[Controls] Add drag-to-select marquee to GroupedTileListBox

samples/SampleApp/DemoPages/GroupedTileListBoxDemo.axaml

@@ -304,6 +304,7 @@
         <TextBlock TextWrapping="Wrap" Foreground="{DynamicResource TextControlPlaceholderForeground}">
           SelectionMode="Multiple" with SelectedItems bound two-way.
           Pointer: click replaces, Ctrl/Cmd+click toggles, Shift+click selects a range.
+          Drag on empty space to marquee-select (Ctrl/Cmd+drag adds to the selection); drag past the top/bottom edge to auto-scroll.
           Keyboard: arrows move, Shift+arrows extend the range, Ctrl/Cmd+Space toggles the current tile, Ctrl/Cmd+A selects all.
         </TextBlock>
 
@@ -344,11 +345,10 @@
         <!-- Resizable Grid -->
         <Grid Height="400" ColumnDefinitions="2*,Auto,*">
           <GroupedTileListBox Grid.Column="0"
-                              ItemsSource="{Binding GroupedItems}"
+                              ItemsSource="{Binding MultiSelectGroupedItems}"
                               SelectionMode="Multiple"
                               SelectedItems="{Binding MultiSelectedItems}"
                               GroupSelector="{Binding CategoryGroupSelector}"
-                              GroupOrderSelector="{Binding CategoryGroupOrderSelector}"
                               GroupOrderAlphabetical="True"
                               ItemWidth="120"
                               ItemHeight="100"

samples/SampleApp/ViewModels/GroupedTileListBoxViewModel.cs

@@ -78,6 +78,38 @@ private void SelectLargeAtTarget()
     // the control mutates this collection directly and the demo observes it live.
     public AvaloniaList<FoodItem> MultiSelectedItems { get; } = new();
 
+    // Scenario 5 source. Deliberately large and multi-grouped (many groups, many items per group,
+    // uneven counts so last rows are partial) so marquee drag-selection, cross-group rectangles, and
+    // edge auto-scroll can all be exercised. Kept separate from GroupedItems so Scenario 1 is unaffected.
+    public List<FoodItem> MultiSelectGroupedItems { get; } = GenerateMultiSelectItems();
+
+    private static List<FoodItem> GenerateMultiSelectItems()
+    {
+        // (group label, item count) — varied counts on purpose to produce partial last rows.
+        (string Group, int Count)[] groups =
+        {
+            ("Group A", 42),
+            ("Group B", 55),
+            ("Group C", 63),
+            ("Group D", 48),
+            ("Group E", 70),
+            ("Group F", 39),
+            ("Group G", 58),
+            ("Group H", 51),
+        };
+
+        var items = new List<FoodItem>();
+        foreach ((string group, int count) in groups)
+        {
+            for (int i = 1; i <= count; i++)
+            {
+                items.Add(new FoodItem($"{group} · {i:D2}", group));
+            }
+        }
+
+        return items;
+    }
+
     // Scenario 1: Named groups
     public List<FoodItem> GroupedItems { get; } = new()
     {

src/Devolutions.AvaloniaControls/Controls/GroupedTileListBox/GroupedTileListBox.axaml

@@ -2,6 +2,13 @@
     xmlns="https://github.com/avaloniaui"
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
 
+    <!-- Marquee (drag-to-select rectangle) brushes. Accent-tinted to match the Explorer-style
+         rubber-band; theme-aware via SystemAccentColor, so no per-theme override is required. -->
+    <SolidColorBrush x:Key="GroupedTileListBoxSelectionRectangleFill"
+                     Color="{DynamicResource SystemAccentColor}" Opacity="0.2" />
+    <SolidColorBrush x:Key="GroupedTileListBoxSelectionRectangleBorderBrush"
+                     Color="{DynamicResource SystemAccentColor}" Opacity="0.8" />
+
     <ControlTheme x:Key="{x:Type GroupedTileListBox}" TargetType="GroupedTileListBox">
         <Setter Property="Background" Value="Transparent" />
         <Setter Property="Padding" Value="4" />
@@ -29,10 +36,25 @@
                         BorderThickness="{TemplateBinding BorderThickness}"
                         CornerRadius="{TemplateBinding CornerRadius}"
                         ClipToBounds="True">
-                        <ScrollViewer Name="PART_ScrollViewer"
-                                      Background="{TemplateBinding Background}"
-                                      HorizontalScrollBarVisibility="Disabled"
-                                      VerticalScrollBarVisibility="Auto" />
+                        <Panel>
+                            <ScrollViewer Name="PART_ScrollViewer"
+                                          Background="{TemplateBinding Background}"
+                                          HorizontalScrollBarVisibility="Disabled"
+                                          VerticalScrollBarVisibility="Auto" />
+                            <!--
+                                Marquee overlay. Shares the ScrollViewer's origin so a pointer
+                                position relative to PART_ScrollViewer maps directly onto this Canvas
+                                (viewport space) and onto content space (viewport + Offset). It never
+                                scrolls with content and never intercepts pointer input.
+                            -->
+                            <Canvas Name="PART_SelectionLayer" IsHitTestVisible="False">
+                                <Border Name="PART_SelectionRectangle"
+                                        IsVisible="False"
+                                        BorderThickness="1"
+                                        Background="{DynamicResource GroupedTileListBoxSelectionRectangleFill}"
+                                        BorderBrush="{DynamicResource GroupedTileListBoxSelectionRectangleBorderBrush}" />
+                            </Canvas>
+                        </Panel>
                     </Border>
                 </Panel>
             </ControlTemplate>