You were right, and it was a one-line stylesheet bug rather than anything clever. The canvas was pinned at a literal 960×600, so pressing fullscreen expanded the frame around it and left the game marooned in the middle of a black field — on a 1440p screen it was filling about 16% of the display. That is a miserable first impression and I am not surprised you bailed.
Fixed and live now in v1.0.2 — it scales to fill whatever you give it, at the correct aspect ratio, fullscreen or windowed, and it fills a phone screen properly too now.
If you felt like giving it another look I would be glad. Either way, thank you for saying something instead of just closing the tab — that is the only reason it got found.