If I extend the length of a life or time line on a sequence diagram it drags all the attached arrow heads down with it. Then I have to go back and move all the arrow heads back ‘up’ to their correct location.
Having to extend a sequence diagram is a very common use case. It would better if the arrow attachment points stayed absolutely positioned relative to the tops of whatever they are attached to. That way if I extend it ‘down’ they remain in the same location.