bug#12251: 24.2.50; crash in note_mouse_highlight

[martin rudalics]

>> The error was obviously thrown from Lisp which per se is harmless.  But
>> I'm quite convinced that it was thrown by a Lisp function directly or
>> indirectly called from C.
>
> Why do you think that?

An error with code attempting to operate on a dead window within
"normal" Lisp is a strange case.  It can be due to users leaving a
dangling reference to such a window somewhere in their code but I doubt
that many people do that.  And when such an error happens, there is no
reason why it should expand into a crash.  The (implicit) invariants of
a window tree should not be violated by Elisp code.

OTOH, calling an Elisp function from C in a state where the window tree
invariants are temporarily violated can easily lead to a crash.

>> How could we explain a crash if it's all within Lisp code?
>
> It's not within Lisp anymore; the error throws to top-level, and Emacs
> tries to read input, at which time it gets a mouse event.

That would be a _very_ strange coinicidence.

martin