What about switching from one monitor to another monitor while keeping windowed mode?
I am also concerned about usage of iScreen. On Linux systems we have one ore more screens, then outputs (Monitors) defining the viewports to this screen. Resolutions are bound to outputs not to screens. 
I know multi-head setups have never worked for Linux in mainline but I have fixed this long ago on top of dropsdl. Further usage of iScreen drives this in a wrong directions.
Abusing iScreen for monitor would mean we need an integer pair (windowd / fullscreen) for each connected monitor. Furhter we can't expect a user to identify a monitor by an arbitrary integer.