Okay, found it. Compiler bug, I think.

(screen blanker failing at 100% sensitivity due to a phantom screen byte "differing").

I was declaring a static 2048 byte screen buffer on the same line as a 1-byte variable, and the compiler just aligned them next to each other, giving a horribly misaligned 2048 byte buffer. Funny thing is, this results in memcpy not working correctly on the buffer, and neither does a hand coded for-loop doing the same copy. ARrrrggg.

Anyway, I'll hand tweak it so it works again.

UPDATE:

The scary part of this, was that the "static" local variable moved around in memory on successive calls.. static vars ain't supposed to do that! So I moved it to outside of the function, and now it stays put, and is correctly aligned as a bonus.

v47 will follow