Fog of war disengaging hide/reveal at borders

I find that if you turn on fog of war on and you click and drag the box past a border it disengages the fog of war feature whether it be hiding or revealing and you have to re-enable hide or reveal. I have only reproduced it on one map, but it is consistent. I use firefox under Windows 7. Suggestion, if it truly is a bug and not a feature, can you get it to lock the fog of war at the edge without disengaging. It is kind of tough trying to get the fog of war to reveal or hide right at the border of the drawing area.

It is really annoying isn't it? It took me forever to figure out that is what was wrong, as I kept trying the FoW tool and nothing would happen.

Thanks for the suggestion. I'll throw this up on the bug list.