Remade the entire game from scratch in Gamemaker Studio (was previously in GM8.1) over the last few days. In GM8.1 I was using drag and drop junk 95% of the time and I figured it would be easier in the long run to just translate it all into GML, and at that point I might as well be using the newer software.

There is at least one new feature: the shadow now stops short if it attempts to extend past a wall! For the most part. It’s still a bit finicky when jumping (probably because the shadow is attempting to move so far past the walls) but it’s a start.
Also the room is darker when the light is switched off.