title: How to check if a NSWindow is visible
body: <p>Is there a way to check if a <code>NSWindow</code> is visible or not? I want to display a sheet controller once the first window of my app became visible (the animation on 10.7 ended and the user can see the window!). If I just show the sheet in <code>windowDidLoad</code>, it results in a stupid looking animation (sheet rolling out, window popping out from the back). I know that <code>NSWindowDelegate</code> provides two methods which are invoked when a window either became the key window or the main window, however, this doesn't have to mean that the window is already fully visible at the time. This is even more noticeable on Lion where windows tend to pop up with this stupid animation.</p>