I use focus follows mouse/mouse precedence with separate screen focus enabled.  However, separate screen focus does not switch focus to the active screen (defined by me, at any rate, as the screen in which the mouse pointer is over) until a window in the newly active screen is activated.

Concrete example: I use multiple desktops.  I keep a VNC session on screen 1 on all desktops, while screen 0 contains all other windows.  If I move the mouse from screen 1 to screen 0, the VNC session retains focus until I move the mouse over an existing window in screen 0 or click on the desktop.

I would like the (or an optional) behavior to be that as soon as I move the mouse off screen 1 that any window on that screen with focus loses it, regardless of whether there is another window with focus.   This would in essence be focus strictly under mouse applied to screens, while the user's chosen focus policy would apply to windows within the current screen.