Took me a minute to realize the button is not in view of the embedded iframe, and you actually have to go fullscreen. That’s an odd choice, I mean, there’s plenty of room in the iframe to fit the whole scene.
Alas, I have no idea why it's doing that. If I change the embedded iframe dimensions, the positioning of the UI elements is just completely off. Any idea what's up with it?
Sorry, no idea! My games always use a canvas with explicit width and height eg 320x180, and they have no choice but to line up right. I’m guessing your framework does something more complex.