mildy related but wasn't there an emulator (maybe not GB but NES or SNES?) which had a visual panel showing each CPU cycle step by step? afaik it was very slow but the 1000% accuracy was the goal not playability.
I'm not sure if this is the one you are talking about but I remember seeing this a little while ago. https://mtmc.cs.montana.edu/
My guess is you're thinking of no$gmb, a very early Game Boy emulator:
https://gbatemp.net/threads/no-gmb-2-5-dos-full-version.6039...
I've got fond memories of using this to get a preview of Pokemon Gold before it was released in NA!