platforms allow you to use the Tab key to move from one text field to
another within a window. Some (e.g. Gtk, Windows) also allow you to tab
into non-text controls such as buttons and check boxes. Some (e.g.
MacOSX) do not do this by default, but provide a user preference
setting for enabling it.
By default, PyGUI follows the platform conventions concerning the behaviour of the Tab key, but allows the user to change it.
On Gtk and Windows, setting the PYGUI_KEYBOARD_NAVIGATION
environment variable to a true value ("True" or "1") enables tabbing
into all controls, and setting it to a false value ("False" or "0")
On MacOSX, this environment variable is not used,
and the System Preferences settings under Keyboard Shortcuts for tab
key behaviour are always followed.