The native output resolution is the max vertical resolution of all currently displayed modes.

For instance, if you have an interlaced mode showing under a non-interlaced mode, then the entire screen displays at ~480i (to use modern terms) with the non-interlaced portion doubling its scanlines. But if the interlaced screen gets covered up, then it snaps to ~240p non-scandoubled.

The biggest modern problem with this is that displays turn themselves off to resync on mode changes, which always annoyed me greatly.