How to make nested flex boxes full width?

Question

css

There is an example on jsfiddle .
There is a list of elements that are rotated vertically:



I'm trying to make the width of the element 100%, but I recently adopted the flex properties, literally today, and I can't enter them yet, maybe it's not about them at all in the example. In general, you need to make sure that there is no white space between the blocks and the element fills everything with color. How to solve it?