yep

Shogi.UI/Pages/Home/GameBoard/SeatedGameBoard.razor

@@ -9,18 +9,26 @@
                        Perspective="Perspective"
                        OnClickHand="OnClickHand"
                        OnClickTile="OnClickTile"
-                       OnClickJoinGame="OnClickJoinGame" />
+                       SelectedPosition="@selectedBoardPosition" />
 
 @code {
     [Parameter, EditorRequired]
     public WhichPlayer Perspective { get; set; }
     [Parameter, EditorRequired]
     public Session Session { get; set; }
-    [Parameter] public Func<Task>? OnRefetchSession { get; set; }
     private bool IsMyTurn => Session?.BoardState.WhoseTurn == Perspective;
     private string? selectedBoardPosition;
     private WhichPiece? selectedPieceFromHand;
 
+    protected override void OnParametersSet()
+    {
+        base.OnParametersSet();
+        if (Session == null)
+        {
+            throw new ArgumentException($"{nameof(Session)} cannot be null.", nameof(Session));
+        }
+    }
+
     bool ShouldPromptForPromotion(string position)
     {
         if (Perspective == WhichPlayer.Player1 && Regex.IsMatch(position, ".[7-9]"))
@@ -36,12 +44,16 @@
 
     async Task OnClickTile(Piece? piece, string position)
     {
+        Console.WriteLine("Is my turn?");
+        Console.WriteLine(true);
         if (!IsMyTurn) return;
 
         if (selectedBoardPosition == null || piece?.Owner == Perspective)
         {
             // Select a position.
+            Console.WriteLine("Position {0}", position);
             selectedBoardPosition = position;
+            StateHasChanged();
             return;
         }
         if (selectedBoardPosition == position)
@@ -77,16 +89,4 @@
         selectedPieceFromHand = piece.WhichPiece;
         await Task.CompletedTask;
     }
-
-    async Task OnClickJoinGame()
-    {
-        if (Session != null && OnRefetchSession != null)
-        {
-            var status = await ShogiApi.PatchJoinGame(Session.SessionName);
-            if (status == HttpStatusCode.OK)
-            {
-                await OnRefetchSession.Invoke();
-            }
-        }
-    }
 }