Once in a while, the yellow minimize button on some windows is greyed out, and the Window → Minimize menu entry is likewise disabled. Right now, for example, this is the case with two open Terminal windows. If I open a new Terminal window, it can be minimized, but the old ones remain unminimizable (if that's a word). I tend to think of this as a bug. Or is it a feature? If so, what causes it, and what can I do to enable minimization of these windows?