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apps/desktop/src/lib/useCallPresence.ts

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+import { useEffect, useState } from 'react';
+
+import { supabase } from './supabase';
+
+// Watches the `call-presence:<conversationId>` realtime channel and returns
+// the set of userIds currently in that call. Independent of whether the
+// viewer is in the room — used to show "Active call · Join" affordances.
+//
+// Supabase realtime 2.103+ dedupes channels by topic: calling
+// `supabase.channel(topic)` returns an existing channel if one already exists.
+// That means an observer can't safely register `.on('presence', ...)` because
+// the tracker (CallContext) may have already subscribed it. We side-step this
+// by polling `presenceState()` on the (shared or fresh) channel.
+export function useCallPresence(conversationId: string | undefined): string[] {
+  const [activeUserIds, setActiveUserIds] = useState<string[]>([]);
+
+  useEffect(() => {
+    if (!conversationId) {
+      setActiveUserIds([]);
+      return;
+    }
+
+    const channel = supabase.channel('call-presence:' + conversationId, {
+      config: {
+        presence: {
+          key: 'observer-' + Math.random().toString(36).slice(2, 8),
+          // Supabase realtime only sends presence_state/diff events to a
+          // channel that has presence enabled. Without `enabled: true` (and
+          // no `.on('presence', ...)` bindings) the server treats the channel
+          // as non-presence and `presenceState()` never populates.
+          enabled: true,
+        },
+      },
+    });
+
+    // If Supabase returned a fresh (closed) channel, we own it and must
+    // subscribe/remove it. If it returned an already-subscribed channel
+    // (tracker owns it), we just read state.
+    const ownsChannel = channel.state === 'closed';
+    if (ownsChannel) {
+      void channel.subscribe();
+    }
+
+    const resync = () => {
+      const state = channel.presenceState() as Record<string, Array<Record<string, unknown>>>;
+      const ids = new Set<string>();
+      for (const list of Object.values(state)) {
+        for (const entry of list) {
+          const uid = entry?.userId;
+          if (typeof uid === 'string') ids.add(uid);
+        }
+      }
+      setActiveUserIds((prev) => {
+        const next = Array.from(ids).sort();
+        if (prev.length === next.length && prev.every((v, i) => v === next[i])) return prev;
+        return next;
+      });
+    };
+
+    resync();
+    const pollId = window.setInterval(resync, 1500);
+
+    return () => {
+      window.clearInterval(pollId);
+      if (ownsChannel) {
+        void supabase.removeChannel(channel);
+      }
+    };
+  }, [conversationId]);
+
+  return activeUserIds;
+}