Why are flexbox blocks cut off?

Question

css

codepen.io/max_minimus/pen/zKGWEq
I can't figure out why flexible blocks are not flexible
bottom borders of two blocks (which have red and green strokes) are cut off
in the firefox, chrome, opera, opera coast mobile browser...
the problem is not on all browsers - for example, in firefox on a poppy everything is fine

Answer 1

Russian ID...