Show per-aircraft peer count in detail panel

app/static/detail_panel.js

@@ -326,6 +326,9 @@ export function buildPopupContent(a, now, airports) {
         `<span class="stat-val pop-signal"></span></div>` +
       `<div class="stat"><span class="stat-label">First seen</span>` +
         `<span class="stat-val pop-first-seen"></span></div>` +
+      `<div class="stat pop-peers-stat" hidden>` +
+        `<span class="stat-label">Also seen by</span>` +
+        `<span class="stat-val pop-peers-val"></span></div>` +
     `</div>`;
   injectHelpIcons(root);
   updatePopupContent(root, a, now, airports);
@@ -605,6 +608,18 @@ export function updatePopupContent(root, a, now, airports) {
   const firstSeen = entry?.sessionFirstSeen || a.first_seen;
   q('.pop-first-seen').textContent = relativeAge(firstSeen, now);
 
+  // P2P "also seen by N peers" — surfaced only when the relay reported a
+  // non-zero count for this aircraft. Locally-only aircraft (e.g. P2P off,
+  // or no peer is currently reporting this ICAO) leave the stat hidden.
+  const peersStat = q('.pop-peers-stat');
+  const seenN = a.seen_by_others;
+  if (typeof seenN === 'number' && seenN > 0) {
+    q('.pop-peers-val').textContent = `${seenN} peer${seenN === 1 ? '' : 's'}`;
+    peersStat.hidden = false;
+  } else {
+    peersStat.hidden = true;
+  }
+
   renderCommBSection(root, q, a, now);
 }
 

dotnet/src/FlightJar.Api/Endpoints/P2PEndpoints.cs

@@ -120,7 +120,10 @@ private static string Sanitise(RegistrySnapshot snap)
         foreach (var ac in snap.Aircraft)
         {
             if (ac.Peer == true) continue;
-            aircraft.Add(ac with { DistanceKm = null });
+            // SeenByOthers is filled by the cloud relay per-recipient and
+            // doesn't make sense over a direct same-LAN /p2p/ws stream;
+            // strip it so consumers can't mistake it for our own count.
+            aircraft.Add(ac with { DistanceKm = null, SeenByOthers = null });
         }
         var sanitised = snap with
         {

dotnet/src/FlightJar.Api/Hosting/P2PRelayClientService.cs

@@ -279,7 +279,11 @@ private string BuildSanitisedPayload()
         foreach (var ac in snap.Aircraft)
         {
             if (ac.Peer == true) continue; // don't echo back what we received
-            aircraft.Add(ac with { DistanceKm = null });
+            // Strip receiver-specific + relay-computed fields:
+            // - DistanceKm describes our reception, not the aircraft itself.
+            // - SeenByOthers is filled by the relay per-recipient; if we
+            //   echoed it back the relay would treat it as our reading.
+            aircraft.Add(ac with { DistanceKm = null, SeenByOthers = null });
         }
 
         // Explicitly null out receiver location — never share it with the relay.

dotnet/src/FlightJar.Core/State/PeerMerge.cs

@@ -106,6 +106,11 @@ public static SnapshotAircraft Combine(SnapshotAircraft local, SnapshotAircraft
             // Peer flag stays null — we have direct contact, so this
             // record renders as a local aircraft (no peer styling).
             Peer = null,
+
+            // Relay-computed: how many other peers also report this ICAO.
+            // The relay calculated this per-recipient (excluding us), so
+            // we just carry the peer record's value through.
+            SeenByOthers = peer.SeenByOthers,
         };
     }
 }

dotnet/src/FlightJar.Core/State/RegistrySnapshot.cs

@@ -122,6 +122,14 @@ public sealed record SnapshotAircraft
     /// for locally-observed aircraft.</summary>
     public bool? Peer { get; init; }
 
+    /// <summary>How many OTHER P2P peers also see this aircraft (excluding
+    /// this receiver). Computed by the relay per-recipient and surfaced via
+    /// the aggregate broadcast — null for locally-only aircraft when the
+    /// relay isn't connected, or any aircraft outside the federation. The
+    /// detail panel renders an "also seen by N peers" stat when this is
+    /// > 0.</summary>
+    public int? SeenByOthers { get; init; }
+
     // Enrichment fields populated by the snapshot pusher from the external
     // clients. Names match the wire schema the frontend reads directly:
     // `origin`, `destination`, `phase`, `operator`, `operator_iata`,

dotnet/tests/FlightJar.Core.Tests/State/PeerMergeTests.cs

@@ -214,6 +214,21 @@ public void ExtendAirports_SkipsAircraftWithNullAirportInfo()
         Assert.Empty(airports);
     }
 
