No idea what the repro is, it just happens. This has happened to me ever since 18.1 RC but it has annoyed me enough to report it now. When this happens you will be unable to scroll or click buttons.
It doesn’t happen every time and it’s pretty rare, every once in a while. I am only seeing this on Meta.
I have seen something similar, and the fact you’re seeing this only on meta makes me think it is related to an issue in the ActivityPub plugin. It publishes topics/posts on a schedule to ActivityPub subscribers, and in the process, it updates the post’s status. It’s possible that it is too eager to reload a topic when it does that.