bug#20353: M-. sometimes jumps in another window

[Stefan Monnier]

> Maybe not deleting the frames is not the real problem, but them the problem
> is in some other window-related functionality.

I think the issue is much simpler: xref should explicitly remember the
window from which *xref* comes, so that it can explicitly go back to it
(if it still exists) when we select an entry.  


        Stefan