Fixed position sidebar that stays relative to parent

Question

I have a mockup of a little CSS quandary I can't puzzle out (see image).

Basically I have a sidebar (blue) that I want to have position: fixed, but I want this sidebar to respect the parent (red) and always only take up 25% of that parent's width, and never go outside the bounds of the red parent (so red all feels like one big centered element).

If I fix the sidebar, it will jump out of the red container, obviously. If I nest blue in an absolute position container (and give red position: relative) blue will stay fixed but it won't fill the full 25%.

Answer 1

Have you tried with position: sticky? It should stay relative to parent.