callback
The command procedure to be executed when a sub-window is interactively selected via one of its tab-buttons. This is invoked only on an interactive button click, not when a sub-window is programmatically selected by setting its visibility or by the window's selected element.
The window identifier of the tab window is passed to the procedure as parameter 1. The sub-window (also a window identifier) is passed as parameter 2.
New tab windows have a .select procedure of null.
<window>.select(<window>,<sub-window>)