feat(ui): folders slide open and closed, rows below follow

src/ClaudeDo.Ui/Design/IslandStyles.axaml

@@ -867,19 +867,6 @@
     <Setter Property="BorderBrush"     Value="Transparent" />
   </Style>
   <!-- Folder tree rows sit tighter than the smart lists — a deep tree gets long fast. -->
-  <!-- Folder expand: children drop down from under the header. Collapse stays instant. -->
-  <Style Selector="Panel.folder-children">
-    <Setter Property="RenderTransformOrigin" Value="50%,0%" />
-    <Setter Property="RenderTransform" Value="scaleY(1)" />
-  </Style>
-  <Style Selector="Panel.folder-children.animate[IsVisible=True]">
-    <Style.Animations>
-      <Animation Duration="0:0:0.16" Easing="CubicEaseOut" FillMode="Forward">
-        <KeyFrame Cue="0%">  <Setter Property="ScaleTransform.ScaleY" Value="0.4"/><Setter Property="Opacity" Value="0"/></KeyFrame>
-        <KeyFrame Cue="100%"><Setter Property="ScaleTransform.ScaleY" Value="1"/><Setter Property="Opacity" Value="1"/></KeyFrame>
-      </Animation>
-    </Style.Animations>
-  </Style>
   <Style Selector="Border.list-item.tree-row">
     <Setter Property="Padding" Value="4,4,10,4" />
   </Style>

src/ClaudeDo.Ui/ViewModels/Islands/ListFolderNavItemViewModel.cs

@@ -18,8 +18,6 @@ public sealed partial class ListFolderNavItemViewModel : ViewModelBase
     [ObservableProperty] private string _iconKey = "Folder";
     /// <summary>Hex colour for name + guide line; null = theme default.</summary>
     [ObservableProperty] private string? _color;
-    /// <summary>Set on the first user toggle so the expand animation skips folders that load already open.</summary>
-    [ObservableProperty] private bool _animateExpand;
     [ObservableProperty]
     [NotifyPropertyChangedFor(nameof(ShowCount))]
     private bool _isCollapsed;

src/ClaudeDo.Ui/ViewModels/Islands/ListsIslandViewModel.cs

@@ -730,7 +730,6 @@ public sealed partial class ListsIslandViewModel : ViewModelBase, IDisposable
     private void ToggleFolder(ListFolderNavItemViewModel? folder)
     {
         if (folder is null) return;
-        folder.AnimateExpand = true;
         folder.IsCollapsed = !folder.IsCollapsed;
         if (_folderCollapseCts.TryGetValue(folder.Id, out var previous)) previous.Cancel();
         var cts = new CancellationTokenSource();

