Firstly, a change had to be made to the container_at functions. If you
create layout `T[view H[view view]]` and hover the second tab, the
container_at functions would return the focus_inactive child. They now
return the split container itself. To compensate for this,
dispatch_cursor_button has been adjusted to find the focus_inactive
child before focusing it.
The actual implementation of wheel scrolling is pretty straightforward.
This uses handle_cursor_axis, so I took a similar approach to
handle_cursor_button (ie. creating a dispatch_cursor_axis function).
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