When inserting the Video9000 extension, the video9000 videosource will be selected by default, where the V9990 output is superimposed over the MSX one.

As for the first I/O detection to a certain VDP auto-switching to it, might seem to be a good idea, because for example some Gfx9000 software can still use the MSX VDP to display status. In such a case that auto feature can be very annoying as it'll constantly switch between these two. The same could happen with MSX and external V99x8 controlled by the secondary I/O range.