bug#56561: 29.0.50; Infloop in try_window

[Eli Zaretskii <eliz@gnu.org>]

> From: Michael Welsh Duggan <md5i@md5i.com>
> Date: Thu, 14 Jul 2022 14:57:40 -0400
> 
> I've connected to this in gdb.  `try_window' is calling `display_line'
> in a loop until the iterator hits a certain point.  But `display_line',
> in this case, isn't modifying the iterator, so the loop never ends.
> Here follows part of the debugging session demonstrating this.  I'll
> keep this gdb session alive for a while.

I need to see what's in the buffer and some other variables.  These
are for the call-stack frame inside display_line:

  (gdb) p current_buffer->text->beg
  (gdb) p it->current
  (gdb) p it->w->desired_matrix->nrows
  (gdb) p MATRIX_ROW_VPOS(row, it->w->desired_matrix)
  (gdb) thread apply all bt
  (gdb) xbacktrace

And finally, any idea what you were doing when this happened?

Thanks.