I was playing around a bit by using the movie to start ghosted and discovered that you can also unghost yourself (and probably get rid of the invisible line) by hitting the trigger point for walking diagonally on
any staircase, not just the one you used to start the glitch. Although, if you hit the trigger on the top of a staircase, you have to be careful to only hit the stairs walking trigger point once, otherwise the game forces you to walk up diagonally for a bit, and then you're stuck because you can only walk diagonally now and running into the staircase again does the same thing.
Here is the proper way to do it. Here is the wrong way to do it. (both of these movies had the characters ghosted with the blue staircase used in the example movie) Notice that I am not pressing anything at one point when Ness is walking diagonally in the wrong movie.
Walking below the invisible line appears to simply put you in staircase diagonal walking mode and you can't get out (unless you are lucky enough to be lined up with a staircase or maybe a door). Warping below that line is a different story though, I suppose.
Oh, and this is what going out of bounds looks like. The game kind of freezes for a bit, and then...
HUH??? (note: if a different tileset was loaded, there would be a lot of graphical glitches while the game is frozen, but the one I used here wasn't interesting enough)