Window ThemeMode sync on resource dictionary update

src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/ThemeManager3.cs

@@ -1,6 +1,7 @@
 using Standard;
 using Microsoft.Win32;
 using System.Collections;
+using System.Collections.ObjectModel;
 using System.Collections.Generic;
 using System.Windows.Interop;
 using System.Windows.Media;
@@ -156,6 +157,40 @@ internal static void SyncDeferredThemeModeAndResources()
         }
     }
 
+    internal static void SyncWindowThemeModeAndResources(Window window)
+    {
+        Collection<ResourceDictionary> windowResources = window.Resources.MergedDictionaries;
+        bool containsFluentResource = false;
+
+        for (int i = windowResources.Count - 1 ; i >= 0 ; i--) 
+        {
+            if (windowResources[i].Source.ToString().Contains("Fluent")) 
+            {
+                containsFluentResource = true;
+
+                if(windowResources[i].Source.ToString().Contains("Light"))
+                {
+                    window.ThemeMode = ThemeMode.Light;
+                }
+                else if(windowResources[i].Source.ToString().Contains("Dark"))
+                {
+                    window.ThemeMode = ThemeMode.Dark;
+                }
+                else
+                {
+                    window.ThemeMode = ThemeMode.System;
+                }
+
+                break;
+            }
+        }
+
+        if(!containsFluentResource)
+        {
+            window.ThemeMode = ThemeMode.None;
+        }
+    }
+
     internal static void ApplyStyleOnWindow(Window window)
     {
         if(!IsFluentThemeEnabled && window.ThemeMode == ThemeMode.None) return;
@@ -231,7 +266,7 @@ private static void ApplyFluentOnWindow(Window window)
 
         bool useLightColors = GetUseLightColors(window.ThemeMode);
         var fluentThemeResourceUri = GetFluentThemeResourceUri(useLightColors);
-        AddOrUpdateThemeResources(window.Resources, fluentThemeResourceUri);
+        AddOrUpdateThemeResources(window.Resources, fluentThemeResourceUri, true);
         ApplyStyleOnWindow(window, useLightColors);
 
         if(!FluentEnabledWindows.HasItem(window))
@@ -300,6 +335,8 @@ internal static bool IsFluentThemeEnabled
 
     internal static bool IgnoreAppResourcesChange { get; set; } = false;
 
+    internal static bool IgnoreWindowResourcesChange { get; set; } = false;
+
     internal static double DefaultFluentThemeFontSize => 14;
 
     internal static WindowCollection FluentEnabledWindows { get; set; } = new WindowCollection();
@@ -365,7 +402,7 @@ private static Uri GetFluentThemeResourceUri(bool useLightMode)
 
     }
 
-    private static void AddOrUpdateThemeResources(ResourceDictionary rd, Uri dictionaryUri)
+    private static void AddOrUpdateThemeResources(ResourceDictionary rd, Uri dictionaryUri, bool isWindowUpdate = false)
     {
         if (rd == null) return;
 
@@ -374,14 +411,23 @@ private static void AddOrUpdateThemeResources(ResourceDictionary rd, Uri diction
         var newDictionary = new ResourceDictionary() { Source = dictionaryUri };
         int index = FindLastFluentThemeResourceDictionaryIndex(rd);
 
+        IgnoreWindowResourcesChange = true;
+
         if (index >= 0)
         {
+            if(isWindowUpdate && rd.MergedDictionaries[index].Source.ToString().Equals(dictionaryUri.ToString(), StringComparison.OrdinalIgnoreCase))
+            {
+                return;
+            }
+
             rd.MergedDictionaries[index] = newDictionary;
         }
         else
         {
             rd.MergedDictionaries.Insert(0, newDictionary);
         }
+
+        IgnoreWindowResourcesChange = false;
     }
 
     private static int FindLastFluentThemeResourceDictionaryIndex(ResourceDictionary rd)

src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/TreeWalkHelper.cs

@@ -680,6 +680,13 @@ private static void InvalidateResourceReferences(
         {
             Debug.Assert(d != null, "Must have non-null current node");
 
+            Window currentWindow = d as Window;
+
+            if(currentWindow != null && !ThemeManager3.IgnoreWindowResourcesChange)
+            {
+                ThemeManager3.SyncWindowThemeModeAndResources(currentWindow);
+            }
+
             // Find properties that have resource reference value
             LocalValueEnumerator localValues = d.GetLocalValueEnumerator();
             int localValuesCount = localValues.Count;