diff --git a/src/OneWare.Chat/ChatHeightEstimation.cs b/src/OneWare.Chat/ChatHeightEstimation.cs
new file mode 100644
index 00000000..50c6ff00
--- /dev/null
+++ b/src/OneWare.Chat/ChatHeightEstimation.cs
@@ -0,0 +1,34 @@
+using System;
+
+namespace OneWare.Chat;
+
+/// <summary>
+/// Heuristics for estimating a chat message's rendered pixel height from its markdown content,
+/// before it has been realized and measured. Used to give the virtualizing message list a stable,
+/// content-accurate scroll extent.
+/// </summary>
+public static class ChatHeightEstimation
+{
+    private const double CharWidth = 7.5;
+    private const double LineHeight = 18;
+    private const double VerticalPadding = 16;
+
+    /// <summary>
+    /// Estimates the height of a block of markdown/plain text for the given available width by
+    /// approximating wrapped line counts. The real measured height replaces this once realized,
+    /// so the estimate only needs to be in the right ballpark.
+    /// </summary>
+    public static double EstimateMarkdown(string? text, double width, double extra = 0)
+    {
+        if (string.IsNullOrEmpty(text))
+            return VerticalPadding + LineHeight;
+
+        var charsPerLine = Math.Max(1, width / CharWidth);
+        double lines = 0;
+
+        foreach (var hardLine in text.Split('\n'))
+            lines += Math.Max(1, Math.Ceiling(hardLine.Length / charsPerLine));
+
+        return VerticalPadding + extra + lines * LineHeight;
+    }
+}
diff --git a/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageAssistantViewModel.cs b/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageAssistantViewModel.cs
index ed165d0b..e67e8839 100644
--- a/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageAssistantViewModel.cs
+++ b/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageAssistantViewModel.cs
@@ -1,9 +1,10 @@
 using System.Runtime.Serialization;
 using CommunityToolkit.Mvvm.ComponentModel;
+using OneWare.Essentials.Controls;
 
 namespace OneWare.Chat.ViewModels.ChatMessages;
 
-public class ChatMessageAssistantViewModel : ObservableObject, IChatMessage
+public class ChatMessageAssistantViewModel : ObservableObject, IChatMessage, IEstimatedHeightItem
 {
     public ChatMessageAssistantViewModel(string? messageId = null)
     {
@@ -27,4 +28,6 @@ public bool IsStreaming
         get;
         set => SetProperty(ref field, value);
     }
+
+    public double EstimateHeight(double width) => ChatHeightEstimation.EstimateMarkdown(Content, width);
 }
diff --git a/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageReasoningViewModel.cs b/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageReasoningViewModel.cs
index 5249f1cb..6e4f56a9 100644
--- a/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageReasoningViewModel.cs
+++ b/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageReasoningViewModel.cs
@@ -1,9 +1,10 @@
 using System.Runtime.Serialization;
 using CommunityToolkit.Mvvm.ComponentModel;
+using OneWare.Essentials.Controls;
 
 namespace OneWare.Chat.ViewModels.ChatMessages;
 
-public class ChatMessageReasoningViewModel : ObservableObject, IChatMessage
+public class ChatMessageReasoningViewModel : ObservableObject, IChatMessage, IEstimatedHeightItem
 {
     public ChatMessageReasoningViewModel(string? reasoningId = null)
     {
@@ -27,4 +28,12 @@ public bool IsStreaming
         get;
         set => SetProperty(ref field, value);
     }
+
+    public double EstimateHeight(double width)
+    {
+        const double header = 36;
+        if (!IsStreaming)
+            return header;
+        return header + System.Math.Min(200, ChatHeightEstimation.EstimateMarkdown(Content, width)) + 8;
+    }
 }
diff --git a/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageToolViewModel.cs b/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageToolViewModel.cs
index b52ff621..7fa55b71 100644
--- a/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageToolViewModel.cs
+++ b/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageToolViewModel.cs
@@ -1,9 +1,10 @@
 using System.Runtime.Serialization;
 using CommunityToolkit.Mvvm.ComponentModel;
+using OneWare.Essentials.Controls;
 
 namespace OneWare.Chat.ViewModels.ChatMessages;
 
