This is really minor, but maybe it's also really easy to do. Often, when I'm working quickly, I end up pressing the UV Editor button instead of the Material Editor button: Then I have to go find the little "x" and close it. Maybe clicking on the button could toggle the window, instead of simply opening it? So if the window is already open, it could close? Maybe others would find that annoying, if they tend to dock their windows, so maybe closing the window could be a modifier-based behavior, such as holding alt- when clicking would close the window?