Description of problem:

System locks up, no ping, NMI watchdog produces no output.

Version-Release number of selected component (if applicable):

gtk2-2.8.20-1

How reproducible:

100% reproducible with the pan binary. I use pan-0.14.2.91-4.fc5 which was
not update in this update cycle. However, it takes some patience before
it happens. Follow Steps for easiest way to reproduce.

Steps to Reproduce:
0. Install and configure pan-0.14.2.91-4.fc5 from Extras
1. Go into a group, find an article
2. Observe that tabs behave erratically, they appear and disappear.
   This indicates the affected gtk2 release.
3. Try to follow-up to an article
4. In built-in editor, type anything, select, and do other activities.
   Eventually, the system locks up hard.
   A slower computer is recommended to catch this.

Actual results:

System locks up.

Expected results:

System works.

Additional info:

I am surprised that a bug in gtk2 causes a hard lock-up. I suppose
that it's the shared fault of kernel and X11 that the box locks up
hard. That NMI Watchdog fails to work is a problem too. HOWEVER,
it's very clear that:
 1. Everything works fine with gtk2-2.8.19-2
 2. System breaks with gtk2 2.8.20-1
Therefore, an update to gtk2 can fix the problem.

I do not know if any other applications trigger this problem besides pan.
Not many use similarly looking tabs, so perhaps that's the key.