-public class ChatMessageToolViewModel : ObservableObject, IChatMessage
+public class ChatMessageToolViewModel : ObservableObject, IChatMessage, IEstimatedHeightItem
 {
     public ChatMessageToolViewModel(string id, string toolName)
     {
@@ -34,4 +35,12 @@ public bool IsSuccessful
         get;
         set => SetProperty(ref field, value);
     }
+
+    public double EstimateHeight(double width)
+    {
+        const double header = 36;
+        if (!IsToolRunning)
+            return header;
+        return header + System.Math.Min(200, ChatHeightEstimation.EstimateMarkdown(ToolOutput, width)) + 8;
+    }
 }
diff --git a/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageUserViewModel.cs b/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageUserViewModel.cs
index 98b817c0..9687baf8 100644
--- a/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageUserViewModel.cs
+++ b/src/OneWare.Chat/ViewModels/ChatMessages/ChatMessageUserViewModel.cs
@@ -1,9 +1,10 @@
 using System.Runtime.Serialization;
 using CommunityToolkit.Mvvm.ComponentModel;
+using OneWare.Essentials.Controls;
 
 namespace OneWare.Chat.ViewModels.ChatMessages;
 
-public class ChatMessageUserViewModel : ObservableObject, IChatMessage
+public class ChatMessageUserViewModel : ObservableObject, IChatMessage, IEstimatedHeightItem
 {
     public ChatMessageUserViewModel(string message)
     {
@@ -15,4 +16,6 @@ public ChatMessageUserViewModel(string message)
     public string Message { get; }
     
     public DateTimeOffset Timestamp { get; }
+
+    public double EstimateHeight(double width) => ChatHeightEstimation.EstimateMarkdown(Message, width);
 }
diff --git a/src/OneWare.Chat/Views/ChatView.axaml b/src/OneWare.Chat/Views/ChatView.axaml
index aba6e18d..035163fb 100644
--- a/src/OneWare.Chat/Views/ChatView.axaml
+++ b/src/OneWare.Chat/Views/ChatView.axaml
@@ -75,65 +75,68 @@
         </Border>
 
         <Panel Grid.Row="1">
-            <ScrollViewer Name="ScrollViewer" HorizontalScrollBarVisibility="Disabled"
-                          VerticalScrollBarVisibility="Auto">
-                <Interaction.Behaviors>
-                    <AutoScrollDuringDragBehavior ScrollDelta="4" />
-                </Interaction.Behaviors>
-                <Border Padding="10 10 10 4">
-                    <Grid RowDefinitions="Auto, Auto, Auto" RowSpacing="5">
-                        <ItemsControl Name="ItemsControl" ItemsSource="{Binding Messages}">
-                            <ItemsControl.Styles>
-                                <Style Selector="Expander /template/ ToggleButton">
-                                    <Setter Property="Background" Value="{DynamicResource ThemeAccentBrush4}" />
-                                    <Setter Property="CornerRadius" Value="3" />
-                                </Style>
-                                <Style Selector="Expander /template/ ToggleButton:pointerover /template/ Border">
-                                    <Setter Property="Background" Value="{DynamicResource ThemeAccentBrush4}" />
-                                </Style>
-                                <Style Selector="Expander /template/ ToggleButton:pointerover /template/ Border Border">
-                                    <Setter Property="Background" Value="Transparent" />
-                                </Style>
-                                <Style Selector="Expander /template/ ToggleButton#PART_toggle /template/ Border">
-                                    <Setter Property="CornerRadius" Value="3" />
-                                    <Setter Property="VerticalAlignment" Value="Center" />
-                                </Style>
-                                <Style Selector="Expander /template/ ToggleButton#PART_toggle /template/ Grid > Border">
-                                    <Setter Property="Margin" Value="0 2" />
-                                    <Setter Property="VerticalAlignment" Value="Center" />
-                                </Style>
-                            </ItemsControl.Styles>
+            <DockPanel>
+                <!-- Loading indicator + queued messages are pinned below the conversation and kept
+                     outside the scrolling region so the virtualizing message list above can virtualize
+                     against the viewport (it must be the direct content of the ScrollViewer). -->
+                <StackPanel DockPanel.Dock="Bottom" Margin="10 0 10 4" Spacing="5">
+                    <!-- Loading Indicator -->
+                    <Grid ColumnDefinitions="Auto, Auto" ColumnSpacing="5"
+                          IsVisible="{Binding IsBusy}">
+                        <controls:Spinner Width="12" Height="12" VerticalAlignment="Center" />
 
