Terminals no longer get stuck on an old screen after you switch tabs.
If a terminal produced output while its tab was in the background, switching back could show it as it was before you left. After the first scroll it jumped back to that old screen and stopped following new output until you pressed a key. A long command such as apt upgrade looked frozen even though it had finished long ago. Nothing was wrong with the servers: the output had arrived, only the view was out of date. The terminal now runs on xterm.js 6.0, and background tabs stay in sync with their output.