My laptop has 1280 x 800 and external monitor 1680 x 1050 resolution.
I had the same problem. Doesn't occur when no external monitor in use.
Workaround: (?)
When turned the external monitor off (+keep settings) and back on (+keep settings) from monitor preferences, the problem disappeared!
Fixed also strange problem with mouse pointer disappearing when moving from top of external monitor to laptop screen (there was like blank not visible area on top of laptop screen).
My laptop has 1280 x 800 and external monitor 1680 x 1050 resolution.
I had the same problem. Doesn't occur when no external monitor in use.
Workaround: (?)
When turned the external monitor off (+keep settings) and back on (+keep settings) from monitor preferences, the problem disappeared!
Fixed also strange problem with mouse pointer disappearing when moving from top of external monitor to laptop screen (there was like blank not visible area on top of laptop screen).