-                            <ItemsControl.ItemsPanel>
-                                <ItemsPanelTemplate>
-                                    <StackPanel Spacing="5" />
-                                </ItemsPanelTemplate>
-                            </ItemsControl.ItemsPanel>
-                        </ItemsControl>
+                        <TextBlock Grid.Column="1"
+                                   Foreground="{DynamicResource ThemeForegroundLowBrush}" FontSize="11"
+                                   FontStyle="Italic"
+                                   Text="{Binding WorkingStatusText}" />
+                    </Grid>
 
-                        <!-- Loading Indicator -->
-                        <Grid Grid.Row="1" ColumnDefinitions="Auto, Auto" ColumnSpacing="5"
-                              IsVisible="{Binding IsBusy}">
-                            <controls:Spinner Width="12" Height="12" VerticalAlignment="Center" />
+                    <!-- Queued messages (dimmed, pinned below the conversation) -->
+                    <ItemsControl Opacity="0.5" ItemsSource="{Binding QueuedMessages}">
+                        <ItemsControl.ItemsPanel>
+                            <ItemsPanelTemplate>
+                                <StackPanel Spacing="5" />
+                            </ItemsPanelTemplate>
+                        </ItemsControl.ItemsPanel>
+                    </ItemsControl>
+                </StackPanel>
 
-                            <TextBlock Grid.Column="1"
-                                       Foreground="{DynamicResource ThemeForegroundLowBrush}" FontSize="11"
-                                       FontStyle="Italic"
-                                       Text="{Binding WorkingStatusText}" />
-                        </Grid>
+                <ScrollViewer Name="ScrollViewer" HorizontalScrollBarVisibility="Disabled"
+                              VerticalScrollBarVisibility="Auto">
+                    <Interaction.Behaviors>
+                        <AutoScrollDuringDragBehavior ScrollDelta="4" />
+                    </Interaction.Behaviors>
+                    
+                    <ItemsControl Name="ItemsControl" ItemsSource="{Binding Messages}" Margin="10 0">
+                        <ItemsControl.Styles>
+                            <Style Selector="Expander /template/ ToggleButton">
+                                <Setter Property="Background" Value="{DynamicResource ThemeAccentBrush4}" />
+                                <Setter Property="CornerRadius" Value="3" />
+                            </Style>
+                            <Style Selector="Expander /template/ ToggleButton:pointerover /template/ Border">
+                                <Setter Property="Background" Value="{DynamicResource ThemeAccentBrush4}" />
+                            </Style>
+                            <Style Selector="Expander /template/ ToggleButton:pointerover /template/ Border Border">
+                                <Setter Property="Background" Value="Transparent" />
+                            </Style>
+                            <Style Selector="Expander /template/ ToggleButton#PART_toggle /template/ Border">
+                                <Setter Property="CornerRadius" Value="3" />
+                                <Setter Property="VerticalAlignment" Value="Center" />
+                            </Style>
+                            <Style Selector="Expander /template/ ToggleButton#PART_toggle /template/ Grid > Border">
+                                <Setter Property="Margin" Value="0 2" />
+                                <Setter Property="VerticalAlignment" Value="Center" />
+                            </Style>
+                        </ItemsControl.Styles>
 
