Haven't managed to stack divs when creating a flexbox layout. Either they push each other within the container div, or I have to use "position:absolute"
on the overlay which gets the div out of its container context.
z-indices seem to not work at all.
I am using chrome latest version and was expecting this behaviour: http://dev.w3.org/csswg/css-flexbox/#painting
Here is a plunker with a basic flexbox layout: http://plnkr.co/edit/o2BAwvg3XV4YjwAwwmYR?p=preview to illustrate.
You will have to comment/uncomment the positioning in the definition of the .overlay
class which is in the css.