GitHub uses selector-observer to enable logic on simple classes, which is why adding or removing classes like .js-fit-content will enable or disable their features.

It's built around MutationObserver with various optimizations on top of it. Its performance might not be super (https://bugzilla.mozilla.org/show_bug.cgi?id=1422522) but it seems to work well enough for GitHub.

It's worth investigating if anyone has time, it would elegantly solve issues around dynamic content and all the specific listeners we have.

I think observe calls would go inside each feature’s init, which could only be called once.