> Try to change $5200

Why? That affects vertical split mode, NOT the extended attribute rendering mode ($5104)

> The screen must be turned on via $2001 when you write to $5c00-$5fff, and don't write to it during VBlank

This seems completely contrary to common sense, which dictates that you would NOT want to write to it when something's trying to read from it. But oddly enough, one of the games I checked DID write to $5C00-$5FFF during rendering. It may be safe to write here while rendering, but I don't see why there would be ANY sorts of problems related to writing to it when it is *NOT* rendering...

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Quietust
P.S. If you don't get this note, let me know and I'll write you another.