Try to reproduce this by making the external-monitor have a much higher resolution than your primary-monitor, and in the display-gui, try moving one monitor's "altitude" (vertical positioning) a little higher than the other and then try it "flush with tops" , and then try it "flush with bottoms". One of these positioning combinations will likely reproduce this bug in your multi-monitor set-up as well.
I think Greg is right.
Try to reproduce this by making the external-monitor have a much higher resolution than your primary-monitor, and in the display-gui, try moving one monitor's "altitude" (vertical positioning) a little higher than the other and then try it "flush with tops" , and then try it "flush with bottoms". One of these positioning combinations will likely reproduce this bug in your multi-monitor set-up as well.