bug#32825: 27.0.50; Deterministic window management

[martin rudalics]

> Ehm - Noam, since you seem the most knowing of us all, could you maybe
> please care about this report?

So we got a hot potato here.  Let me try to recapitulate what should
be done:

(1) Try to never display the *Backtrace* buffer in the selected (aka
same) window.  So far, we all agreed on that and nobody asked for an
exception from this rule.

(2) Try to keep 'debugger-previous-window' around even after the
debugger session has been closed.  Doing that naively will get us back
Bug#17882.  So we probably don't want to use
'debugger-previous-window' in a new session when it is on another
frame (or display).  But we could use it even in a new session when it
is on the selected frame and not the selected window.

This should take care of the default value of 'debugger-bury-or-kill'
- the immediate problem that caused this bug report.  Does anyone see
a problem with this?

I attach a preliminary patch.  Please try it and tell me whether you
see any difficulties with it.

Thanks, martin

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