revVideoGrabber: stop preview doesn't hide video window

[Ben Rubinstein]

Docs for revStopPreviewingVideo says:
"Stops showing input from a video camera in the video grabber window."
"Use the revStopPreviewingVideo command to stop showing video input."
"When you stop previewing video, the video grabber window shows a blank screen."

When I try this (rev 4.0 or later, on MacOS X 10.6) it doesn't show a blank 
screen, instead I'm left with a still of the last frame.  The only way to make 
it go away is to execute revCloseVideoGrabber.

Is this a known bug?  Am I doing something wrong?  Does anyone else experience 
this, or conversely has anyone experience of it working as documented?

TIA,

Ben