Feature: Added Tree View sidebar

src/Files.App.Controls/Sidebar/FlatSidebarItem.cs

@@ -0,0 +1,46 @@
+// Copyright (c) Files Community
+// Licensed under the MIT License.
+
+namespace Files.App.Controls
+{
+	/// <summary>
+	/// Per-row wrapper used by the sidebar's virtualized ItemsRepeater. Carries the row's tree depth and section-gap flags so the underlying data items don't have to.
+	/// </summary>
+	public sealed class FlatSidebarItem : INotifyPropertyChanged
+	{
+		public ISidebarItemModel Item { get; }
+
+		public int Depth { get; }
+
+		// Caller-supplied at construction; hidden filesystem items are dimmed to match the file list's dimming convention.
+		public double RowOpacity { get; }
+
+		private static readonly PropertyChangedEventArgs SectionGapMarginChangedArgs = new(nameof(SectionGapMargin));
+
+		private bool _hasExpandedPredecessor;
+		public bool HasExpandedPredecessor
+		{
+			get => _hasExpandedPredecessor;
+			set
+			{
+				if (_hasExpandedPredecessor == value)
+					return;
+				_hasExpandedPredecessor = value;
+				PropertyChanged?.Invoke(this, SectionGapMarginChangedArgs);
+			}
+		}
+
+		public Thickness SectionGapMargin => _hasExpandedPredecessor
+			? new Thickness(0, 12, 0, 0)
+			: new Thickness(0);
+
+		public event PropertyChangedEventHandler? PropertyChanged;
+
+		public FlatSidebarItem(ISidebarItemModel item, int depth, double rowOpacity = 1.0)
+		{
+			Item = item;
+			Depth = depth;
+			RowOpacity = rowOpacity;
+		}
+	}
+}

src/Files.App.Controls/Sidebar/ISidebarItemModel.cs

@@ -17,13 +17,18 @@ public interface ISidebarItemModel : INotifyPropertyChanged
 		bool IsExpanded { get; set; }
 
 		/// <summary>
-		/// Indicates whether the children should have an indentation or not.
+		/// Optional path associated with this sidebar item for drag/drop scenarios.
 		/// </summary>
-		bool PaddedItem { get; }
+		string? Path { get; }
 
 		/// <summary>
-		/// Optional path associated with this sidebar item for drag/drop scenarios.
+		/// Renders as expandable even when Children is empty (children load lazily on first expansion).
 		/// </summary>
-		string? Path { get; }
+		bool HasUnrealizedChildren => false;
+
+		/// <summary>
+		/// Expansion participant that keeps the regular row appearance (icon + normal text) instead of the section-header style.
+		/// </summary>
+		bool IsLeafWithChildren => false;
 	}
 }

src/Files.App.Controls/Sidebar/SidebarItem.Properties.cs

@@ -13,7 +13,14 @@ public SidebarView? Owner
 			set { SetValue(OwnerProperty, value); }
 		}
 		public static readonly DependencyProperty OwnerProperty =
-			DependencyProperty.Register(nameof(Owner), typeof(SidebarView), typeof(SidebarItem), new PropertyMetadata(null));
+			DependencyProperty.Register(nameof(Owner), typeof(SidebarView), typeof(SidebarItem), new PropertyMetadata(null, OnOwnerChanged));
+
+		// Owner is assigned by the hosting ItemsRepeater's ElementPrepared (top-level rows) or by the parent row (flyout children) — recycled containers can carry a stale Owner across realizations, so the chevron-column visual state must re-apply whenever Owner flips.
+		private static void OnOwnerChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
+		{
+			if (d is SidebarItem item && item.Owner is { } owner)
+				VisualStateManager.GoToState(item, owner.SupportsExpansion ? "OwnerSupportsExpansion" : "OwnerDoesNotSupportExpansion", false);
+		}
 
 		public bool IsSelected
 		{
@@ -31,6 +38,37 @@ public bool IsExpanded
 		public static readonly DependencyProperty IsExpandedProperty =
 			DependencyProperty.Register(nameof(IsExpanded), typeof(bool), typeof(SidebarItem), new PropertyMetadata(true, OnPropertyChanged));
 
+		public int NestingLevel
+		{
+			get { return (int)GetValue(NestingLevelProperty); }
+			set { SetValue(NestingLevelProperty, value); }
+		}
+		public static readonly DependencyProperty NestingLevelProperty =
+			DependencyProperty.Register(nameof(NestingLevel), typeof(int), typeof(SidebarItem), new PropertyMetadata(0, OnNestingLevelChanged));
+
+		public double IndentWidth
+		{
+			get { return (double)GetValue(IndentWidthProperty); }
+			set { SetValue(IndentWidthProperty, value); }
+		}
+		public static readonly DependencyProperty IndentWidthProperty =
+			DependencyProperty.Register(nameof(IndentWidth), typeof(double), typeof(SidebarItem), new PropertyMetadata(0d));
+
+		private static void OnNestingLevelChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
+		{
+			if (d is SidebarItem item && e.NewValue is int level)
+				item.IndentWidth = level * 16d;
+		}
+
+		// Dims icon + text + chevron + decorator only; the selection indicator and pointer-over fill stay at full opacity so a selected hidden row still reads as selected.
+		public double ContentOpacity
+		{
+			get { return (double)GetValue(ContentOpacityProperty); }
+			set { SetValue(ContentOpacityProperty, value); }
+		}
+		public static readonly DependencyProperty ContentOpacityProperty =
+			DependencyProperty.Register(nameof(ContentOpacity), typeof(double), typeof(SidebarItem), new PropertyMetadata(1.0));
+
 		public bool IsInFlyout
 		{
 			get { return (bool)GetValue(IsInFlyoutProperty); }
@@ -39,15 +77,6 @@ public bool IsInFlyout
 		public static readonly DependencyProperty IsInFlyoutProperty =
 			DependencyProperty.Register(nameof(IsInFlyout), typeof(bool), typeof(SidebarItem), new PropertyMetadata(false));
 
-		public double ChildrenPresenterHeight
-		{
-			get { return (double)GetValue(ChildrenPresenterHeightProperty); }
-			set { SetValue(ChildrenPresenterHeightProperty, value); }
-		}
-		// Using 30 as a default in case something goes wrong
-		public static readonly DependencyProperty ChildrenPresenterHeightProperty =
-			DependencyProperty.Register(nameof(ChildrenPresenterHeight), typeof(double), typeof(SidebarItem), new PropertyMetadata(30d));
-
 		public ISidebarItemModel? Item
 		{
 			get { return (ISidebarItemModel)GetValue(ItemProperty); }
@@ -94,17 +123,6 @@ public SidebarDisplayMode DisplayMode
 		[GeneratedDependencyProperty]
 		public partial object? ToolTip { get; set; }
 
-		public static void SetTemplateRoot(DependencyObject target, FrameworkElement value)
-		{
-			target.SetValue(TemplateRootProperty, value);
-		}
-		public static FrameworkElement GetTemplateRoot(DependencyObject target)
-		{
-			return (FrameworkElement)target.GetValue(TemplateRootProperty);
-		}
-		public static readonly DependencyProperty TemplateRootProperty =
-			DependencyProperty.Register("TemplateRoot", typeof(FrameworkElement), typeof(SidebarItem), new PropertyMetadata(null));
-
 		public static void OnPropertyChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
 		{
 			if (sender is not SidebarItem item) return;