SQL scripts open in new instance of SQL Navigator instead of within the same session. (4229562)

In SQL Navigator, there is the option to associate .SQL files to open in Navigator, with the option to open new scripts in an existing instance of Nav, or to open a new instance of Nav. But in Windows 7, double clicking a SQL file in Windows Explorer always opens a new instance of SQL Navigator, regardless of the setting.

How can SQL scripts be forced to open in the most recently opened instance of SQL Navigator when working in Windows 7? This was not an issue when working in XP.