Add project files.

Hyperbar.Desktop.Controls/DesktopFlyout/DesktopFlyout.cs

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+using Microsoft.UI.Xaml;
+
+namespace Hyperbar.Desktop.Controls;
+
+public class DesktopFlyout :
+    DependencyObject
+{
+    public static readonly DependencyProperty ContentProperty =
+        DependencyProperty.Register(nameof(Content),
+            typeof(object), typeof(DesktopFlyout),
+            new PropertyMetadata(null));
+
+    public static readonly DependencyProperty PlacementProperty =
+        DependencyProperty.Register(nameof(Placement),
+            typeof(DesktopFlyoutPlacement), typeof(DesktopFlyout),
+            new PropertyMetadata(DesktopFlyoutPlacement.Left, OnPlacementPropertyChanged));
+
+    private readonly DesktopFlyoutHost host;
+    private readonly DesktopFlyoutPresenter presenter;
+
+    public DesktopFlyout()
+    {
+        presenter = new DesktopFlyoutPresenter
+        {
+            Parent = this
+        };
+
+        host = new DesktopFlyoutHost(presenter);
+        host.Activate();
+    }
+
+    public object Content
+    {
+        get => GetValue(ContentProperty);
+        set => SetValue(ContentProperty, value);
+    }
+
+    public DesktopFlyoutPlacement Placement
+    {
+        get => (DesktopFlyoutPlacement)GetValue(PlacementProperty);
+        set => SetValue(PlacementProperty, value);
+    }
+
+    private static void OnPlacementPropertyChanged(DependencyObject dependencyObject,
+        DependencyPropertyChangedEventArgs args)
+    {
+        if (dependencyObject is DesktopFlyout sender)
+        {
+            sender.OnPlacementPropertyChanged();
+        }
+    }
+
+    private void OnPlacementPropertyChanged() => UpdatePlacement();
+
+    private void UpdatePlacement() => host.UpdatePlacement(Placement);
+}