Explore using `selector-observer` to run features

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.

[fregante]

@yakov116 if you also want to look into any other features that use selector-observer but don’t have this class, please send a PR for that

[fregante]

I confirmed the bug and found a workaround:

• if the selector has :not(.rgh-this-feature) and we add the class right away, selector-observer will no longer call the add method because the original selector no longer applies. This avoids the loop.

We almost always do that already, but #3517 didn’t because it didn’t seem necessary.

• I’ll open the previously-locked PRs
• @kidonng you can resubmit #3517 with the workaround described #3531 already exists 👍
• @yakov116 if you want to explore Monitoring anyway, try it out on linkify-code, which is the heaviest feature making use of selector-observer