I'm using xScope's Overlay tool, but whenever my browser's scroll bars are displayed the overlay's measurements are off.
 

This is a bug we're aware of and might be able to fix in a future release. The difficulty is in being able to detect when exactly the browser's toolbars are displayed, which is what creates the insets. For the time being, you can work around this problem by going into your computer's System Preferences > General Settings. From there, select "Show scroll bars: When scrolling". This should prevent the scroll bars from displaying in most situations that would disrupt your work in the Overlay tool.