-                        <!-- Queued messages (dimmed, pinned below the conversation) -->
-                        <ItemsControl Grid.Row="2" Opacity="0.5"
-                                      ItemsSource="{Binding QueuedMessages}">
-                            <ItemsControl.ItemsPanel>
-                                <ItemsPanelTemplate>
-                                    <StackPanel Spacing="5" />
-                                </ItemsPanelTemplate>
-                            </ItemsControl.ItemsPanel>
-                        </ItemsControl>
-                    </Grid>
-                </Border>
-            </ScrollViewer>
+                        <ItemsControl.ItemsPanel>
+                            <ItemsPanelTemplate>
+                                <controls:EstimatingVirtualizingStackPanel Spacing="5"/>
+                            </ItemsPanelTemplate>
+                        </ItemsControl.ItemsPanel>
+                    </ItemsControl>
+                </ScrollViewer>
+            </DockPanel>
 
             <!--
             <Border VerticalAlignment="Center" HorizontalAlignment="Center"
diff --git a/src/OneWare.Essentials/Controls/EstimatingVirtualizingStackPanel.cs b/src/OneWare.Essentials/Controls/EstimatingVirtualizingStackPanel.cs
new file mode 100644
index 00000000..33c1257a
--- /dev/null
+++ b/src/OneWare.Essentials/Controls/EstimatingVirtualizingStackPanel.cs
@@ -0,0 +1,532 @@
+using System.Collections.Generic;
+using System.Collections.Specialized;
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Generators;
+using Avalonia.Input;
+using Avalonia.Layout;
+using Avalonia.VisualTree;
+
+namespace OneWare.Essentials.Controls;
+
+/// <summary>
+/// A vertical virtualizing panel that keeps a per-item height cache to provide a stable,
+/// content-accurate scroll extent.
+/// </summary>
+/// <remarks>
+/// Unlike <see cref="VirtualizingStackPanel"/>, which estimates the total scroll extent from the
+/// running average height of the currently realized items (causing the scrollbar to jump when
+/// items vary a lot in height), this panel tracks every item's height individually:
+/// <list type="bullet">
+/// <item>Realized items use their real measured height.</item>
+/// <item>Unrealized items use an estimate, ideally provided by <see cref="IEstimatedHeightItem"/>
+/// (e.g. derived from a chat message's markdown), otherwise <see cref="EstimatedItemHeight"/>.</item>
+/// </list>
+/// The reported <see cref="Layoutable.DesiredSize"/> is the sum of all cached heights, so the
+/// scrollbar reflects the real content size. Realized children are registered with the ancestor
+/// <see cref="IScrollAnchorProvider"/> so the surrounding <see cref="ScrollViewer"/> keeps the
+/// visible content anchored when a cached estimate is replaced by a real measurement.
+/// </remarks>
+public class EstimatingVirtualizingStackPanel : VirtualizingPanel
+{
+    public static readonly StyledProperty<double> SpacingProperty =
+        AvaloniaProperty.Register<EstimatingVirtualizingStackPanel, double>(nameof(Spacing));
+
+    public static readonly StyledProperty<double> EstimatedItemHeightProperty =
+        AvaloniaProperty.Register<EstimatingVirtualizingStackPanel, double>(
+            nameof(EstimatedItemHeight), 48);
+
+    public static readonly StyledProperty<double> CacheLengthProperty =
+        AvaloniaProperty.Register<EstimatingVirtualizingStackPanel, double>(
+            nameof(CacheLength), 0.5);
+
+    private readonly List<double> _sizes = new();
+    private readonly List<bool> _measured = new();
+    private readonly Dictionary<int, RealizedContainer> _realized = new();
+    private readonly List<int> _scratch = new();
+
+    private Rect _viewport;
+    private double _lastWidth = 400;
+    private IScrollAnchorProvider? _scrollAnchorProvider;
+
+    static EstimatingVirtualizingStackPanel()
+    {
