How can I have a position: fixed; behaviour for a flexbox sized element?

Question

I have a div called .side-el which I would like to have in a position: fixed; behavior, but as soon as I apply position fixed the width alternates from the right one. The ri

Answer 1

You can't.

As explained by the CSS2.1 spec:

Absolutely positioned boxes are taken out of the normal flow.

And the Flexible Box Layout spec confirms that:

An absolutely-positioned child of a flex container does not participate in flex layout. However, it does participate in the reordering step (see order), which has an effect in their painting order.

^{(Emphasis mine)}