Why does "window = window.parent;" create an infinite loop?

You should also check if window.parent == window​ is false. Otherwise you will end up with an infinite loop. If there is no parent, the parent property will reference to itself (infinite loop).

var win = window;
while (win.parent && win.parent != win) {
    //perform actions on win
    win = win.parent;
}​

http://jsfiddle.net/EZfHf/

I found this on MDN:

If a window does not have a parent, its parent property is a reference to itself.

top's parent is itself.

top == top.parent //true