Command Bound to Keyboard Input not Firing

Question

I'm using the MVVM Community toolkit (v 8.2) in a learning project that implements the Risk board game.

I have a working command for some custom buttons, so I know the basics are functioning. But when I add commands bound to Keyboard input, they never fire (breakpoints are never reached), and I can't tell why.

I worried about Focus for a while, but the problem persists even when Window.Focusable=True explicitly, or when the InputBindings are on child elements like a background image or custom buttons (mentioned previously).

I've found many helpful answers to closely related questions here and elsewhere online, but nothing has resolved my specific issue.

MainWindow View XAML:

<Window x:Class="RiskMVVM2.MainWindow"
        ...
        Width="1820" Height="1062" 
        ResizeMode="NoResize" Name="GameWindow"
        WindowStartupLocation="CenterScreen" WindowStyle="None"
        FontFamily="Courier New" FontSize="12" >

    <Window.InputBindings>
        <KeyBinding Key="Return" Command="{Binding ConfirmInputCommand}" />
        <KeyBinding Gesture="Shift+Return" Command="{Binding SkipPlayerTurnCommand}" />
    </Window.InputBindings>

MainWindow Constructor (View code-behind):

public MainWindow(RiskBoardVM VM, List<Brush> playerColors)
{
     this.DataContext = VM;

     ...

     InitializeComponent();

     ...
}

Relevant ViewModel Code:

...
     private bool CanConfirmInput()
     {
          return true;
     }

     [RelayCommand(CanExecute = nameof(CanConfirmInput))]
     private void ConfirmInput()
     {
         CurrentGame.State.IncrementPhase();
     }

     private bool CanSkipPlayerTurn()
     {
         if ((int)CurrentPhase > (int)GamePhase.Place) return true;
         else return false;
     }


     [RelayCommand(CanExecute = nameof(CanSkipPlayerTurn))]
     private void SkipPlayerTurn()
     {
         CurrentGame.State.IncrementPlayerTurn();
     }
...