The current behaviour of `focus{up,down,left,right}` is to show the
indicator if and only if the screen onto which the focus has just been
moved (or stayed) has more than 1 frame on it.
This however is rather unintuitive, especially in multi-screen setups,
where the focus can move from one screen
When compared with the existing overlap check in
`find_frame_{up,down}`, it appears that the original implementation of
the overlap test for `find_frame_{left,right}` is erroneous. Indeed,
the wrong boundaries are used, which causes issues like allowing
finding a frame in one direction, but not fin
