A FocusPolicy?, available in most X/Unix environments, which acts like FocusFollowsMouse except that if the mouse pointer goes over the background (the RootWindow?, in X parlance) the application/window which most recently had KeyboardFocus? loses focus. Compare with strict FocusFollowsMouse, which then assigns focus to the RootWindow?. In most configurations, the RootWindow? only responds to a fixed set of key combinations which applications generally don't handle (instead they are passed up the window stack); thus there is usually no reason to specifically assign focus to the root window.
SloppyFocus generally only applies to keyboard events; if the pointer is in the root window and a ContextSensitiveMenu? is invoked, the default CSM is brought up and not the application which last had focus.
See also ClickToFocusPointToFocusCategoryUserInterface