fix(text): resolve app-bundled font families by passing a DirectWrite font collection

change/react-native-windows-fix-app-bundled-fonts.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "Fabric: resolve app-bundled fonts (Assets, Assets\\Fonts) during text layout by merging them with the system font set into the DirectWrite font collection used by WindowsTextLayoutManager",
+  "packageName": "react-native-windows",
+  "email": "collindanielschneide@gmail.com",
+  "dependentChangeType": "patch"
+}

vnext/Microsoft.ReactNative/Fabric/DWriteHelpers.cpp

@@ -5,6 +5,11 @@
 
 #include "DWriteHelpers.h"
 
+#include <dwrite_3.h>
+#include <windows.h>
+#include <cstdint>
+#include <string>
+
 namespace Microsoft::ReactNative {
 
 winrt::com_ptr<::IDWriteFactory> DWriteFactory() noexcept {
@@ -16,4 +21,103 @@ winrt::com_ptr<::IDWriteFactory> DWriteFactory() noexcept {
   return s_dwriteFactory;
 }
 
+namespace {
+
+// Directory that contains the running module, including the trailing separator: the
+// package root for packaged (MSIX) apps and the directory next to the .exe for
+// unpackaged apps. Bundled font assets are deployed below this directory.
+std::wstring AppDirectory() noexcept {
+  wchar_t modulePath[MAX_PATH]{};
+  const DWORD length = ::GetModuleFileNameW(nullptr, modulePath, MAX_PATH);
+  if (length == 0 || length >= MAX_PATH) {
+    return {};
+  }
+  std::wstring path(modulePath, length);
+  const auto lastSeparator = path.find_last_of(L"\\/");
+  if (lastSeparator == std::wstring::npos) {
+    return {};
+  }
+  path.resize(lastSeparator + 1);
+  return path;
+}
+
+// Adds every file matching <directory> + <pattern> to the font-set builder and returns
+// the number of files added. Per-file failures are skipped so that one bad font file
+// cannot break font resolution for the rest of the app.
+uint32_t AddFontFiles(
+    ::IDWriteFactory5 *factory,
+    ::IDWriteFontSetBuilder1 *builder,
+    const std::wstring &directory,
+    const wchar_t *pattern) noexcept {
+  uint32_t count = 0;
+  const std::wstring searchPattern = directory + pattern;
+  WIN32_FIND_DATAW findData{};
+  const HANDLE findHandle = ::FindFirstFileW(searchPattern.c_str(), &findData);
+  if (findHandle == INVALID_HANDLE_VALUE) {
+    return count;
+  }
+  do {
+    if (!(findData.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY)) {
+      const std::wstring fontPath = directory + findData.cFileName;
+      winrt::com_ptr<::IDWriteFontFile> fontFile;
+      if (SUCCEEDED(factory->CreateFontFileReference(fontPath.c_str(), nullptr, fontFile.put())) &&
+          SUCCEEDED(builder->AddFontFile(fontFile.get()))) {
+        ++count;
+      }
+    }
+  } while (::FindNextFileW(findHandle, &findData));
+  ::FindClose(findHandle);
+  return count;
+}
+
+winrt::com_ptr<::IDWriteFontCollection> CreateAppFontCollection() noexcept {
+  try {
+    const std::wstring appDirectory = AppDirectory();
+    if (appDirectory.empty()) {
+      return nullptr;
+    }
+
+    const auto factory5 = DWriteFactory().as<::IDWriteFactory5>();
+
+    winrt::com_ptr<::IDWriteFontSetBuilder1> builder;
+    winrt::check_hresult(factory5->CreateFontSetBuilder(builder.put()));
+
+    // Include the system font set so that system families keep resolving when this
+    // collection is used in place of the system collection.
+    winrt::com_ptr<::IDWriteFontSet> systemFontSet;
+    winrt::check_hresult(factory5->GetSystemFontSet(systemFontSet.put()));
+    winrt::check_hresult(builder->AddFontSet(systemFontSet.get()));
+
+    uint32_t fontFileCount = 0;
+    for (const auto *subdirectory : {L"Assets\\", L"Assets\\Fonts\\"}) {
+      for (const auto *pattern : {L"*.ttf", L"*.otf"}) {
+        fontFileCount += AddFontFiles(factory5.get(), builder.get(), appDirectory + subdirectory, pattern);
+      }
+    }
+    if (fontFileCount == 0) {
+      // Nothing bundled: report "no app collection" so callers pass nullptr to DirectWrite
+      // and keep using DirectWrite's own (cached, updatable) system font collection.
+      return nullptr;
+    }
+
+    winrt::com_ptr<::IDWriteFontSet> fontSet;
+    winrt::check_hresult(builder->CreateFontSet(fontSet.put()));
+    winrt::com_ptr<::IDWriteFontCollection1> collection;
+    winrt::check_hresult(factory5->CreateFontCollectionFromFontSet(fontSet.get(), collection.put()));
+    return collection.as<::IDWriteFontCollection>();
+  } catch (...) {
+    // Fail closed: callers fall back to the system font collection (previous behavior).
+    return nullptr;
+  }
+}
+
+} // namespace
+
+winrt::com_ptr<::IDWriteFontCollection> DWriteAppFontCollection() noexcept {
+  // Thread-safe (magic static) one-time initialization. Bundled font assets cannot
+  // change for the lifetime of the process, so the collection never needs rebuilding.
+  static const winrt::com_ptr<::IDWriteFontCollection> s_appFontCollection = CreateAppFontCollection();
+  return s_appFontCollection;
+}
+
 } // namespace Microsoft::ReactNative

vnext/Microsoft.ReactNative/Fabric/DWriteHelpers.h

@@ -10,4 +10,11 @@ namespace Microsoft::ReactNative {
 
 winrt::com_ptr<::IDWriteFactory> DWriteFactory() noexcept;
 
+// Font collection that merges the system font set with every font file bundled in the
+// application's Assets\ and Assets\Fonts\ directories (*.ttf / *.otf), so app-bundled
+// font families resolve during text layout exactly like installed fonts. Built once on
+// first use. Returns nullptr when the app bundles no fonts or when the collection
+// cannot be built; callers should treat nullptr as "use the system font collection".
+winrt::com_ptr<::IDWriteFontCollection> DWriteAppFontCollection() noexcept;
+
 } // namespace Microsoft::ReactNative

vnext/Microsoft.ReactNative/Fabric/platform/react/renderer/textlayoutmanager/WindowsTextLayoutManager.cpp

@@ -116,7 +116,9 @@ void WindowsTextLayoutManager::GetTextLayout(
       outerFragment.textAttributes.fontFamily.empty()
           ? L"Segoe UI"
           : Microsoft::Common::Unicode::Utf8ToUtf16(outerFragment.textAttributes.fontFamily).c_str(),
-      nullptr, // Font collection (nullptr sets it to use the system font collection).
+      // Bundled app fonts merged over the system font set (nullptr when the app bundles
+      // no fonts, which selects the system font collection as before).
+      Microsoft::ReactNative::DWriteAppFontCollection().get(),
       static_cast<DWRITE_FONT_WEIGHT>(outerFragment.textAttributes.fontWeight.value_or(
           static_cast<facebook::react::FontWeight>(DWRITE_FONT_WEIGHT_REGULAR))),
       style,