Position and Size value changes not being retained

Hi just bought the license yesterday so I might be missing something obvious.
I'm noticing that the Position and Size values are not retaining the changed values when I press Enter. In this case, I was trying to modify the Browser Window object's properties in order to position it at 0,0 with a 1024,768 size. Every time I edit size and press enter, it reverts back to the default. Is 0,0 not a valid location? The online help seems to suggest that it is (http://www.balsamiq.com/products/mock...).

The field appears to accept values sometimes and reject it other times. Is there a particular way to edit the Position and Size properties?