Feedback.

Highlight the edge that would be clicked on if the user clicked, as the mouse hovers. If it wouldn't count as an edge by the click filter, don't.

Sadly, one of the platforms I'm hoping to (eventually) have this collection ported to is stylus-based PDAs, so any control mechanism that depends on mouse hover events for its effectiveness is a bad plan...

Well, the control/feedback mechanism doesn't *have* to be identical on all platforms surely?

You could also subtly shade active areas so they're visually distinct (without interfering with the larger distinctions on the board.)

I'd like the control mechanism to work similarly on all platforms, because that way adding a new platform front end is O(1) effort rather than O(number of games), and adding a new game back end is O(1) rather than O(number of platforms). Since my ultimate aim is to end up with loads of games all running on loads of platforms, this will eventually become a serious concern :-)

Anyway, I think I've improved matters now without having to resort to mouse hovering.