+        AffectsMeasure<EstimatingVirtualizingStackPanel>(SpacingProperty);
+    }
+
+    /// <summary>Gets or sets the gap, in pixels, between successive items.</summary>
+    public double Spacing
+    {
+        get => GetValue(SpacingProperty);
+        set => SetValue(SpacingProperty, value);
+    }
+
+    /// <summary>
+    /// Fallback estimated height (pixels) for items that do not implement
+    /// <see cref="IEstimatedHeightItem"/>.
+    /// </summary>
+    public double EstimatedItemHeight
+    {
+        get => GetValue(EstimatedItemHeightProperty);
+        set => SetValue(EstimatedItemHeightProperty, value);
+    }
+
+    /// <summary>
+    /// How many viewports worth of items to realize on each side of the visible area. A larger
+    /// value reduces realization churn (and markdown re-rendering) while scrolling.
+    /// </summary>
+    public double CacheLength
+    {
+        get => GetValue(CacheLengthProperty);
+        set => SetValue(CacheLengthProperty, value);
+    }
+
+    protected override Size MeasureOverride(Size availableSize)
+    {
+        var items = Items;
+        var count = items.Count;
+        SyncCache(count);
+
+        if (count == 0)
+        {
+            RecycleAll();
+            return default;
+        }
+
+        var width = double.IsInfinity(availableSize.Width) ? _lastWidth : availableSize.Width;
+        if (System.Math.Abs(width - _lastWidth) > 0.5)
+        {
+            _lastWidth = width;
+            for (var i = 0; i < _sizes.Count; i++)
+                if (!_measured[i])
+                    _sizes[i] = EstimateAt(i);
+        }
+        else
+        {
+            _lastWidth = width;
+        }
+        var spacing = Spacing;
+
+        var viewportStart = _viewport.Top;
+        var viewportEnd = _viewport.Bottom;
+        var viewportHeight = viewportEnd - viewportStart;
+
+        // Before any effective viewport has been reported we still realize the first item so that
+        // the layout system has something to measure; a real pass follows once the viewport is known.
+        if (viewportHeight <= 0)
+        {
+            viewportStart = 0;
+            viewportEnd = 0;
+        }
+
+        var cache = System.Math.Max(0, CacheLength) * viewportHeight;
+        var realizeStart = viewportStart - cache;
+        var realizeEnd = viewportEnd + cache;
+
+        var first = IndexAt(realizeStart, spacing);
+        var keep = new HashSet<int>();
+        var maxWidth = 0.0;
+
+        var index = first;
+        var u = TopOf(first, spacing);
+
+        while (index < count)
+        {
+            var container = Realize(index);
+            container.Measure(new Size(width, double.PositiveInfinity));
+            var h = container.DesiredSize.Height;
+            _sizes[index] = h;
+            _measured[index] = true;
+            maxWidth = System.Math.Max(maxWidth, container.DesiredSize.Width);
+            keep.Add(index);
+
+            u += h + spacing;
+            index++;
+
+            if (u >= realizeEnd)
+                break;
+        }
+
+        RecycleExcept(keep);
+
+        return new Size(System.Math.Max(width, maxWidth), TotalHeight(spacing));
+    }
+
+    protected override Size ArrangeOverride(Size finalSize)
+    {
+        var spacing = Spacing;
+
+        foreach (var pair in _realized)
+        {
+            var i = pair.Key;
+            var container = pair.Value.Container;
+            var top = TopOf(i, spacing);
+            container.Arrange(new Rect(0, top, finalSize.Width, _sizes[i]));
+            _scrollAnchorProvider?.RegisterAnchorCandidate(container);
+        }
+
+        return new Size(finalSize.Width, System.Math.Max(finalSize.Height, TotalHeight(spacing)));
+    }
+
+    protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
+    {
+        base.OnAttachedToVisualTree(e);
+        EffectiveViewportChanged += OnEffectiveViewportChanged;
