[klodolph]

This definitely does not appear to be an “unsupported use-case”. This technique is used in many first-party launch titles. As far as I can tell, this is the reason that sprite zero tests exist in the first place.

If you want an example of something that’s really an unsupported use case, consider mid-frame palette changes—in order to do this, you actually have to disable and re-enable rasterization during the horizontal blanking interval. It works, but it is difficult to get right and the PPU is clearly not designed with this in mind.

Short of having an actual spec in hand, it would appear that mid-frame scroll register updates were entirely normal and an intended way to use the device, based on the evidence (no other reason for sprite zero check, lots of first-party titles using this feature, other mid-frame updates are much more difficult).

[makomk]

The sprite zero check is useful for changing the horizontal scroll position mid-frame, which is something the hardware seems to be designed to support - it helpfully copies the value written to the scroll register to the internal horizontal scroll position at the end of each scanline. It does not seem to be designed to make the same thing possible for vertical scrolling. The programmed register value for vertical scroll is read once at the start of each frame.