How can I make Flexbox children 100% height of their parent?

If I understand correctly, you want flex-2-child to fill the height and width of its parent, so that the red area is fully covered by the green?

If so, you just need to set flex-2 to use Flexbox:

.flex-2 {
    display: flex;
}

Then tell flex-2-child to become flexible:

.flex-2-child {
    flex: 1;
}

See http://jsfiddle.net/2ZDuE/10/

The reason is that flex-2-child is not a Flexbox item, but its parent is.

Use align-items: stretch

Similar to David Storey's answer, my workaround is:

.flex-2 {
    display: flex;
    align-items: stretch;
}

Note that height: 100% should be removed from the child component (see comments).

Alternatively to align-items, you can use align-self just on the .flex-2-child item you want stretched.

I have answered a similar question here.

I know you have already said position: absolute; is inconvenient, but it works. See below for further information on fixing the resize issue.

Also see this jsFiddle for a demo, although I have only added WebKit prefixes so open in Chrome.

You basically have two issues which I will deal with separately.

1. Getting the child of a flex-item to fill height 100%

• Set position: relative; on the parent of the child.
• Set position: absolute; on the child.
• You can then set width/height as required (100% in my sample).

2. Fixing the resize scrolling "quirk" in Chrome

• Put overflow-y: auto; on the scrollable div.
• The scrollable div must have an explicit height specified. My sample already has height 100%, but if none is already applied you can specify height: 0;

See this answer for more information on the scrolling issue.