fix: Chats doesn't sync for multi client users

agents-docs/LESSONS.md

@@ -25,6 +25,13 @@ Durable rules for AI agents working on this project. Read this file at session s
 
 ## Lessons
 
+### Don't bump E2E timeouts for sync flakes - gate on presence and read server logs [testing] [realtime]
+
+- **Trigger:** a multi-client chat-sync E2E flaked on "message not visible" and the first instinct was to raise `toBeVisible` timeouts or add waits; the user correctly rejected this ("it's not a timeout issue").
+- **Rule:** when a cross-user E2E assertion flakes, first gate the assertion on an observable precondition (peer visible in the members panel), then diff the signaling-server logs of a passing vs failing run (`joined server`, `user_joined`, `user_left`, `Removing dead connection`) before touching any timeout.
+- **Why:** the flake was a server race — `identify` + `join_server` arriving in one TCP segment were processed concurrently, the join was dropped as unauthenticated, and room membership silently vanished; no timeout can fix a message that is never broadcast. Fixed by serializing per-connection message handling in `server/src/websocket/handler.ts`.
+- **Example:** failing run showed one `joined server` for Ludde then `user_left` on sibling-client close; passing run showed two. `expectServerPeerVisible(page, displayName)` in `e2e/helpers/multi-device-session.ts` is the presence gate.
+
 ### When renaming an Angular route, sweep every navigate/url-match/doc reference [routing]
 
 - **Trigger:** the find-servers route was renamed `/search` → `/servers` in `app.routes.ts`, but `servers-rail.component.ts` still called `router.navigate(['/search'])` (leave-server) and matched `startsWith('/search')` for the user-bar visibility signal, throwing `NG04002: 'search'` on leave and never showing the user-bar on the discovery page.