docs: the superseded fallback answers its own session; who uses beacon()

backend/app/plex/stream.py

@@ -744,8 +744,9 @@ def start_session(
         # `replace_only` makes the supersede identity-aware for the runtime fallback: the whole
         # check-and-replace happens under ONE _lock, so a newer same-key session (a concurrent
         # seek-restart) installed between the fallback's decision and here is never clobbered. When the
-        # registered session is no longer `replace_only`, we abort and return None (the caller yields to
-        # whoever is registered now). The INITIAL spawn never gets None — see its call site.
+        # registered session is no longer `replace_only`, we abort and return None, and the caller
+        # answers with its own dead session — never the newer one, whose id a stale requester would
+        # stop. The INITIAL spawn never gets None — see its call site.
         # Software-fallback encode tuning — read only on the SW transcode path (the VAAPI path and
         # remux ignore preset/crf/max_height, and remux must not touch the DB for them).
         preset, crf, max_height = "veryfast", 20, 0

frontend/src/lib/api/core.ts

@@ -285,11 +285,12 @@ export function identityHeaders(): Record<string, string> {
   };
 }
 
-// Fire-and-forget POST that survives page unload (keepalive) — shared by the resume-position
-// beacons (YT + Plex players). A normal fetch is cancelled on unload and React effect cleanup
-// doesn't run on a full reload, so without keepalive the last position (esp. right after a seek)
-// would be lost. `onSettled`: the request has been answered (or failed) — for a caller whose cached
-// view must be read again once the write is on the server.
+// Fire-and-forget POST that survives page unload (keepalive) — for every write a closing page must
+// still make: the resume checkpoints of the YouTube, Plex and library players, the Plex session stop,
+// the diagnostics flush. A normal fetch is cancelled on unload and React effect cleanup doesn't run
+// on a full reload, so without keepalive the last position (esp. right after a seek) would be lost.
+// `onSettled`: the request has been answered (or failed) — for a caller whose cached view must be
+// read again once the write is on the server.
 export function beacon(url: string, body: Record<string, unknown>, onSettled?: () => void): void {
   try {
     void send(url, {