This is a mock-up of a simple cross-fade interface proposal for Audacity.

Take a track with two clips:

When you slide the clips toward each other, they initially "stick" together and stop moving:

But as you keep sliding the clips together, they "unstick" and resume moving. When the clips overlap, Audacity does an automatic cross-fades. The diagonal line shows the region where the first track is fading out and the second track is fading in: