Wow, yeah this is very bad. As far as I can tell it’s not directly flexbox related, because if I remove flexbox from the container it still happens.
Definitely seems to be yet another Safari rendering bug. There are some similar iOS Safari bugs that can be fixed by adding -webkit-transform: translatez(0); to the CSS of the fixed container… and it looks like that fix works for me here too. Generally not regarded as a great option, as it apparently forces GPU acceleration so it could impact performance.
I can only reproduce it when creating a new topic… so there must be something unique going on there. Investigating a bit more…