[btn]

It's the "zoom" button, and it's supposed to toggle between two sizes: user state (whatever you last dragged the window borders to be), and standard state (whatever the application thinks makes sense for the window's content [1].

This works for applications where the content has a natural fixed size (for example, Photoshop snaps the window to the size of the image), but breaks down for flexible content (web browsers, terminal windows, etc.). Also, it's up to application developers to get this right, which isn't always the case.

[1] https://developer.apple.com/library/mac/documentation/UserEx...

[chaz]

Except Apple's own iTunes. I know it's a mess of an app to start with, but for iTunes 10 or earlier, the green button used to throw it into mini player mode. Now, it maximizes/restores the window, ignoring the content dimensions. And not to be confused with full screen.