Make column headers reachable and usable from the keyboard

docs/docs/accessibility.md

@@ -119,6 +119,13 @@ Uses `AutomationProperties.Name` if set on the `TableViewRowHeader`, otherwise d
 | **Ctrl+Shift+A** | Deselect all |
 | **Ctrl+C** | Copy to clipboard |
 
+The column header row is one tab stop: **Tab** to it, then move between headers with the **Left** / **Right** arrow keys.
+
+| Key (on a column header) | Action |
+|---|---|
+| **Enter** / **Space** | Sort by the column (cycles like a click); **Ctrl** adds to the existing sort, like **Ctrl**+click |
+| **Alt+Down**, **Shift+F10** or the **Menu** key | Open the column's options (sort, group, filter) |
+
 ## Screen reader behavior
 
 When navigating with a screen reader such as Narrator:

src/TableViewColumnHeader.cs

@@ -61,6 +61,7 @@ public TableViewColumnHeader()
         ManipulationMode = ManipulationModes.TranslateX;
         RegisterPropertyChangedCallback(WidthProperty, OnWidthChanged);
         RightTapped += OnRightTapped;
+        ContextRequested += OnContextRequested;
     }
 
     /// <summary>
@@ -312,6 +313,52 @@ internal void HideFlyout()
         _optionsFlyout?.Hide();
     }
 
+    /// <summary>
+    /// Keyboard access to what a click does: Enter or Space sorts (Ctrl adds to the existing sort, as Ctrl+click does),
+    /// and Alt+Down opens the column's options. Shift+F10 and the Menu key open them too (see OnContextRequested).
+    /// </summary>
+    protected override void OnKeyDown(KeyRoutedEventArgs e)
+    {
+        if (e.Key is VirtualKey.Enter or VirtualKey.Space && CanSort && Column is not null)
+        {
+            DoSort(GetNextSortDirection(), !IsKeyDown(VirtualKey.Control));
+            e.Handled = true;
+        }
+        else if (e.Key is VirtualKey.Down && IsKeyDown(VirtualKey.Menu) && ShowOptions())
+        {
+            e.Handled = true;
+        }
+
+        base.OnKeyDown(e);
+    }
+
+    /// <summary>
+    /// Opens the column's options for a keyboard context request (Shift+F10, the Menu key). A right-click also
+    /// raises this, with a pointer position; those keep going through <see cref="TableView.UseRightClickForColumnFilter"/>.
+    /// </summary>
+    private void OnContextRequested(UIElement sender, ContextRequestedEventArgs e)
+    {
+        if (!e.TryGetPosition(this, out _) && ShowOptions())
+        {
+            e.Handled = true;
+        }
+    }
+
+    /// <summary>Opens the options flyout (sort, group, filter), if the column has one.</summary>
+    private bool ShowOptions()
+    {
+        if (_optionsButton?.Flyout is not { } flyout || !CanFilter)
+        {
+            return false;
+        }
+
+        flyout.ShowAt(_optionsButton);
+        return true;
+    }
+
+    private static bool IsKeyDown(VirtualKey key) =>
+        InputKeyboardSource.GetKeyStateForCurrentThread(key).HasFlag(CoreVirtualKeyStates.Down);
+
     /// <inheritdoc/>
     protected override void OnTapped(TappedRoutedEventArgs e)
     {

src/Themes/TableViewColumnHeader.xaml

@@ -33,7 +33,7 @@
         <Setter Property="HorizontalContentAlignment"
                 Value="Stretch" />
         <Setter Property="IsTabStop"
-                Value="False" />
+                Value="True" />
         <Setter Property="Background">
             <Setter.Value>
                 <SolidColorBrush Opacity="0" />

src/Themes/TableViewHeaderRow.xaml

@@ -15,6 +15,8 @@
             <Setter.Value>
                 <ControlTemplate TargetType="local:TableViewHeaderRow">
                     <Grid x:Name="root"
+                          TabFocusNavigation="Once"
+                          win:XYFocusKeyboardNavigation="Enabled"
                           Background="{TemplateBinding Background}"
                           BorderBrush="{TemplateBinding BorderBrush}"
                           BorderThickness="{TemplateBinding BorderThickness}">