<?xml version="1.0" encoding="UTF-8"?> <!-- Any copyright is dedicated to the Public Domain. http://creativecommons.org/publicdomain/zero/1.0/ --> <!-- Testcase with fixed-position flex container & children. All positioning is with respect to the viewport. --> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <style> div.containingBlock { top: 15px; left: 20px; height: 400px; position: absolute; border: 2px dashed purple; } .fixedpos { position: fixed; left: 5px; border: 2px dotted black; } div.flexbox { top: 30px; left: 40px; width: 200px; height: 100px; display: flex; position: fixed; border: 2px dashed teal; } div.a { flex: 1 0 auto; width: 30px; height: 100px; background: lightgreen; } div.b { flex: 2 0 20px; height: 100px; background: yellow; } </style> </head> <body> <div class="containingBlock"> <div class="flexbox"><div class="a fixedpos"/><div class="b"/></div> </div> </body> </html>