+        _scrollAnchorProvider = this.FindAncestorOfType<IScrollAnchorProvider>();
+    }
+
+    protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
+    {
+        base.OnDetachedFromVisualTree(e);
+        EffectiveViewportChanged -= OnEffectiveViewportChanged;
+        _scrollAnchorProvider = null;
+    }
+
+    protected override void OnItemsChanged(IReadOnlyList<object?> items, NotifyCollectionChangedEventArgs e)
+    {
+        switch (e.Action)
+        {
+            case NotifyCollectionChangedAction.Add:
+            {
+                var start = e.NewStartingIndex;
+                var n = e.NewItems!.Count;
+                ShiftRealized(start, n);
+                for (var k = 0; k < n; k++)
+                {
+                    _sizes.Insert(start + k, EstimateAt(start + k));
+                    _measured.Insert(start + k, false);
+                }
+                break;
+            }
+            case NotifyCollectionChangedAction.Remove:
+            {
+                var start = e.OldStartingIndex;
+                var n = e.OldItems!.Count;
+                for (var i = start; i < start + n; i++)
+                    RecycleIndex(i);
+                if (start < _sizes.Count)
+                {
+                    var removable = System.Math.Min(n, _sizes.Count - start);
+                    _sizes.RemoveRange(start, removable);
+                    _measured.RemoveRange(start, removable);
+                }
+                ShiftRealized(start + n, -n);
+                break;
+            }
+            case NotifyCollectionChangedAction.Replace:
+            {
+                var start = e.OldStartingIndex;
+                var n = e.OldItems!.Count;
+                for (var i = start; i < start + n; i++)
+                {
+                    RecycleIndex(i);
+                    if (i < _sizes.Count)
+                    {
+                        _sizes[i] = EstimateAt(i);
+                        _measured[i] = false;
+                    }
+                }
+                break;
+            }
+            default:
+            {
+                RecycleAll();
+                _sizes.Clear();
+                _measured.Clear();
+                SyncCache(items.Count);
+                break;
+            }
+        }
+
+        InvalidateMeasure();
+    }
+
+    protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
+    {
+        base.OnPropertyChanged(change);
+        if (change.Property == EstimatedItemHeightProperty || change.Property == CacheLengthProperty)
+            InvalidateMeasure();
+    }
+
+    private void OnEffectiveViewportChanged(object? sender, EffectiveViewportChangedEventArgs e)
+    {
+        var oldStart = _viewport.Top;
+        var oldEnd = _viewport.Bottom;
+
+        _viewport = e.EffectiveViewport.Intersect(new Rect(Bounds.Size));
+
+        if (!AreClose(oldStart, _viewport.Top) || !AreClose(oldEnd, _viewport.Bottom))
+            InvalidateMeasure();
+    }
+
+    private static bool AreClose(double a, double b) => System.Math.Abs(a - b) < 0.5;
+
+    // --- Height cache helpers -------------------------------------------------------------
+
+    private void SyncCache(int count)
+    {
+        if (_sizes.Count == count)
+            return;
+
+        if (count < _sizes.Count)
+        {
+            _sizes.RemoveRange(count, _sizes.Count - count);
+            _measured.RemoveRange(count, _measured.Count - count);
+        }
+        else
+        {
+            for (var k = _sizes.Count; k < count; k++)
+            {
+                _sizes.Add(EstimateAt(k));
+                _measured.Add(false);
+            }
+        }
+    }
+
+    private double EstimateAt(int index)
+    {
+        var item = index >= 0 && index < Items.Count ? Items[index] : null;
+        var height = item is IEstimatedHeightItem e ? e.EstimateHeight(_lastWidth) : EstimatedItemHeight;
