This is for a mobile menu, which of course needs to be responsive. The longest child should set the width for all the other children .index needs to be full width and height with a background-color
Some ways to align the last row in a flexbox grid
Aligning items in a flex container - CSS: Cascading Style Sheets
html - How to wrap smaller elements around larger elements with flexbox - Stack Overflow
html - flex-flow: column wrap doesn't stretch the parent element's width - Stack Overflow
html - Aligning two flex items: one to the top, the other centered - Stack Overflow
Flexbox Visual Guide Tutorial - Learn the basics of CSS Flexbox Module
A Guide to CSS Flexbox - CoderPad
Center elements with flexbox - Webflow University Documentation
html - Shrink-wrap flexbox with wrapping flex-items so it can be centered - Stack Overflow
Some ways to align the last row in a flexbox grid