When you first place an operator, the properties menu appears, and there is a minimize button at the top right of this menu. If you use the minimize button, the only way to unminimize is to click the button at the top right of the screen. I found it more natural to double-click the operator to bring back the menu, as moving my mouse to the top right of my screen is inconvenient and unintuitive (at least for me). It would be good to add that as a feature.