Hi.
I don’t have an example but I think you can break it down in to simpler parts and see how they could be done.
I guess I should play the game, but from watching videos this is what I can gather.
1. The enemy moves around in the empty area and won’t leave the boundaries.
2. The player can move anywhere inside the boundary too, but when it’s on the edge it’s safe.
3. When the player leaves the edge it draws lines behind itself.
4. The player can’t cross this edge, and if the enemy touches this edge or the player it kills the player.
5. When the player makes it back to the original edge the area the enemy isn’t in is filled and the boundary edge is updated.
I may be overlooking some aspects of the game, and it does look like some things can be more complex but as a start that covers most of it.
Number 5 is the most novel. There are a few ways to think of it. One way that seems to work well in my mind is to have the boundary of the level be a polygon. When the player moves from one edge and makes it to another it splits the polygon into two. We then test which polygon the enemy is in and filling the other.
Filling a polygon can either be done with a flood fill or more efficiently a scan line at a time. Canvas sounds like it would work well, or maybe a bunch of 1px thick sprites.
So the laundry list of things to do would be:
Point inside polygon test.
Split polygon by poly line
Fill polygon
Others may have different ideas.