Programs not accepting input?

[Greg 'groggy' Lehey]

In the last month or two I've seen increasing occurrences of programs
refusing keyboard input after they've been running for a while
(between hours and days).  They still respond to the mouse.  At first
I thought it was hardware, but it happens on a number of different
machines, and only with certain programs, all of them X clients.
Here's an overview (system names are simply to show that they're
different machines).

- System echunga, running a version of the klondike game that used to
  come with the X distribution.
- System wantadilla, running firefox.  After it refuses keyboard
  input, I can open a new window with the mouse, and the new window
  accepts input.  The old one still doesn't.
- The same problem again with system teevee.

wantadilla had what I think were hardware problems a couple of weeks
ago, so I changed the entire system board and memory (but kept the
disks).  The new system is stable, but the problems continue.  echunga
and teevee have been up for months:

echunga       up  49+02:25,     1 user,   load 0.60, 0.50, 0.35
teevee        up 100+22:08,     0 users,  load 0.06, 0.31, 0.28
tvremote      up  36+21:07,     0 users,  load 0.18, 0.04, 0.01
wantadilla    up  18+02:44,     3 users,  load 0.46, 0.38, 0.49

One thing that the machines have in common is that they all run x2x
(which joins X servers).  echunga and wantadilla run one instance, and
teevee runs another instance with tvermote, which I don't think has
shown any problems.  I've been running x2x for years as well, and only
one of the machines has had a software upgrade anywhere near the time
when the problem began.  Here are the versions:

FreeBSD echunga.lemis.com 6.1-PRERELEASE FreeBSD 6.1-PRERELEASE #0: Sun Feb  5 14:15:02 CST 2006     grog at echunga.lemis.com:/usr/obj/src/FreeBSD/6-STABLE/src/sys/ECHUNGA  i386
FreeBSD teevee.lemis.com 5.3-RELEASE-p5 FreeBSD 5.3-RELEASE-p5 #4: Sun Mar 13 14:58:24 CST 2005     grog at teevee.lemis.com:/usr/obj/src/FreeBSD/TEEVEE/src/sys/TEEVEE  i386
FreeBSD tvremote.lemis.com 5.3-RELEASE FreeBSD 5.3-RELEASE #0: Fri Nov  5 04:19:18 UTC 2004     root at harlow.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC  i386
FreeBSD wantadilla.lemis.com 6.0-CURRENT FreeBSD 6.0-CURRENT #1: Tue Jul 12 11:57:56 CST 2005     grog at wantadilla.lemis.com:/usr/obj/src/FreeBSD/6-CURRENT/src/sys/WANTADILLA  i386

Does this ring a bell with anybody?  Any idea how to start debugging?
The fact that new firefox windows accept input suggests that it's
somewhere in X.

Greg
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