ah - figured it out! Had an old optimization for unloading characters that were offscreen that was unnecessary and despawning the boss. It should be fixed now!
Regarding the border, I was a little lazy about the map and visuals (as you can tell, it's pretty awful), and punted on getting procedural generation working. The game should "work" out of bounds, it's just visual.
I removed the ability to maximize in the latest build, just because it's tricky to support scaling the UI and keeping a consistent/fair experience with the viewport