We don't need columns of divs to equally size themselves, so use easier layout techniques to make the list fit in the container. We have to take a hit with `height:100%`, but the hit is much more insignificant than confusing the layout engine.
The layout engine has a hard time with dynamically-but-statically-sized stuff like `width: 100%; display: flex;`, particularly when it is nested so badly. Overall this should improve performance for the app by not having to re-paint so often.
Fixes https://github.com/vector-im/riot-web/issues/14639
Travis Ralston