The behavior selector should popup on mouseDown rather than mouseUp. I'd also be keen to be able to drag a line out to the button, stack or project browser similar to Interface Builder but I'm guessing that would be rather interesting to implement in a multi-window environment. It's worth noting that the reason I thought of the latter is because of the mouseDown highlight and no popup made me thing it was how it worked and it wasn't until I finally mouseUp'ed on on the widget that I realised there's a popup.