What is "Recently viewed topics" showing a user?

It's personal. "Recently viewed topics" shows only what you have recently viewed — never what other users of the site have viewed. Talkyard has no site-wide recently-viewed list anywhere.

Details, from the source of the version running here (v1.2026.003):

• The list is the recent-topics section of the watchbar (the left sidebar). Whenever you open or read a topic, the server moves that topic to the top of your own list: each full page load does this while building your me data (debiki/ReactJson.scala → watchbarAddRecentMarkSeen), and in-forum navigation does it via the reading-progress tracker (controllers/UserController.scala). The logic lives in BareWatchbar.tryAddRecentTopicMarkSeen (appsv/model/…/watchbar.scala).
• It's stored per user id — in the app's memory cache and in Redis under the key <userId>|Watchbar (debiki/dao/WatchbarDao.scala). Deliberately not in Postgres: the code comments explain that would cost one database write per member and page view. Practical consequence: the list survives app restarts (Redis persists), but it's cache-like state — flushing Redis resets everyone's list back to just their joined chats and direct messages.
• It holds at most 9 topics (MaxRecentTopics = 9), newest first; older ones fall off.
• Privacy-relevant behavior in the same code:
  • Private chats and direct messages are deliberately never added to the recent list, precisely because they're private — they appear only in the Joined Chats / Direct Messages sections below it.
  • Before a topic is added, the server re-checks that the user is actually allowed to see that page (the [WATCHSEC] check in WatchbarDao.scala).
  • The list is only ever sent to the browser as part of your own user data; no UI or API exposes anyone else's.
• Logged-out visitors: the server gives all strangers one shared watchbar containing only globally-pinned chats — no recent topics from the server. The "recently viewed" entries a logged-out visitor sees are tracked purely inside their own browser's sessionStorage (client/app-slim/ReactStore.ts), so those are also private to that browser and disappear when the browser session ends.

So each person sees their own private list. The only site-wide topic lists are the forum index sort orders (Active / New / Top), and those rank by posting activity, not by who viewed what.