I can't go in to fullscreen mode on the official website, using opera GX.
U can't save either.
Hi Gabe! I gotta say I had never tried Opera GX, I just did and the game renders amazing!
I also tried on the official site htopia.online and I was able to make the game fullscreen pressing F11, also Local saves are working properly after giving permission to the browser. I believe Opera GX runs on Chromium so it should behave pretty much the same as Chrome where I've done most of my tests.
Are you running the game on mobile by any chance?