I implemented a sticky sisebar library that aims to be easy to use as you are free in setting up the sidebar and its CSS. The library only gives you an additional value as custom property (--push-down) you can use in CSS to push the sidebar to its current position.

The problem I'm facing is that the script causes massive jitters in the sidebar position on some devices. These do only occur on the first scroll - if you've scrolled through the site once, it's working fine.

I would like to know why that could be the case (and if it's possible to be fixed. If not I still want to find out what's happening here.)

Using throttle functions does not make a difference here, as described here: https://stackoverflow.com/a/44779316/6336728. I'm already using the suggested method of doing layout read on scroll and layout write with requestAnimationFrame.

I put the code of my GitHub repo in this CodePen to try it out quickly; only the JS part is relevant.