working!

NoteContext.cs

@@ -0,0 +1,7 @@
+namespace BinaryDad.Notes
+{
+    public static class NoteContext
+    {
+        public static IDictionary<string, string> ClientNotes { get; } = new Dictionary<string, string>();
+    }
+}

NoteHub.cs

@@ -12,11 +12,27 @@ namespace BinaryDad.Notes
             this.noteService = noteService;
         }
 
+        public override Task OnConnectedAsync()
+        {
+            var noteName = Context.GetHttpContext().Request.Query["noteName"];
+
+            NoteContext.ClientNotes[Context.ConnectionId] = noteName;
+
+            return base.OnConnectedAsync();
+        }
+
         public async Task SaveNote(string content, string? noteName)
         {
             noteService.Save(content, noteName);
 
-            await Clients.Others.SendAsync("updateNote", content);
+            var clientConnections = NoteContext.ClientNotes
+                .Where(c => c.Value == noteName && c.Key != Context.ConnectionId)
+                .Select(c => c.Key)
+                .ToList();
+
+            await Clients
+                .Clients(clientConnections)
+                .SendAsync("updateNote", content);
         }
     }
 }

wwwroot/js/site.js

@@ -2,7 +2,7 @@
 
 let connection = new signalR.HubConnectionBuilder()
     .withAutomaticReconnect()
-    .withUrl("/noteHub")
+    .withUrl(`/noteHub?noteName=${noteName}`)
     .build();
 
 function start() {