It seems to be caused by the FediAct addon, which doesn’t make any sense. Disabling the addon fixes the page without needing a reload, so it must be a CSS issue rather than an exception. Edit: it seems the addon is overriding the fadeIn
keyframe, but it looks fixed in the github version.
It looks like it’s the addon’s fault, and has already been fixed in the github version. It’s also been abandoned, so it’s probably not worth keeping around anyway.