then draws a single patch: with the request dropped the draw had too few vertices for one patch, produced nothing at all, and the case read back the clear colour. The value is context state on both sides and ES 3.2 spells the entry point exactly the same way, so it is stored in the render state (where glGetIntegerv(GL_PATCH_VERTICES) now finds it) and forwarded. Validation needs the real bound, so GL_MAX_PATCH_VERTICES and GL_MAX_TESS_GEN_LEVEL are probed off the host driver alongside the other limits and answered from there too; the defaults are the GL 4.0 core minimums. KHR-GL40.texture_gather is now 75/75.