feat: wip.

src/Ursa.Themes.Semi/Controls/ToolBar.axaml

@@ -23,8 +23,8 @@
                             Fill="Gray" />
                         <ContentPresenter Margin="4 0" Content="{TemplateBinding Header}" ContentTemplate="{TemplateBinding HeaderTemplate}" />
                         <ItemsPresenter VerticalAlignment="Center" HorizontalAlignment="Left" ItemsPanel="{TemplateBinding ItemsPanel}"/>
-                        <Button Content="More"></Button>
-                        <Popup>
+                        <ToggleButton Name="button" Content="More"></ToggleButton>
+                        <Popup IsOpen="{Binding #button.IsChecked, Mode=TwoWay}" PlacementTarget="{Binding #button}" IsLightDismissEnabled="True">
                             <Border Theme="{DynamicResource CardBorder}">
                                 <StackPanel Name="{x:Static u:ToolBar.PART_OverflowPanel}"></StackPanel>
                             </Border>

src/Ursa/Controls/ToolBar/ToolBar.cs

@@ -36,6 +36,12 @@ public class ToolBar: HeaderedItemsControl
         set => SetValue(BandProperty, value);
     }
 
+    public static readonly AttachedProperty<OverflowMode> OverflowModeProperty =
+        AvaloniaProperty.RegisterAttached<ToolBar, Control?, OverflowMode>("OverflowMode");
+
+    public static void SetOverflowMode(Control? obj, OverflowMode value) => obj.SetValue(OverflowModeProperty, value);
+    public static OverflowMode GetOverflowMode(Control? obj) => obj.GetValue(OverflowModeProperty);
+
     static ToolBar()
     {
         IsTabStopProperty.OverrideDefaultValue<ToolBar>(false);

src/Ursa/Controls/ToolBar/ToolBarPanel.cs

@@ -28,7 +28,24 @@ public class ToolBarPanel: StackPanel
 
     protected override Size MeasureOverride(Size availableSize)
     {
-        return base.MeasureOverride(availableSize);
-        
+        var size = base.MeasureOverride(availableSize);
+        var children = this.Children;
+        var children2 = this.OverflowPanel?.Children;
+        var all = children.ToList();
+        if (children2 != null)
+        {
+            all.AddRange(children2);
+        }
+        this.Children.Clear();
+        OverflowPanel?.Children.Clear();
+        for (int i = 0; i < all.Count - 1; i++)
+        {
+            this.Children.Add(all[i]);
+        }
+        if (all.Count > 0)
+        {
+            OverflowPanel?.Children.Add(all.Last());
+        }
+        return size;
     }
 }