I right now have a flex grow child container with some overflow in a parent container with display flex. The overflowing container also has a sibling with a set size, and so the size of the overflowing container is not set but defined with flex grow. Though when i do this the container seems to completely ignore the parents height and width constraints. I've recreated it in js fiddle like so: https://jsfiddle.net/5kjoqdzy/32/