X screen blanking strangeness

[Duncan Sands <duncan.sands@math.u-psud.fr>]

I'm running X 4.0.3 on an 8500 powermac (kernel 2.4).  If I leave
the computer alone, after a while the screen goes blank.  Normal!
But not everything blanks: the clock for instance keeps being drawn,
leaving a black screen with a clock on it.  If I wait longer, eventually
everything goes black, even the clock.

Anyone have any idea what's causing this strangeness?

Thanks,

Duncan.