Compare WIND-A and WIND-B, which are selected by clicking, linewise.
With prefix argument, DUMB-MODE, or on a non-windowing display, works as
follows:
If WIND-A is nil, use selected window.
If WIND-B is nil, use window next to WIND-A.
(fn DUMB-MODE &optional WIND-A WIND-B STARTUP-HOOKS)