Using Alt+DblClick is tempting but not a great solution (in Windows at least) because holding down the Alt key, all by itself, causes Windows to do stuff. In particular, it open the first menu item (usually File). In DbVis this happens during a Alt+DblClick, but apparently only when collapsing an object (expanding an object is fine) - you have to hit Alt again, or escape, to get the focus back where it belongs. Maybe you could add code to check if the focus is in the menu when you're finished processing an Alt+Dblclick, and if it is, send an Escape or an Alt-keypress to cancel it.
On two different vanilla installations of Windows 10, no menus are opened or focus changes when holding Alt.
What Windows version are you using?