Earlier quoted context omitted.
This should be fixed by the browser. If mouse is hold-clicked on scrollbar and the height of the document changes. the position of the mouse should change as well. What's the problem with that?
I don't think the browser is technically able to move the mouse cursor, I imagine that happens at the OS level. Anyone have a counterexample?
IIRC it isn't exposed to JS except maybe for some new fullscreen API (not my area, sorry.)