%bx is used, particularly after %tt - Textual Toolbar, %ib - Image Button or %tb - Bitmap Button, in order to provide a grey
toolbar at the top of the window. depth is measured as a fraction of the
character height and this amount is added to the height, one half above and one
half below the buttons.
%bx can only be used once in any window. Its use is not restricted to %tt - Textual Toolbar and
%tb - Bitmap Button. %bx creates a grey background from the point where it is used to the top
of the window. It automatically removes the border (as in %ww - Window Style[no_border]) and
the grey background spans the width of the window.