[Controls] Add CalendarDatePicker inner content slots

samples/SampleApp/DemoPages/CalendarDatePickerDemo.axaml

@@ -27,7 +27,7 @@
           RowSpacing="30">
       <Grid Grid.Column="0" Grid.Row="0"
             ColumnDefinitions="Auto, 250"
-            RowDefinitions="Auto, Auto, Auto, Auto, Auto, Auto, Auto"
+            RowDefinitions="Auto, Auto, Auto, Auto, Auto, Auto, Auto, Auto"
             ColumnSpacing="15"
             RowSpacing="14">
         <TextBlock Grid.Row="0" Grid.Column="0" Text="Placeholder"   VerticalAlignment="Center" />
@@ -37,6 +37,7 @@
         <TextBlock Grid.Row="4" Grid.Column="0" Text="Custom Format" VerticalAlignment="Center" />
         <TextBlock Grid.Row="5" Grid.Column="0" Text="Reset"         VerticalAlignment="Center" />
         <TextBlock Grid.Row="6" Grid.Column="0" Text="Error"         VerticalAlignment="Center" />
+        <TextBlock Grid.Row="7" Grid.Column="0" Text="Inner slots"   VerticalAlignment="Center" />
 
         <CalendarDatePicker Grid.Row="0" Grid.Column="1" HorizontalAlignment="Left" />
         <CalendarDatePicker Grid.Row="1" Grid.Column="1" Name="DatePicker"  HorizontalAlignment="Left" />
@@ -58,6 +59,20 @@
             </system:Exception>
           </DataValidationErrors.Error>
         </CalendarDatePicker>
+        <StackPanel Grid.Row="7" Grid.Column="1" Orientation="Horizontal" Spacing="10">
+          <CalendarDatePicker HorizontalAlignment="Left">
+            <CalendarDatePickerContent.InnerLeftContent>
+              <Button Command="{Binding GreenCommand}"><Border Width="8" Height="8" Background="Green" /></Button>
+            </CalendarDatePickerContent.InnerLeftContent>
+            <CalendarDatePickerContent.InnerLeftOfCalendarButtonContent>
+              <Button Command="{Binding OrangeCommand}"><Border Width="8" Height="8" Background="Orange" /></Button>
+            </CalendarDatePickerContent.InnerLeftOfCalendarButtonContent>
+            <CalendarDatePickerContent.InnerRightContent>
+              <Button Command="{Binding RedCommand}"><Border Width="8" Height="8" Background="Red" /></Button>
+            </CalendarDatePickerContent.InnerRightContent>
+          </CalendarDatePicker>
+          <Border Width="12" Height="12" VerticalAlignment="Center" Background="{Binding SelectedBrush}" />
+        </StackPanel>
       </Grid>
 
       <StackPanel Grid.Row="0" Grid.Column="1"

samples/SampleApp/ViewModels/CalendarDatePickerViewModel.cs

@@ -2,6 +2,7 @@
 
 using System;
 using System.Collections.ObjectModel;
+using Avalonia.Media;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
 
@@ -15,4 +16,16 @@ private void SetToToday()
     {
         this.SelectedDate = DateTime.Today;
     }
+
+    [ObservableProperty]
+    private IBrush selectedBrush = Brushes.Transparent;
+
+    [RelayCommand]
+    private void Green() => this.SelectedBrush = Brushes.Green;
+
+    [RelayCommand]
+    private void Orange() => this.SelectedBrush = Brushes.Orange;
+
+    [RelayCommand]
+    private void Red() => this.SelectedBrush = Brushes.Red;
 }

src/Devolutions.AvaloniaControls/AttachedProperties/CalendarDatePickerContent.cs

@@ -0,0 +1,48 @@
+namespace Devolutions.AvaloniaControls.AttachedProperties;
+
+using System.Collections;
+using Avalonia;
+using Avalonia.Collections;
+using Avalonia.Controls;
+
+public static class CalendarDatePickerContent
+{
+    public static readonly AttachedProperty<IEnumerable> InnerLeftContentProperty =
+        AvaloniaProperty.RegisterAttached<CalendarDatePicker, IEnumerable>("InnerLeftContent", typeof(CalendarDatePickerContent));
+
+    public static readonly AttachedProperty<IEnumerable> InnerLeftOfCalendarButtonContentProperty =
+        AvaloniaProperty.RegisterAttached<CalendarDatePicker, IEnumerable>("InnerLeftOfCalendarButtonContent", typeof(CalendarDatePickerContent));
+
+    public static readonly AttachedProperty<IEnumerable> InnerRightContentProperty =
+        AvaloniaProperty.RegisterAttached<CalendarDatePicker, IEnumerable>("InnerRightContent", typeof(CalendarDatePickerContent));
+
+    public static IEnumerable GetInnerLeftContent(CalendarDatePicker element) => GetOrCreate(element, InnerLeftContentProperty);
+
+    public static void SetInnerLeftContent(CalendarDatePicker element, IEnumerable value) => element.SetValue(InnerLeftContentProperty, value);
+
+    public static IEnumerable GetInnerLeftOfCalendarButtonContent(CalendarDatePicker element) =>
+        GetOrCreate(element, InnerLeftOfCalendarButtonContentProperty);
+
+    public static void SetInnerLeftOfCalendarButtonContent(CalendarDatePicker element, IEnumerable value) =>
+        element.SetValue(InnerLeftOfCalendarButtonContentProperty, value);
+
+    public static IEnumerable GetInnerRightContent(CalendarDatePicker element) => GetOrCreate(element, InnerRightContentProperty);
+
+    public static void SetInnerRightContent(CalendarDatePicker element, IEnumerable value) => element.SetValue(InnerRightContentProperty, value);
+
+    // Attached-property defaults are shared across all instances, which is not what we want for
+    // mutable collections. Lazily create a per-instance AvaloniaList<Control> on first read so
+    // XAML element-syntax (<CalendarDatePickerContent.InnerLeftContent>...</...>) can Add into it.
+    private static IEnumerable GetOrCreate(CalendarDatePicker element, AttachedProperty<IEnumerable> property)
+    {
+        if (element.GetValue(property) is { } existing)
+        {
+            return existing;
+        }
+
+        // ReSharper disable once CollectionNeverUpdated.Local
+        AvaloniaList<Control> list = [];
+        element.SetValue(property, list);
+        return list;
+    }
+}

