The containers become shorter instead of overflowing

I have made a messaging web app a while back and this summer i tried to make another with a bit of a different structure but ran into this issue where the spans’ (messages’) height shrunk instead of overflowing with scroll, and if i set a min-height they wouldn’t get taller when the message was longer, so i used the source code of the old one and changed it a bit, but had the same issue. I would really like to know why this is happening and how to solve it.

I put the code on github, to be easier to see/try: https://github.com/Konstei/stack-overflow-code/.

I also made it into a running website using github’s pages: https://konstei.github.io/stack-overflow-code/new/, https://konstei.github.io/stack-overflow-code/old/