JavaFX lacks the ability to pin windows above/below other native windows (keep-above/keep-below). Exposing this capability would be a very useful addition to the api. It would enable developers to create the following types of applications: OSD notifications (keep-above), desktop widgets (keep-below), media players (keep-above), dock applications (keep-above/keep/below), and quake-style terminals (keep-above).

In terms of implementation, GTK exposes gtk_window_set_keep_above and gtk_window_set_keep_below. Microsoft has SetWindowPos with HWND_BOTTOM and HWND_TOP flags. Mac has setLevel with kCGFloatingWindowLevel / kCGDesktopWindowLevel.

On platforms that don't stack windows (mobile?), no-ops wouldn't hurt anything.