In what manner are you setting the diagram to draw a custom rectangle?

I imagine this could happen if you show an alert from some mouse-related event, as the alert could take the mouse capture away from the diagram so it misses mouse-up event. I'm not sure if this is the case in JavaScript, but I have seen this happen a lot with message boxes in other platforms we support, where the solution is to either cancel the current drawing operation or move the message box to a different event handler.

Stoyan