Avalonia/samples/SampleControls/HamburgerMenu/HamburgerMenu.cs

using Avalonia;
using Avalonia.Controls;
using Avalonia.Controls.Primitives;
using Avalonia.Media;

namespace ControlSamples
{
    public class HamburgerMenu : TabControl
    {
        private SplitView? _splitView;

        public static readonly StyledProperty<IBrush?> PaneBackgroundProperty =
            SplitView.PaneBackgroundProperty.AddOwner<HamburgerMenu>();

        public IBrush? PaneBackground
        {
            get => GetValue(PaneBackgroundProperty);
            set => SetValue(PaneBackgroundProperty, value);
        }

        public static readonly StyledProperty<IBrush?> ContentBackgroundProperty =
            AvaloniaProperty.Register<HamburgerMenu, IBrush?>(nameof(ContentBackground));

        public IBrush? ContentBackground
        {
            get => GetValue(ContentBackgroundProperty);
            set => SetValue(ContentBackgroundProperty, value);
        }

        public static readonly StyledProperty<int> ExpandedModeThresholdWidthProperty =
            AvaloniaProperty.Register<HamburgerMenu, int>(nameof(ExpandedModeThresholdWidth), 1008);

        public int ExpandedModeThresholdWidth
        {
            get => GetValue(ExpandedModeThresholdWidthProperty);
            set => SetValue(ExpandedModeThresholdWidthProperty, value);
        }

        protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
        {
            base.OnApplyTemplate(e);

            _splitView = e.NameScope.Find<SplitView>("PART_NavigationPane");
        }

        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
        {
            base.OnPropertyChanged(change);

            if (change.Property == BoundsProperty && _splitView is not null)
            {
                var (oldBounds, newBounds) = change.GetOldAndNewValue<Rect>();
                EnsureSplitViewMode(oldBounds, newBounds);
            }
        }

        private void EnsureSplitViewMode(Rect oldBounds, Rect newBounds)
        {
            if (_splitView is not null)
            {
                var threshold = ExpandedModeThresholdWidth;

                if (newBounds.Width >= threshold && oldBounds.Width < threshold)
                {
                    _splitView.DisplayMode = SplitViewDisplayMode.Inline;
                    _splitView.IsPaneOpen = true;
                }
                else if (newBounds.Width < threshold && oldBounds.Width >= threshold)
                {
                    _splitView.DisplayMode = SplitViewDisplayMode.Overlay;
                    _splitView.IsPaneOpen = false;
                }
            }
        }
    }
}