Investigate rules for CM scrolling after edits

[njx]

#7458 seemed to be caused by some change in the cmv4 behavior on scrolling after an edit. We worked around that issue by explicitly scrolling, but it would be good to figure out what changed in CMv4 and determine whether it was intentional or not, so that we know what the correct behavior should be as we implement future edits.

[njx]

Low pri to me

[peterflynn]

@njx Think I just found another case of this: if you press Tab while the cursor is scrolled out of view, it only scrolls back into view.if the cursor was at the start of the line. Tabs in the middle or at the end of the line leave the diff ouf of view (though undoing that diff does scroll it back into view).

This was also not the behavior before CMv4 landed.