What is a free way to make a window "reserve" desktop space so other windows cannot maximize themselves into that area on the screen? Just like the taskbar and some sidebars. I do not really mean a "sidebar", I mean like an arbitrary window to reserve the space. (There is "alway-on-top", what is similar but not what I want, due other windows clipping onto the "below" area with that.)
I would like this to work on Windows XP (which didn't have a sidebar built-in, but I recall google desktop work on it as one) and preferably on higher Windows versions too.
Alternatively, how do the sidebars achieve this (via what setting / API functionality)?