How to make stretching columns of the same height like live broadcasts?

Question

css

Hello colleagues.
Please, help me solve the problem:
SUBJECT
There is a module where there is a video and there is a chat module.
PROBLEM:
How to make sure that in any case the HEIGHT of the chat does not exceed the height of the video: it stretches and shrinks after it?
https://screenshots.firefox.com/BrwWZt70duk0UrbF/l... - now it's like this.
Need to -
Tweak the codepen
In general, like on live YouTube broadcasts. (I inspected the code. Too many wrappers... I'm lost in logic...
CONDITION:
Without scripts if possible . THANK YOU in
advance for any advice.

Answer 1

https://jsfiddle.net/anton4ept/3fndu12d/ например так.

Answer 2

Пока Viewport-единицы рулят миром.
https://s.codepen.io/emelyanova/debug/zWaxLM/VJrxx...
Завтра отдебажу и дам ответ.)