Fix low-contrast TreeViewItem chevron in WPF Gallery nav pane

The Fluent nav TreeView expand/collapse chevron renders as a thin FontSize-10 glyph whose anti-aliased strokes only reach ~2.6:1 against the nav background, below the WCAG/MAS 1.4.11 3:1 non-text contrast minimum. The glyph size is baked into the sealed theme template via a StaticResource and cannot be overridden through application resources, so the chevron FontSize is enlarged on each realized nav container in code-behind, rendering the full stroke at ~3.9:1 in every theme.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: amarinov-msft

Sample Applications/WPFGallery/MainWindow.xaml.cs

@@ -206,6 +206,7 @@ private void ControlsList_PreviewMouseLeftButtonUp(object sender, MouseButtonEve
 
     private void ControlsList_Loaded(object sender, RoutedEventArgs e)
     {
+        ResizeNavChevrons();
         if (ControlsList.Items.Count > 0)
         {
             TreeViewItem firstItem = (TreeViewItem)ControlsList.ItemContainerGenerator.ContainerFromItem(ControlsList.Items[0]);
@@ -216,6 +217,52 @@ private void ControlsList_Loaded(object sender, RoutedEventArgs e)
         }
     }
 
+    // The Fluent nav chevron is a thin FontSize-10 glyph that anti-aliases to ~2.6:1 (below the
+    // WCAG/MAS 1.4.11 3:1 minimum); enlarging it renders the full stroke at ~3.9:1 in any theme.
+    private const double NavChevronFontSize = 12d;
+
+    // Only the top-level nav items expand, and their containers are realized by the time the
+    // TreeView is loaded, so resizing their chevrons once here is sufficient. The size is baked into
+    // the sealed theme template via a StaticResource, so it must be set on the realized glyph.
+    private void ResizeNavChevrons()
+    {
+        foreach (object item in ControlsList.Items)
+        {
+            if (ControlsList.ItemContainerGenerator.ContainerFromItem(item) is TreeViewItem tvi
+                && FindChevronIcon(tvi) is TextBlock chevron)
+            {
+                chevron.FontSize = NavChevronFontSize;
+            }
+        }
+    }
+
+    private static TextBlock? FindChevronIcon(DependencyObject root)
+    {
+        int count = VisualTreeHelper.GetChildrenCount(root);
+        for (int i = 0; i < count; i++)
+        {
+            DependencyObject child = VisualTreeHelper.GetChild(root, i);
+
+            // Each item owns a single chevron in its own header; don't descend into nested items.
+            if (child is TreeViewItem)
+            {
+                continue;
+            }
+
+            if (child is TextBlock { Name: "ChevronIcon" } chevron)
+            {
+                return chevron;
+            }
+
+            if (FindChevronIcon(child) is TextBlock found)
+            {
+                return found;
+            }
+        }
+
+        return null;
+    }
+
     private void SelectedItemChanged(TreeViewItem? tvi)
     {
         ControlsList_SelectedItemChanged();