Polypad itself isn’t open source, but many of its underlying math and UI libraries are published by its creator under the Mathigon org [1]. The UI is impressive on its own, but what I find really interesting is that it’s built entirely without a traditional frontend framework. The core is a small library that's a mix of jquery-style DOM wrappers and Vue-like reactivity, templating, and web components [2].

I wish this style of building complex UIs was more common. Having some direct control over the DOM works really well for highly interactive graphics and visual tools.

[1] https://github.com/mathigon [2] https://github.com/mathigon/boost.js