Accoding to cljg FAQ revision 1.17, Java on Mac OS X
can make a Window with a background color which has
alpha element as well as normal RGB elements.

If we couldn't tweak on other platform to get
transparent or translucent Window, then, what is
the peculiar nature of the alpha element that
other color elements(R, G, B values) do not
have?

Is it just that underlying graphics primitives, Windows
or X, does not have a relevant APIs for top level window
configuration?