+    [Fact]
+    public void Combine_TakesSeenByOthersFromPeer()
+    {
+        // The relay computes seen_by_others per-recipient (it knows the
+        // contributor set) and stamps it on the peer record. Combine must
+        // surface that value on the merged record so the detail panel can
+        // render the count even when the aircraft is locally observed.
+        var local = new SnapshotAircraft { Icao = "abc123", Callsign = "BAW123" };
+        var peer = new SnapshotAircraft { Icao = "abc123", SeenByOthers = 3 };
+
+        var merged = PeerMerge.Combine(local, peer);
+
+        Assert.Equal(3, merged.SeenByOthers);
+    }
+
     [Fact]
     public void Combine_FillsAltitudeAndVelocityFromPeer()
     {

relay-worker/src/relay.ts

@@ -3,6 +3,10 @@ import { DurableObject } from 'cloudflare:workers';
 interface AircraftEntry {
   data: Record<string, unknown>;
   receivedAt: number; // epoch seconds
+  // WebSockets that have reported this aircraft, keyed by their last
+  // contribution timestamp. Lets us tell each recipient how many OTHER
+  // peers also see the aircraft, so the UI can render "also seen by N".
+  contributors: Map<WebSocket, number>;
 }
 
 interface TokenRecord {
@@ -143,7 +147,7 @@ export class RelayDurableObject extends DurableObject {
     }));
   }
 
-  override webSocketMessage(_ws: WebSocket, message: string | ArrayBuffer): void {
+  override webSocketMessage(ws: WebSocket, message: string | ArrayBuffer): void {
     if (typeof message !== 'string') return;
 
     let parsed: Record<string, unknown>;
@@ -163,17 +167,31 @@ export class RelayDurableObject extends DurableObject {
       if (typeof ac !== 'object' || ac === null) continue;
       const icao = (ac as Record<string, unknown>)['icao'];
       if (typeof icao !== 'string' || !icao) continue;
-      this.aggregate.set(icao.toLowerCase(), { data: ac as Record<string, unknown>, receivedAt: now });
+      const key = icao.toLowerCase();
+      const existing = this.aggregate.get(key);
+      if (existing) {
+        existing.data = ac as Record<string, unknown>;
+        existing.receivedAt = now;
+        existing.contributors.set(ws, now);
+      } else {
+        this.aggregate.set(key, {
+          data: ac as Record<string, unknown>,
+          receivedAt: now,
+          contributors: new Map([[ws, now]]),
+        });
+      }
     }
   }
 
   override webSocketClose(ws: WebSocket): void {
     this.connections.delete(ws);
+    this.dropContributor(ws);
     console.log(JSON.stringify({ event: 'disconnect', connections: this.connections.size }));
   }
 
   override webSocketError(ws: WebSocket, error: unknown): void {
     this.connections.delete(ws);
+    this.dropContributor(ws);
     try { ws.close(); } catch { /* already closed */ }
     console.log(JSON.stringify({
       event: 'ws_error',
@@ -182,6 +200,15 @@ export class RelayDurableObject extends DurableObject {
     }));
   }
 
+  // Remove `ws` from every aircraft's contributor map. Called on clean
+  // disconnect so the seen_by_others count drops immediately rather than
+  // waiting up to STALE_S for the lazy eviction in broadcast() to catch up.
+  private dropContributor(ws: WebSocket): void {
+    for (const entry of this.aggregate.values()) {
+      entry.contributors.delete(ws);
+    }
+  }
+
   override async alarm(): Promise<void> {
     this.broadcast();
     // Re-schedule unless there are no connections (DO will hibernate)
@@ -196,27 +223,34 @@ export class RelayDurableObject extends DurableObject {
     const now = Date.now() / 1000;
     const cutoff = now - STALE_S;
 
-    // Evict stale and collect fresh
-    const fresh: Record<string, unknown>[] = [];
+    // Evict stale aggregate entries; for each surviving entry also sweep
+    // out per-WS contributor timestamps that have aged past the same
+    // cutoff, so seen_by_others counts a contributor only as long as
+    // they're actively reporting the aircraft.
+    const fresh: AircraftEntry[] = [];
     for (const [icao, entry] of this.aggregate) {
       if (entry.receivedAt < cutoff) {
         this.aggregate.delete(icao);
-      } else {
-        fresh.push(entry.data);
+        continue;
+      }
+      for (const [ws, ts] of entry.contributors) {
+        if (ts < cutoff) entry.contributors.delete(ws);
       }
+      fresh.push(entry);
     }
 
-    if (fresh.length === 0 && this.connections.size === 0) return;
-
-    // Count of "other" peers from any single recipient's perspective —
-    // every connected client is one of `connections`, so each sees
-    // `connections.size - 1` others. Same number for every recipient,
-    // so one shared payload is correct.
+    // Total connected count goes in the envelope; per-aircraft counts go
+    // in `seen_by_others` and are computed per-recipient (each WS subtracts
+    // itself from every aircraft it's contributing to).
     const peers = Math.max(0, this.connections.size - 1);
-    const payload = JSON.stringify({ type: 'aggregate', peers, aircraft: fresh });
     const dead: WebSocket[] = [];
 
     for (const ws of this.connections) {
+      const aircraft = fresh.map(entry => ({
+        ...entry.data,
+        seen_by_others: entry.contributors.size - (entry.contributors.has(ws) ? 1 : 0),
+      }));
+      const payload = JSON.stringify({ type: 'aggregate', peers, aircraft });
       try {
         ws.send(payload);
       } catch {
@@ -226,6 +260,7 @@ export class RelayDurableObject extends DurableObject {
 
     for (const ws of dead) {
       this.connections.delete(ws);
+      this.dropContributor(ws);
       try { ws.close(); } catch { /* already closed */ }
     }
   }