I'm on a macbook. I don't see a fullscreen button.
Sorry, I meant the bottom right corner. The third button there is the fullscreen button. If you don't see it, it probably means it's somehow being cropped off due to your device's resolution or aspect ratio.
If you don't mind, may I know your screen resolution and which browser you're running Serenitrove on? Is it Safari?
I managed to have a look at your screenshot, and it does seem to be cropped off for some reason. I tried running it on my macbook's Safari browser and i couldn't reproduce the issue. One thing's for sure, the itch.io's iframe is limiting the viewport, so you could export your save (from the Settings button that is still accessible), download the offline copy of Serenitrove (v1.2.2), run it and reimport your save over there. Since the offline copy is not rendered within an iframe, it should solve the cropping issue. Hope this helps!