There's a bug somewhere between X, the window manager, and the kernel
(probably X) that causes this to happen when dragging an opaque window
(rather than a window outline). Until it gets fixed, a suitable workaround
is to use twm (barf, but it only does outline moves), or if you use
fvwm or AfterStep, set the OpaqueMove property to 0 (zero) in the
corresponding rc file (.fvwmrc, .fvwm2rc, .steprc, or the appropriate
system-wide rc file).

jonh@cs.dartmouth.edu