(Now the SNES Test program stars sprites are identical when switches from non interlace to interlace, dunno at the current time how they squeeze in half like bsnes does)
If I'm not mistaken, the half-compression is due to enabling OAM interlace.
what about the black square moving in front of the stars?
It isn't a black square, it's range-tile over. There are too many sprites on the scanline to show all at once, so one ends up not getting rendered. They change the OAM base address to determine which is the FirstSprite, allowing them to cycle through all of them.
Support for this goes hand-in-hand with the RTO test in the electronics test, so you can kill two birds with one stone.
