Sticky sidebar

In the first link, the opening sentence and the comments actually say all you need to know. :

It is important that the immediate parent container of the elements (on which one should be sticky) is at display: flex
There are a lot of settings on flex containers – but this is the essential condition

on the image

you can see that for the sidebar (aside) the direct parent element is the div with the class: container.

The sidebar itself got that position sticky – the top value determines the distance to top.
This for sticky position inside the parent container.

But:

/*** the whole thing only works if top containers are set to: ***/
.responsive #top {
    overflow-x: visible;
}

#top #wrap_all {
    overflow: visible;
}