Lemmy keeps a global list of posts you've read on any device and the account preference can be used to "permanently" hide them. It's really useful for faking the way Reddit reworked your homepage at one point, where it keeps feeding you new content every refresh without having to keep clicking "clear read" all the time. But if you then want to see those posts - because you clicked back on accident, want to see a pinnet post etc - you have to go to ´´Settings -> Account Preferences´´ to toggle it on/off there, save, come back, and manually refresh the page you were on.
It would be really handy if there was a "Show read" toggle in the kebab menu right above the current "Show hidden" that would toggle it.
