Thanks for the template!
Scale to fit seems to fail again in Unity 2022. Like ThisIsNik, I’m using manual dimensions since auto-detect fails on itch.io (with any Unity template anyway).
But I think I found the reason: itch.io uses a fixed width for the main content column, so resizing the window won’t affect it.
If you change Integration Options to Click to Play in Fullscreen, then it will open the game in a full tab and it will behave as in your demo GIF, resizing the canvas with the window.
I guess that for embedded frames, we should just keep a fixed size…