Fix compat fluent theme not being applied

src/Avalonia.Themes.Fluent/DensityStyles/Compact.xaml

@@ -1,10 +1,3 @@
-<Styles xmlns="https://github.com/avaloniaui" 
-        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
-        x:ClassModifier="internal">
-    <Style Selector="TextBlock" >
-        <Setter Property="FontSize" Value="14" />
-    </Style>
-    <Styles.Resources>
       <x:Double x:Key="ControlContentThemeFontSize">14</x:Double>
       <x:Double x:Key="ContentControlFontSize">14</x:Double>
       <x:Double x:Key="TextControlThemeMinHeight">24</x:Double>
@@ -18,5 +11,7 @@
       <x:Double x:Key="ComboBoxMinHeight">24</x:Double>
       <Thickness x:Key="ComboBoxPadding">12,1,0,3</Thickness>
       <x:Double x:Key="NavigationViewItemOnLeftMinHeight">32</x:Double>
-    </Styles.Resources>
-</Styles>
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    x:ClassModifier="internal">
+</ResourceDictionary>

src/Avalonia.Themes.Fluent/FluentTheme.xaml

@@ -18,7 +18,7 @@
       </ResourceDictionary.MergedDictionaries>
       
       <!-- These are not part of MergedDictionaries so we can add or remove them easier -->
-      <StyleInclude x:Key="CompactStyles" Source="/DensityStyles/Compact.xaml" />
+      <ResourceInclude x:Key="CompactStyles" Source="/DensityStyles/Compact.xaml" />
     </ResourceDictionary>
   </Styles.Resources>
   

src/Avalonia.Themes.Fluent/FluentTheme.xaml.cs

@@ -16,9 +16,10 @@ namespace Avalonia.Themes.Fluent
     /// <summary>
     /// Includes the fluent theme in an application.
     /// </summary>
-    public class FluentTheme : Styles
+    public class FluentTheme : Styles, IResourceNode
     {
-        private readonly Styles _compactStyles;
+        private readonly ResourceDictionary _compactStyles;
+        private DensityStyle _densityStyle;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="FluentTheme"/> class.
@@ -28,9 +29,7 @@ namespace Avalonia.Themes.Fluent
         {
             AvaloniaXamlLoader.Load(sp, this);
             
-            _compactStyles = (Styles)GetAndRemove("CompactStyles");
-
-            EnsureCompactStyles();
+            _compactStyles = (ResourceDictionary)GetAndRemove("CompactStyles");
 
             Palettes = Resources.MergedDictionaries.OfType<ColorPaletteResourcesCollection>().FirstOrDefault()
                 ?? throw new InvalidOperationException("FluentTheme was initialized with missing ColorPaletteResourcesCollection.");
@@ -43,17 +42,17 @@ namespace Avalonia.Themes.Fluent
                 return val;
             }
         }
-        
-        public static readonly StyledProperty<DensityStyle> DensityStyleProperty =
-            AvaloniaProperty.Register<FluentTheme, DensityStyle>(nameof(DensityStyle));
+
+        public static readonly DirectProperty<FluentTheme, DensityStyle> DensityStyleProperty = AvaloniaProperty.RegisterDirect<FluentTheme, DensityStyle>(
+            nameof(DensityStyle), o => o.DensityStyle, (o, v) => o.DensityStyle = v);
 
         /// <summary>
         /// Gets or sets the density style of the fluent theme (normal, compact).
         /// </summary>
         public DensityStyle DensityStyle
         {
-            get => GetValue(DensityStyleProperty);
-            set => SetValue(DensityStyleProperty, value);
+            get => _densityStyle;
+            set => SetAndRaise(DensityStyleProperty, ref _densityStyle, value);
         }
         
         public IDictionary<ThemeVariant, ColorPaletteResources> Palettes { get; }
@@ -64,20 +63,20 @@ namespace Avalonia.Themes.Fluent
 
             if (change.Property == DensityStyleProperty)
             {
-                EnsureCompactStyles();
+                Owner?.NotifyHostedResourcesChanged(ResourcesChangedEventArgs.Empty);
             }
         }
 
-        private void EnsureCompactStyles()
+        bool IResourceNode.TryGetResource(object key, ThemeVariant? theme, out object? value)
         {
-            if (DensityStyle == DensityStyle.Compact)
+            // DensityStyle dictionary should be checked first
+            if (_densityStyle == DensityStyle.Compact
+                && _compactStyles.TryGetResource(key, theme, out value))
             {
-                Add(_compactStyles);
-            }
-            else
-            {
-                Remove(_compactStyles);
+                return true;
             }
+
+            return base.TryGetResource(key, theme, out value);
         }
     }
 }