a node with double click event.
Who is capturing the mouse? If it is your code, you need to release it.
If it is a diagram tool, you can call myDiagram.CurrentTool.DoCancel().
Actually what you are saying doesn’t make sense to me. Could you be more precise about describing the situation? The ClickSelectingTool doesn’t capture the mouse. So a LeftMouseButtonDown event handler that checks DiagramPanel.IsDoubleClick(MouseButtonEventArgs) shouldn’t get involved with a mouse capture. Only if there’s a drag or a resizing/reshaping/relinking will those tools capture the mouse.