Thabks for the updated core slingshot, it's really good to see this getting some love.
I don't know if this will help with debugging, but I've been working through the Skid Row demo now that it's fixed on the latest core.
One demo, BMT3, is a full overscan screen scroll with a picture of a wolf, but (on my TV at least) the sync is lost and tears part way down the screen, but the central scroll line is stable.
Also, I feel like many key presses are being lost while walking around the main menu, is anyone else finding similar?
BTW, to find that screen, walk through the menu screen keeping to the left wall. It should be the second door you reach.
Tested as both a ST and STe.
Atari equipment all in storage - Now playing with MiST