Queue `on_scroll` to be run on operation

[Koranir]

Fixes #2437

Due to some inherent limitations, the operation can't access the on_scroll function, and so it sets a flag to be consumed on a later event. A better solution might be to give operations access to a Shell, so a message could be triggered manually, but that might not be necessary in this case.

[lufte]

@Koranir I'm gonna thank you on my behalf because this is good enough for me. Really clever.

I've applied to my fork :)