FVWM: Is this fvwm or X or what?

I have a Kensington trackball, which means I can click a mouse
button without touching the ball, and there is absolutely
no mouse movement when I click.

When I do such a mouse click on the window decoration "close"
button, I see the button animation as it appears to be
depressed and spring back up which makes me believe
it did indeed see both the press and release events.

But fairly often, the window will just sit there, not
closing till I finally touch the trackball and cause
some mouse movement event, then the window finally pops
out of existence.

Clearly this is the worlds most unimportant problem, but
I just got curious about it, so I thought I'd ask if
anyone knows something that would cause this.