Instead it moves my arc. Or maybe it's changing the radius AND moving the endpoints?

Is there a trick to dragging the arc radius without moving the endpoints of the arc and not giving it a radius dimension or fixing the endpoints?

It works as expected when first creating the 3-point arc. But when I try to edit it, dragging moves it instead.

It would be nice if the endpoints didn't move unless I pick them and drag. Something doesn't seem right.