Improvements to Weeknumber behavior

samples/ControlCatalog/Pages/CalendarDatePickerPage.xaml

@@ -49,6 +49,11 @@
 
         <TextBlock Text="Validation Example"/>
         <CalendarDatePicker SelectedDate="{CompiledBinding ValidatedDateExample, Mode=TwoWay}"/>
+        
+        <TextBlock Text="Show WeekNumbers" />
+        <CalendarDatePicker ShowWeekNumbers="True" 
+                            FirstDayOfWeek="Monday"
+                            WeekNumberRule="FirstFourDayWeek" />
       </StackPanel>
 
     </StackPanel>

src/Avalonia.Controls/Calendar/Calendar.cs

@@ -390,7 +390,7 @@ namespace Avalonia.Controls
         /// Defines the <see cref="WeekNumberHeader"/> property.
         /// </summary>
         public static readonly StyledProperty<object?> WeekNumberHeaderProperty =
-            AvaloniaProperty.Register<Calendar, object?>(nameof(WeekNumberHeader));
+            AvaloniaProperty.Register<Calendar, object?>(nameof(WeekNumberHeader), "#");
 
         /// <summary>
         /// Gets or sets the content displayed in the week-number column header cell.

src/Avalonia.Controls/Calendar/CalendarItem.cs

@@ -24,7 +24,7 @@ namespace Avalonia.Controls.Primitives
     [TemplatePart(PART_ElementNextButton,     typeof(Button))]
     [TemplatePart(PART_ElementPreviousButton, typeof(Button))]
     [TemplatePart(PART_ElementYearView,       typeof(Grid))]
-    [PseudoClasses(":calendardisabled")]
+    [PseudoClasses(":calendardisabled", ":showweeknumbers")]
     public sealed class CalendarItem : TemplatedControl
     {
         /// <summary>
@@ -632,6 +632,8 @@ namespace Avalonia.Controls.Primitives
             var rule = Owner?.WeekNumberRule ?? DateTimeHelper.GetCurrentDateFormat().CalendarWeekRule;
             var firstDayOfWeek = Owner?.FirstDayOfWeek ?? DateTimeHelper.GetCurrentDateFormat().FirstDayOfWeek;
 
+            PseudoClasses.Set(":showweeknumbers", show);
+
             for (int i = 0; i < _weekNumberLabels.Length; i++)
             {
                 DateTime firstDayOfRow = _calendar.AddDays(firstDateDisplayed, i * NumberOfDaysPerWeek);

src/Avalonia.Controls/CalendarDatePicker/CalendarDatePicker.Properties.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Globalization;
 using Avalonia.Controls.Primitives;
 using Avalonia.Data;
 using Avalonia.Layout;
@@ -140,6 +141,24 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<VerticalAlignment> VerticalContentAlignmentProperty =
             ContentControl.VerticalContentAlignmentProperty.AddOwner<CalendarDatePicker>();
 
+        /// <summary>
+        /// Defines the <see cref="ShowWeekNumbers"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> ShowWeekNumbersProperty =
+            Calendar.ShowWeekNumbersProperty.AddOwner<CalendarDatePicker>();
+
+        /// <summary>
+        /// Defines the <see cref="WeekNumberRule"/> property.
+        /// </summary>
+        public static readonly StyledProperty<CalendarWeekRule> WeekNumberRuleProperty =
+            Calendar.WeekNumberRuleProperty.AddOwner<CalendarDatePicker>();
+
+        /// <summary>
+        /// Defines the <see cref="WeekNumberHeader"/> property.
+        /// </summary>
+        public static readonly StyledProperty<object?> WeekNumberHeaderProperty =
+            Calendar.WeekNumberHeaderProperty.AddOwner<CalendarDatePicker>();
+
         /// <summary>
         /// Gets a collection of dates that are marked as not selectable.
         /// </summary>
@@ -358,5 +377,36 @@ namespace Avalonia.Controls
             get => GetValue(VerticalContentAlignmentProperty);
             set => SetValue(VerticalContentAlignmentProperty, value);
         }
+        
+        /// <summary>
+        /// Gets or sets a value indicating whether week numbers are shown in the month view
+        /// of the drop-down <see cref="Calendar"/>.
+        /// </summary>
+        public bool ShowWeekNumbers
+        {
+            get => GetValue(ShowWeekNumbersProperty);
+            set => SetValue(ShowWeekNumbersProperty, value);
+        }
+
+        /// <summary>
+        /// Gets or sets the rule used to determine the first week of the year for week number display.
+        /// The default is taken from the current culture.
+        /// </summary>
+        public CalendarWeekRule WeekNumberRule
+        {
+            get => GetValue(WeekNumberRuleProperty);
+            set => SetValue(WeekNumberRuleProperty, value);
+        }
+
+        /// <summary>
+        /// Gets or sets the content displayed in the week-number column header cell of the drop-down
+        /// <see cref="Calendar"/>. Set to a localized string such as <c>"CW"</c>, <c>"KW"</c>,
+        /// or <c>"Wk"</c> to give users context. Defaults to <c>null</c> (blank).
+        /// </summary>
+        public object? WeekNumberHeader
+        {
+            get => GetValue(WeekNumberHeaderProperty);
+            set => SetValue(WeekNumberHeaderProperty, value);
+        }
     }
 }

