Just to add a data point:
I see the same behavior but don’t remember if it was ever different. I can scroll without sidebar but can’t scroll when sidebar is enabled.
Assumed it was normal behavior.
No sign of black line for me- or at least, I can’t figure out how to get it to show up
Intel 27″iMac Sequoia 15.3.1
Nitro 2025.02 (77)
Russ