Hi.

Sometime between emacs 23 and emacs 24 the behavior of display-buffer
changed to show the buffer in the current window at times (emacs 23
always used a different window). As a (presumably unwanted) side-effect
it is now possible for a gud user to lose the gud window as one
interacts with gud. I often encounter this when I open a core dump in
gud-gdb, then invoke "up" repeatedly to traverse the call stack.
Eventually a source buffer takes over the gud window and I have to
switch back to gud to keep navigating the stack.

Trivial recipe:

 1. Create any C program in /tmp/tst.c. Can be as simple as

  int main(void)
  {
      return 0;
  }

 2. gcc -g -o tst tst.c

 3. emacs -Q

 4. C-x 3 (split window horizontally)

 5. C-x C-f tst.c (open tst.c)

 6. M-x gud-gdb, 'gdb --fullname /tmp/tst' (start up gud in the same
    window that's currently showing tst.c)

 7. b main

 8. r

On my machine this switches to tst.c on top of the gud buffer, replacing
gud in its window. With the attached patch this does not happen.

This is similar, but not identical to bug

 http://debbugs.gnu.org/cgi/bugreport.cgi?bug=17675

The proposed fix in that bug does not handle the above recipe.