bug#19170: 25.0.50; enhancement request: `compare-windows' use across frames

[Drew Adams]

> > (None of the current behavior should be lost, in particular,
> > behavior per `compare-windows-sync'.)
> 
> Thanks for the bug report.  The patch below fixes it in
> `compare-windows-sync' to use the same frame-selecting logic
> like existed for a long time in `compare-windows':

Thanks, that does indeed seem to take care of the sync behavior,
for `compare-windows-sync-default-function', at least.

The main reason for the bug report, however, is the inability
to choose the two windows, especially when they are in different
frames.  Cycling via `other-window' is limited when you have
multiple frames (each possibly with multiple windows).  It is
even somewhat limited in the typical case of two windows in the
same frame.