Why doesn't a block laid out on flexbox scroll?

Question

css

Hello. Why doesn't a block laid out like this (link below) scroll? To check, it is enough to shrink the browser window vertically and see this bug, the block goes above the browser and becomes unreadable. I noticed such a bug when I turned the smartphone on its side. Maybe I missed some feature?
codepen.io/anon/pen/avpqqa