And add to that the screen's going black when the cursor is dragged through the dock ... and comes back to life when dragged back again.
Mystery upon mystery.
Maybe the trigger isn't so much dragging the cursor through the dock as dragging it into a corner. Each of the four corners of the screen can be configured as a "hot corner" to trigger actions like "Put Display to Sleep" (makes screen go black), "Start Screen Saver" (possibly likewise, depending on your choice of screen saver), or "Desktop" (move windows off-screen and put a gray border around the now-exposed desktop).
