Encapsulating XUL elements in and gave different result

I found an XUL code that enables us to put button and textbox on top of the statusbar. It uses that contains and . The sample codes are encapsulated in much like any other XUL files. Since TT codes are using different approaches, that is we encapsulate elements inside , and use it to lay over the default window, I changed the tag to to see what happen. It does gave a different output as shown in these two screenies:

using <window>- the chatbox is on top of statusbar












using <overlay> - the Chatbox is on the right side