Basecamp 4.4.2 has a new drop-down; Show Overlays and Hide Overlays. I can't see what difference it makes though. Should we be getting our hopes up concerning overlay maps? :D
Ah, I figured it out. Turns on/off custom maps and Birdseye. I was really hoping it would work with transparent overlay maps like My Trails, too. Oh well, maybe next time.
