Display problem with margin (was Re: gdb-ui / fring coredump question)

[Nick Roberts]

 > Another problem I saw in passing with the both-fringe-and-margins mode
 > is that for some reason once the margin had been displayed, it would
 > sometimes start blinking on and off (I mean, it would display first
 > without the margin, then redisplay with the margin, etc).  I don't
 > have an easy test case for this though.

Here's a similar scenario:

Start emacs in an xterm (emacs -nw).
Type M-x gdb and give it a C program to debug.
Type 'break main' and then 'run' in the GUD buffer.
The window should split in two and the source display in the lower window.
The code shunts back and forth a few times as the margin disappears and
appears again, and then stops with no margin.
Clicking on the source buffer makes the margin re-appear.
Clicking on the GUD buffer makes it disappear.

Nick