feat: mirror html elements with the :root's scroll position

[hirasso]

Description

Adds experimental support for mirroring the scroll position between the root and HTML elements.

API

new ScrollMirror(document.querySelectorAll(':root, .scroller1, .scroller2'));

This is being achieved like this: If the library detects either html or body elements in the provided elements option, it will normalize the behavior like this:

1. Add a scroll event handler to the window
2. Measure the offsets and dimensions from the document.documentElement

Notes

• In my manual tests, this is working fine on desktop (Chromium, FF, Safari).
• on iOS Safari, momentum scrolling is sometimes becoming a bit flaky

Demo

https://deploy-preview-18--scrollmirror.netlify.app/root/

TODO

• Make sure only one of :root, html and body makes it into the final elements array

Checks

• The PR is submitted to the main branch
• The code was formatted before pushing (npm run format)
• All tests are passing (npm run test)
• New or updated tests are included
• The documentation was updated as required

Additional information

[netlify]

✅ Deploy Preview for scrollmirror ready!

Name	Link
🔨 Latest commit	161d596
🔍 Latest deploy log	https://app.netlify.com/sites/scrollmirror/deploys/658b4a3e58e6f10008c535c2
😎 Deploy Preview	https://deploy-preview-18--scrollmirror.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.