src/Devolutions.AvaloniaControls/Behaviors/CalendarDatePickerBehavior.cs

@@ -1,9 +1,12 @@
-using Avalonia;
+using Avalonia;
 
 namespace Devolutions.AvaloniaControls.Behaviors;
 
+using System;
+using System.Linq;
 using Avalonia.Controls;
 using Avalonia.Controls.Primitives;
+using Avalonia.Threading;
 using Avalonia.VisualTree;
 
 public static class CalendarDatePickerBehavior
@@ -15,30 +18,62 @@ static CalendarDatePickerBehavior()
     {
         OpenOnSelectedDateProperty.Changed.Subscribe(args =>
         {
-            if (args.Sender is CalendarDatePicker datePicker)
+            if (args.Sender is not CalendarDatePicker datePicker) return;
+
+            if (args.NewValue.GetValueOrDefault<bool>())
+            {
+                datePicker.CalendarOpened += OnCalendarOpened;
+                datePicker.CalendarClosed += OnCalendarClosed;
+            }
+            else
             {
-                if (args.NewValue.GetValueOrDefault<bool>())
-                {
-                    datePicker.CalendarOpened += OnCalendarOpen;
-                }
-                else
-                {
-                    datePicker.CalendarOpened -= OnCalendarOpen;
-                }
+                datePicker.CalendarOpened -= OnCalendarOpened;
+                datePicker.CalendarClosed -= OnCalendarClosed;
             }
         });
     }
 
-    private static void OnCalendarOpen(object? sender, EventArgs e)
+    private static void OnCalendarOpened(object? sender, EventArgs e)
     {
-        if (sender is CalendarDatePicker datePicker)
-        {
-            Popup? popup = datePicker.GetVisualDescendants().OfType<Popup>().FirstOrDefault();
-            Calendar? calendar = popup?.Child?.FindDescendantOfType<Calendar>() ?? popup?.Child as Calendar;
+        if (sender is not CalendarDatePicker datePicker) return;
 
-            if (calendar == null) return;
+        Popup? popup = datePicker.GetVisualDescendants().OfType<Popup>().FirstOrDefault();
+        Calendar? calendar = popup?.Child?.FindDescendantOfType<Calendar>() ?? popup?.Child as Calendar;
+
+        if (calendar == null) return;
+
+        calendar.DisplayDate = datePicker.SelectedDate ?? DateTime.Now;
+
+        // Move keyboard focus into the calendar when the popup opens. The calendar button now lives inside
+        // PART_TextBox (to host inner-content slots), so opening no longer takes focus off the text box on
+        // its own — focus would otherwise stay in the box and its segmented arrow-keys would look like
+        // calendar input. Focusing the popup also lets Avalonia's light-dismiss close it on Tab again.
+        // (PART_Calendar sets Focusable="True" in each theme so this Focus() takes effect.)
+        Dispatcher.UIThread.Post(() => calendar.Focus(), DispatcherPriority.Input);
+    }
 
-            calendar.DisplayDate = datePicker.SelectedDate ?? DateTime.Now;
+    private static void OnCalendarClosed(object? sender, EventArgs e)
+    {
+        if (sender is not CalendarDatePicker datePicker) return;
+
+        // Focus was moved into the popup calendar on open (see OnCalendarOpened). The calendar lives in the
+        // popup's own tree, outside the window's tab order, so once the popup closes we return focus to the
+        // picker's text box — restoring the original tab flow (Tab then continues from the picker).
+        Dispatcher.UIThread.Post(() => ReturnFocusToPicker(datePicker), DispatcherPriority.Background);
+    }
+
+    // Returns focus to the picker's primary control. Prefers the text box, but on themes where it is hidden
+    // and non-focusable (Yaru shows a button instead) falls back to the button.
+    private static void ReturnFocusToPicker(CalendarDatePicker picker)
+    {
+        TextBox? textBox = picker.GetVisualDescendants().OfType<TextBox>().FirstOrDefault(tb => tb.Name == "PART_TextBox");
+        if (textBox is { Focusable: true, IsVisible: true })
+        {
+            textBox.Focus();
+            return;
         }
+
+        Button? button = picker.GetVisualDescendants().OfType<Button>().FirstOrDefault(b => b.Name == "PART_Button");
+        button?.Focus();
     }
-}
+}