It was the UI scale. It is made for 1080p.
Viewing post in Deep Water Fish jam comments
the UI scale is not the problem. the entire game’s scale is the problem. more specifically: you’ve set an unnecessarily large size for the embed window under the misguided impression that everyone will have their browser set to a size significantly larger than 1920x1080 (in order to contain the frame and its controls)
the game should be embedded in a significantly smaller frame, ideally the default width of 640px. when i eventually did manage to reach the fullscreen button, it was running just fine at 4480x2520, so it’s not like it only works at 1080p
i don’t know how much clearer i can make this statement. your game’s browser-embed is set, in the itch.io settings, to be 1920x1080. If you want a 16:9 aspect ratio, set it to 640x360, not 1920x1080. Why? Because 1366x768 is an extremely common resolution and anyone playing on such a display will have to scroll horizontally in order to reach the fullscreen button in order for your game to be fully visible.
After doing so the game will play fine, but doing so will be difficult, because the fullscreen button only exists after they have pressed “Run Game” which disables scrolling.
tl;dr: 640x360, not 1920x1080