When we handle a mousewheel event targeting a line or line number, we
assign the mousewheelScreenRow to prevent the removal of the target
node, which interferes with velocity scrolling.
However, the ::mousewheelScreenRow is only cleared 100ms after we stop
scrolling vertically. This means that if we're only scrolling
horizontally, it's never cleared. This causes the line node associated
with this screen row to hang around longer until the mousewheel screen
row is cleared again, which is not what we want.
This commit only assigns the ::mousewheelScreenRow when scrolling
vertically, so we can be sure it will be cleared.
89c57b6d52