+        return height > 1 ? height : 1;
+    }
+
+    private double TopOf(int index, double spacing)
+    {
+        var u = 0.0;
+        var max = System.Math.Min(index, _sizes.Count);
+        for (var i = 0; i < max; i++)
+            u += _sizes[i] + spacing;
+        return u;
+    }
+
+    private double TotalHeight(double spacing)
+    {
+        var total = 0.0;
+        for (var i = 0; i < _sizes.Count; i++)
+            total += _sizes[i];
+        if (_sizes.Count > 1)
+            total += spacing * (_sizes.Count - 1);
+        return total;
+    }
+
+    private int IndexAt(double position, double spacing)
+    {
+        if (position <= 0 || _sizes.Count == 0)
+            return 0;
+
+        var u = 0.0;
+        for (var i = 0; i < _sizes.Count; i++)
+        {
+            var bottom = u + _sizes[i];
+            if (bottom > position)
+                return i;
+            u = bottom + spacing;
+        }
+
+        return _sizes.Count - 1;
+    }
+
+    // --- Realization ----------------------------------------------------------------------
+
+    private Control Realize(int index)
+    {
+        if (_realized.TryGetValue(index, out var existing))
+            return existing.Container;
+
+        var item = Items[index];
+        var generator = ItemContainerGenerator!;
+        Control container;
+        object? recycleKey = null;
+        var ownContainer = false;
+
+        if (generator.NeedsContainer(item, index, out recycleKey))
+        {
+            container = generator.CreateContainer(item, index, recycleKey);
+        }
+        else
+        {
+            ownContainer = true;
+            container = (Control)item!;
+        }
+
+        generator.PrepareItemContainer(container, item, index);
+        AddInternalChild(container);
+        generator.ItemContainerPrepared(container, item, index);
+
+        _realized[index] = new RealizedContainer(container, recycleKey, ownContainer);
+        return container;
+    }
+
+    private void RecycleExcept(HashSet<int> keep)
+    {
+        if (_realized.Count == 0)
+            return;
+
+        _scratch.Clear();
+        foreach (var index in _realized.Keys)
+        {
+            if (!keep.Contains(index))
+                _scratch.Add(index);
+        }
+
+        foreach (var index in _scratch)
+            RecycleIndex(index);
+    }
+
+    private void RecycleAll()
+    {
+        if (_realized.Count == 0)
+            return;
+
+        _scratch.Clear();
+        foreach (var index in _realized.Keys)
+            _scratch.Add(index);
+
+        foreach (var index in _scratch)
+            RecycleIndex(index);
+    }
+
+    private void RecycleIndex(int index)
+    {
+        if (!_realized.TryGetValue(index, out var realized))
+            return;
+
+        _realized.Remove(index);
+        _scrollAnchorProvider?.UnregisterAnchorCandidate(realized.Container);
+
+        if (realized.IsOwnContainer)
+        {
+            RemoveInternalChild(realized.Container);
+        }
+        else
+        {
+            ItemContainerGenerator!.ClearItemContainer(realized.Container);
+            RemoveInternalChild(realized.Container);
+        }
+    }
+
+    private void ShiftRealized(int fromIndex, int delta)
+    {
+        if (delta == 0 || _realized.Count == 0)
+            return;
+
+        var generator = ItemContainerGenerator;
+        var moved = new List<KeyValuePair<int, RealizedContainer>>();
+
+        _scratch.Clear();
+        foreach (var pair in _realized)
+        {
+            if (pair.Key >= fromIndex)
+            {
+                moved.Add(pair);
+                _scratch.Add(pair.Key);
+            }
+        }
+
+        foreach (var index in _scratch)
+            _realized.Remove(index);
+
+        foreach (var pair in moved)
+        {
+            var newIndex = pair.Key + delta;
+            _realized[newIndex] = pair.Value;