src/ClaudeDo.Ui/Views/Controls/ExpandPanel.cs

@@ -0,0 +1,99 @@
+using System;
+using System.Threading;
+using Avalonia;
+using Avalonia.Animation;
+using Avalonia.Animation.Easings;
+using Avalonia.Controls;
+using Avalonia.Styling;
+
+namespace ClaudeDo.Ui.Views.Controls;
+
+/// <summary>
+/// Accordion host: animates its height between 0 and the child's natural height, so content
+/// below slides instead of jumping. The first value (from the binding) applies without animation.
+/// </summary>
+public sealed class ExpandPanel : Decorator
+{
+    private static readonly TimeSpan Duration = TimeSpan.FromMilliseconds(180);
+
+    public static readonly StyledProperty<bool> IsExpandedProperty =
+        AvaloniaProperty.Register<ExpandPanel, bool>(nameof(IsExpanded));
+
+    /// <summary>0 = collapsed, 1 = fully open; drives the measured height.</summary>
+    public static readonly StyledProperty<double> ProgressProperty =
+        AvaloniaProperty.Register<ExpandPanel, double>(nameof(Progress));
+
+    private CancellationTokenSource? _cts;
+    private bool _arranged;
+
+    static ExpandPanel()
+    {
+        AffectsMeasure<ExpandPanel>(ProgressProperty);
+        ClipToBoundsProperty.OverrideDefaultValue<ExpandPanel>(true);
+    }
+
+    /// <summary>Whether the content is shown.</summary>
+    public bool IsExpanded
+    {
+        get => GetValue(IsExpandedProperty);
+        set => SetValue(IsExpandedProperty, value);
+    }
+
+    /// <summary>Current open fraction, animated between 0 and 1.</summary>
+    public double Progress
+    {
+        get => GetValue(ProgressProperty);
+        set => SetValue(ProgressProperty, value);
+    }
+
+    protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
+    {
+        base.OnPropertyChanged(change);
+        if (change.Property == IsExpandedProperty)
+            Animate(IsExpanded ? 1 : 0);
+        else if (change.Property == ProgressProperty && Child is { } child)
+            child.IsVisible = Progress > 0;
+    }
+
+    protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
+    {
+        base.OnAttachedToVisualTree(e);
+        if (Child is { } child) child.IsVisible = Progress > 0;
+    }
+
+    private void Animate(double to)
+    {
+        _cts?.Cancel();
+        var from = Progress;
+        Progress = to;
+        if (!_arranged || from == to) return;
+
+        _cts = new CancellationTokenSource();
+        var animation = new Animation
+        {
+            Duration = Duration,
+            Easing = new CubicEaseOut(),
+            Children =
+            {
+                new KeyFrame { Cue = new Cue(0), Setters = { new Setter(ProgressProperty, from) } },
+                new KeyFrame { Cue = new Cue(1), Setters = { new Setter(ProgressProperty, to) } },
+            },
+        };
+        _ = animation.RunAsync(this, _cts.Token);
+    }
+
+    protected override Size MeasureOverride(Size availableSize)
+    {
+        if (Child is not { } child) return default;
+        child.Measure(availableSize.WithHeight(double.PositiveInfinity));
+        return new Size(child.DesiredSize.Width, child.DesiredSize.Height * Progress);
+    }
+
+    protected override Size ArrangeOverride(Size finalSize)
+    {
+        _arranged = true;
+        // Full natural height, top-aligned; ClipToBounds cuts off what isn't revealed yet.
+        Child?.Arrange(new Rect(0, 0, finalSize.Width, Child.DesiredSize.Height));
+        return finalSize;
+    }
+}

src/ClaudeDo.Ui/Views/Islands/ListsIslandView.axaml

@@ -3,6 +3,7 @@
              xmlns:vm="using:ClaudeDo.Ui.ViewModels.Islands"
              xmlns:converters="using:ClaudeDo.Ui.Converters"
              xmlns:loc="using:ClaudeDo.Ui.Localization"
+             xmlns:ctl="using:ClaudeDo.Ui.Views.Controls"
              x:Class="ClaudeDo.Ui.Views.Islands.ListsIslandView"
              x:DataType="vm:ListsIslandViewModel">
   <!-- Implicit, type-matched templates: the sidebar's top level is one mixed ItemsControl over
@@ -162,13 +163,14 @@
 
           <!-- Guide line drops from under the chevron's centre (tree-row padding 4 + half the 9px
                glyph); children start past it so their highlight never touches the line. -->
-          <Panel Classes="folder-children" Classes.animate="{Binding AnimateExpand}"
-                 IsVisible="{Binding !IsCollapsed}">
-            <Border Width="1" HorizontalAlignment="Left" Margin="8,0,0,4"
-                    Background="{Binding Color, Converter={x:Static converters:HexBrushConverter.Instance}, ConverterParameter=LineBrush}"
-                    IsHitTestVisible="False"/>
-            <ItemsControl ItemsSource="{Binding Children}" Margin="14,0,0,0"/>
-          </Panel>
+          <ctl:ExpandPanel IsExpanded="{Binding !IsCollapsed}">
+            <Panel>
+              <Border Width="1" HorizontalAlignment="Left" Margin="8,0,0,4"
+                      Background="{Binding Color, Converter={x:Static converters:HexBrushConverter.Instance}, ConverterParameter=LineBrush}"
+                      IsHitTestVisible="False"/>
+              <ItemsControl ItemsSource="{Binding Children}" Margin="14,0,0,0"/>
+            </Panel>
+          </ctl:ExpandPanel>
         </StackPanel>
 
         <Border Grid.Row="2" Height="2" VerticalAlignment="Center" Margin="4,0"