Everything old is new again. Back in the Windows 95 days, there were a handful of freeware games that played with no window and entirely by displaying a changing icon in the system tray. I remember having executables for clones of Pong, Space Invaders, Tetris, and chess that played this way with the icon as their only 'screen'.
I guess because it's nothing more than a transparent window that is reading out information about other windows and changing its own position. So pretty basic WinAPI calls.
