Diagramscene example line rendering bug

Steps to reproduce:
Compile and run the Diagramscene example program.
Add two shapes and a line between them, like this: A---B
Drag shape A directly over the other and place on the other side, like this: B--A
Notice while dragging that the line is not drawn anymore when the node is moved to the other side.

Notes:
-this glitch does not appear if moving the shape around instead of over the other shape.
-the line reapperars if the shape A is moved back to original position.
-the line reapperars if the shape B is selected

Please fix this bug as it makes this example rather useless as starting point for diagram editor creation.