Use `ResizeObserver` in `sticky-sidebar` #3582

Instead of relying on order of executing, we can use this new API to ensure that the sidebar’s stickiness is updated when it changes size. You'd have to combine ResizeObserver with selector-observer

I'm automatically closing this to be picked up after #3084 is resolved.

The current feature’s code might be a little hard to adapt/understand, so feel free to write it from scratch; This is probably easier and it's probably only 15 lines of code or less.