Gutters break mouse based scrolling.
I'm using 21.2.31 to do some real work today and this has been
annoying me a bit.
Take a large buffer
Goto (point-max)
Start selecting a region with the mouse (i.e. drag button-1).
When you leave the window to hit the gutter, nothing happens.
Continue until mouse is over the toolbar. Then scrolling starts.
This is probably another remnant of the frame gutter is part of the
window problem.
Jan