src/Avalonia.Themes.Fluent/Controls/CalendarDatePicker.xaml

@@ -133,7 +133,10 @@
                           SelectedDate="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=SelectedDate, Mode=TwoWay}"
                           DisplayDate="{TemplateBinding DisplayDate}"
                           DisplayDateStart="{TemplateBinding DisplayDateStart}"
-                          DisplayDateEnd="{TemplateBinding DisplayDateEnd}" />
+                          DisplayDateEnd="{TemplateBinding DisplayDateEnd}"
+                          ShowWeekNumbers="{TemplateBinding ShowWeekNumbers}"
+                          WeekNumberRule="{TemplateBinding WeekNumberRule}"
+                          WeekNumberHeader="{TemplateBinding WeekNumberHeader}" />
               </Popup>
             </Grid>
           </Panel>

src/Avalonia.Themes.Fluent/Controls/CalendarItem.xaml

@@ -73,9 +73,10 @@
           <!-- To keep calendar from resizing when switching DisplayMode
                  In WinUI Min-Width from TemplateSettings
                  basically...MinWidth of DayItem = 40, 40 * 7 = 280 + margins/padding = ~294
+                 With week numbers: 40 * 8 = 320 + margins/padding = ~334 (see :showweeknumbers style below)
                  Viewport height is set from # of rows displayed (2-8) in Month mode, = ~290 for 6 weeks (+ day names)
                  -->
-          <Grid VerticalAlignment="Stretch" HorizontalAlignment="Stretch" RowDefinitions="40,*" MinWidth="294">
+          <Grid x:Name="LayoutRoot" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" RowDefinitions="40,*" MinWidth="294">
             <Grid ColumnDefinitions="5*,*,*">
               <Button Name="PART_HeaderButton"
                       Theme="{StaticResource FluentCalendarButton}"
@@ -159,5 +160,12 @@
     <Style Selector="^ /template/ Button:pressed > Path">
       <Setter Property="Stroke" Value="{DynamicResource CalendarViewNavigationButtonForegroundPressed}" />
     </Style>
+    <!-- When week numbers are visible, the MonthView gains an extra 40px column.
+         Increase the LayoutRoot MinWidth to prevent the calendar from shrinking
+         when switching to year/decade view (which does not have that extra column).
+         334 = 40 (week-number col) + 294 (7 day-cols + margins/padding) -->
+    <Style Selector="^:showweeknumbers /template/ Grid#LayoutRoot">
+      <Setter Property="MinWidth" Value="334" />
+    </Style>
   </ControlTheme>
 </ResourceDictionary>

src/Avalonia.Themes.Simple/Controls/CalendarDatePicker.xaml

@@ -128,7 +128,10 @@
                       FirstDayOfWeek="{TemplateBinding FirstDayOfWeek}"
                       IsTodayHighlighted="{TemplateBinding IsTodayHighlighted}"
                       SelectedDate="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=SelectedDate,
-                                                     Mode=TwoWay}" />
+                                                     Mode=TwoWay}"
+                      ShowWeekNumbers="{TemplateBinding ShowWeekNumbers}"
+                      WeekNumberRule="{TemplateBinding WeekNumberRule}"
+                      WeekNumberHeader="{TemplateBinding WeekNumberHeader}" />
           </Popup>
         </Grid>
       </ControlTemplate>

src/Avalonia.Themes.Simple/Controls/CalendarItem.xaml

@@ -176,5 +176,12 @@
     <Style Selector="^:calendardisabled /template/ Rectangle#DisabledVisual">
       <Setter Property="IsVisible" Value="True" />
     </Style>
+    <!-- When week numbers are visible the MonthView is wider than the YearView.
+         Lock CalendarItem's own MinWidth so the size stays stable when switching
+         between month and year/decade views.
+         37px per CalendarButton × 4 cols = 148 + the week-number column (~30px) -->
+    <Style Selector="^:showweeknumbers">
+      <Setter Property="MinWidth" Value="200" />
+    </Style>
   </ControlTheme>
 </ResourceDictionary>