Annoying mouse binding problem

[Harald Hanche-Olsen <hanche math ntnu no>]

I have a four button trackball. The four buttons are 1, 2, 3, and 6,
after a trivial bit of xmodmap hackery. I have bound button 6 to move
and resize windows.

Here is a highly edited excerpt from .sawfish/custom:

(custom-set-keymap
 (quote window-keymap)
 (quote
  (keymap (move-window-interactively . "Button6-Click")
	  (resize-window-interactively . "Button6-S-Click"))))

This works just fine most of the time. But once in a while, it stops
working, and clicks on button 6 go right throught to the window below
instead of letting me move or resize windows. Restarting sawfish does
not help, and this sad state of affairs remains until I log out and
back in. [*]

My question is twofold: (1) does anybody have a notion what can be the
reason? and barring that, (2) how can I debug the problem myself?

Let me add that I am a reasonably proficient common lisp and emacs
lisp hacker, but haven't quite wrapped my brain around the repl module
system. And despite using X11 for well over a decade and a half, I am
still not too well versed in the X protocol, to put it mildly.

- Harald

[*] Actually, it's worse, because the X server doesn't survive
resetting, so I end up rebooting the machine. But this is not
sawfish's fault.