I think in this case the automatic constraint engine is a bit too eager to add a horizontal/vertical constraint. It makes sense that a horizontal/vertical constraint would be added automatically when the line is created (if it's close enough to horizontal or vertical), but once it's created it should assume that you don't want the constraint and never try to add it again. Dragging an endpoint should only create another automatic constraint if you drag and snap to an already existing point, which would of course be a coincident constraint and not horizontal/vertical.
Also, I think when adding automatic constraints it should put the indicator symbol (the one that shows that a constraint will be added if you release the mouse button) right next to the mouse cursor instead of farther away like it does now. Currently if you are zoomed in close to a point that you are snapping to something, the auto constraint indicator can actually be off screen so that you can't see it, and therefore could add a constraint without you knowing about it. This violates the Principle of Least Surprise.
C|
