This repros easily 100% of the time, you need to scroll slowly.
Happens cause cloak/uncloak sequence in posts is not accounting for spoilers somehow.
I am adding a pr-welcome here, would be nice to fix this but it is going to be pretty tricky
@eviltrout maybe worth you having a 2 min look at this in case there is something more sinister going on with cloak/uncloak, maybe post decorators are not firing right on uncloaked posts?
Uncloaking a post will save its height at that time. If you’ve expanded details, the height will have changed. When you uncloak it, it uses the previously stored height, which is now incorrect.
I think to solve this the details plugin will have to remember what was expanded, and when re-rendering that post re-apply the opened state.
Otherwise it feels weird - why is discourse collapsing details just because a post scrolled out of view?