Is it possible for inline-block element to auto fill the available width?

Answer 1

Ever heard of flex box model!!

It is made just for that.

Note in flexbox model all child elements act as flex box model you cant opt out certain things. Which mean if page has navigation and under it content div + side div. You can't make top navigation out of it. Which has implications. So solution is to have all things only that need flex box in one div.