Another interface quirk


Licensed Computer Geek
It seems that Apple wasn't completely consistent when programming window control button behavior. If a window is in the background, clicking and holding over its close or hide button doesn't bring the window to the foreground. If you try it with the zoom button, the window immediately leaps into the foreground.
Hold down the Command key to zoom in the background. This is consistent with the background scrolling and resizing behaviors.

In the sense that zooming means "Show me all you can of this document" zooming in the background by default makes little sense.