+            generator?.ItemContainerIndexChanged(pair.Value.Container, pair.Key, newIndex);
+        }
+    }
+
+    // --- VirtualizingPanel abstract members -----------------------------------------------
+
+    protected override Control? ScrollIntoView(int index)
+    {
+        if (index < 0 || index >= Items.Count)
+            return null;
+
+        var container = Realize(index);
+        container.Measure(new Size(_lastWidth, double.PositiveInfinity));
+        _sizes[index] = container.DesiredSize.Height;
+        _measured[index] = true;
+        container.BringIntoView();
+        return container;
+    }
+
+    protected override Control? ContainerFromIndex(int index)
+    {
+        return _realized.TryGetValue(index, out var realized) ? realized.Container : null;
+    }
+
+    protected override int IndexFromContainer(Control container)
+    {
+        foreach (var pair in _realized)
+        {
+            if (ReferenceEquals(pair.Value.Container, container))
+                return pair.Key;
+        }
+
+        return -1;
+    }
+
+    protected override IEnumerable<Control>? GetRealizedContainers()
+    {
+        var result = new List<Control>(_realized.Count);
+        foreach (var pair in _realized)
+            result.Add(pair.Value.Container);
+        return result;
+    }
+
+    protected override IInputElement? GetControl(NavigationDirection direction, IInputElement? from, bool wrap)
+    {
+        var count = Items.Count;
+        if (count == 0)
+            return null;
+
+        var fromIndex = from is Control c ? IndexFromContainer(c) : -1;
+        int toIndex;
+
+        switch (direction)
+        {
+            case NavigationDirection.First:
+                toIndex = 0;
+                break;
+            case NavigationDirection.Last:
+                toIndex = count - 1;
+                break;
+            case NavigationDirection.Next:
+            case NavigationDirection.Down:
+                toIndex = fromIndex + 1;
+                break;
+            case NavigationDirection.Previous:
+            case NavigationDirection.Up:
+                toIndex = fromIndex - 1;
+                break;
+            default:
+                return null;
+        }
+
+        if (wrap)
+        {
+            if (toIndex < 0)
+                toIndex = count - 1;
+            else if (toIndex >= count)
+                toIndex = 0;
+        }
+
+        if (toIndex < 0 || toIndex >= count)
+            return null;
+
+        return ScrollIntoView(toIndex);
+    }
+
+    private readonly struct RealizedContainer
+    {
+        public RealizedContainer(Control container, object? recycleKey, bool isOwnContainer)
+        {
+            Container = container;
+            RecycleKey = recycleKey;
+            IsOwnContainer = isOwnContainer;
+        }
+
+        public Control Container { get; }
+        public object? RecycleKey { get; }
+        public bool IsOwnContainer { get; }
+    }
+}
diff --git a/src/OneWare.Essentials/Controls/IEstimatedHeightItem.cs b/src/OneWare.Essentials/Controls/IEstimatedHeightItem.cs
new file mode 100644
index 00000000..1b7168e7
--- /dev/null
+++ b/src/OneWare.Essentials/Controls/IEstimatedHeightItem.cs
@@ -0,0 +1,15 @@
+namespace OneWare.Essentials.Controls;
+
+/// <summary>
+/// Implemented by items that can provide an estimated pixel height before they are realized
+/// and measured. Used by <see cref="EstimatingVirtualizingStackPanel"/> to compute a stable,
+/// content-accurate scroll extent (e.g. estimating a chat message's height from its markdown).
+/// </summary>
+public interface IEstimatedHeightItem
+{
+    /// <summary>
+    /// Returns an estimated height in pixels for the given available width.
+    /// The estimate is replaced by the real measured height once the item is realized.
+    /// </summary>
+    double EstimateHeight(double width);
+}