This looks fantastic. I will definitely give it a spin. I've been tracking what I call "computational scratchpad" apps for a while now but haven't found one that fits my environment/workflow yet. Maybe Heynote will. Here are some others that I've looked at:

* https://soulver.app Granddad of them all, Mac-only, proprietary, expensive

* https://numi.app Mac-only, proprietary, semi-expensive. Has a Github and claims to be MIT-licensed but I don't see how you could build a working application with what's in the repo.

* https://calca.io Windows- and Mac-only, proprietary, not expensive, nice docs.

* https://notepadcalculator.com Web-based, not open source, hosted but uses local storage. You can optionally create an account to sign in and have your notes saved in plaintext on his server.

* https://github.com/bbodi/notecalc3 Web-based, open source, self-hostable. But it seems to save your document in the URL string itself, which means the URL gets updated with almost every keystroke. Worth it for quick calculations and very small notes, I guess.

* https://numpad.io Web-based, hosted, not open source. Also stores entire doc in URL, but doesn't update the URL bar the whole time you're typing.

* https://numbr.dev/ Web-based, hosted. Has a Github but is not open source and the repo does not have all the bits needed to self-host it. Stores entire doc in URL.

* https://github.com/metakirby5/codi.vim Vim/NeoVim plugin that is less like a "smart notepad" and more like Jupyter but with results printed on the right side of the screen instead of in a cell below. Supports lots of programming languages.