bug#8789: 23.3; debug backtrace buffer changes window on step-through

[Drew Adams]

> Thanks for your fixes.  I'll check the next Windows binary. - Drew

I have not yet been able to do that (no Windows binary), but here is some more
info.  (I have not customized `debugger-bury-or-kill' - the value is still
`bury'.)

Sometimes I can grab the border of the *Backtrace* frame, as usual, and resize
it (even though it goes back to the default size when I hit `d', as described
earlier).

But sometimes I cannot: as soon as I touch the mouse to the frame edge and try
to drag it, the frame disappears!  I can just touch it (e.g. click it) without
it disappearing, but as soon as I try to drag the touched edge, the frame
disappears.  It does not matter which edge (e.g. bottom or right) I try to drag.

No idea what is going on here - I have never seen anything like this.

This happpens systematically when I enter the debugger in a certain context, and
it never seems to happen otherwise.  But that context is far too complex to try
to communicate.  Suffice it to say that this happens.

When it happens I see nothing additional in *Messages*, and there is no crash.
The *Backtrace* frame just disappears, and the mode line indicates that I am no
longer in a recursive edit - IOW, the debugger is exited.  And if I explicitly
visit buffer *Backtrace*, I see that it is indeed empty.

It is as if trying to drag the frame edge is (sometimes) the equivalent of
hitting `q' in the debugger.

Very weird.  I'm sorry that I cannot offer more info about this.  But clearly
something is very wrong.