Since people here might have relevant experience but might not browse nesemdev, please see viewtopic.php?f=3&t=10029#p111204 .
Basically it's unknown what happens when flags like VBL/sprite 0, etc. are read at different master clock offsets from where the flag changes (blargg's test roms only test CPU behavior in a relative fashion and do not nail down the exact timing), meaning there's still a bit of fudging and guesswork. :/
Might want to reply in that thread if you're interested to avoid the discussion getting spread out. Excuse the spam.
Basically it's unknown what happens when flags like VBL/sprite 0, etc. are read at different master clock offsets from where the flag changes (blargg's test roms only test CPU behavior in a relative fashion and do not nail down the exact timing), meaning there's still a bit of fudging and guesswork. :/
Might want to reply in that thread if you're interested to avoid the discussion getting spread out. Excuse the spam.