libtxt: disable font fallback when test font mode is enabled (#4661)

lib/ui/text/font_collection.cc

@@ -122,6 +122,8 @@ void FontCollection::RegisterTestFonts() {
 
   collection_->PushFront(sk_make_sp<txt::TestFontManager>(
       std::move(asset_data_provider), GetTestFontFamilyName()));
+
+  collection_->DisableFontFallback();
 }
 
 }  // namespace blink

third_party/txt/src/txt/font_collection.cc

@@ -64,7 +64,7 @@ class TxtFallbackFontProvider
   std::shared_ptr<FontCollection> font_collection_;
 };
 
-FontCollection::FontCollection() = default;
+FontCollection::FontCollection() : enable_font_fallback_(true) {}
 
 FontCollection::~FontCollection() = default;
 
@@ -88,6 +88,10 @@ void FontCollection::PushBack(sk_sp<SkFontMgr> skia_font_manager) {
   skia_font_managers_.push_back(std::move(skia_font_manager));
 }
 
+void FontCollection::DisableFontFallback() {
+  enable_font_fallback_ = false;
+}
+
 std::shared_ptr<minikin::FontCollection>
 FontCollection::GetMinikinFontCollectionForFamily(const std::string& family) {
   // Look inside the font collections cache first.
@@ -132,14 +136,18 @@ FontCollection::GetMinikinFontCollectionForFamily(const std::string& family) {
     std::vector<std::shared_ptr<minikin::FontFamily>> minikin_families = {
         minikin_family,
     };
-    for (const auto& fallback : fallback_fonts_)
-      minikin_families.push_back(fallback.second);
+    if (enable_font_fallback_) {
+      for (const auto& fallback : fallback_fonts_)
+        minikin_families.push_back(fallback.second);
+    }
 
     // Create the minikin font collection.
     auto font_collection =
         std::make_shared<minikin::FontCollection>(std::move(minikin_families));
-    font_collection->set_fallback_font_provider(
-        std::make_unique<TxtFallbackFontProvider>(shared_from_this()));
+    if (enable_font_fallback_) {
+      font_collection->set_fallback_font_provider(
+          std::make_unique<TxtFallbackFontProvider>(shared_from_this()));
+    }
 
     // Cache the font collection for future queries.
     font_collections_cache_[family] = font_collection;

third_party/txt/src/txt/font_collection.h

@@ -48,6 +48,10 @@ class FontCollection : public std::enable_shared_from_this<FontCollection> {
 
   const std::shared_ptr<minikin::FontFamily>& MatchFallbackFont(uint32_t ch);
 
+  // Do not provide alternative fonts that can match characters which are
+  // missing from the requested font family.
+  void DisableFontFallback();
+
  private:
   std::deque<sk_sp<SkFontMgr>> skia_font_managers_;
   std::unordered_map<std::string, std::shared_ptr<minikin::FontCollection>>
@@ -55,6 +59,7 @@ class FontCollection : public std::enable_shared_from_this<FontCollection> {
   std::unordered_map<SkFontID, std::shared_ptr<minikin::FontFamily>>
       fallback_fonts_;
   std::shared_ptr<minikin::FontFamily> null_family_;
+  bool enable_font_fallback_;
 
   const std::shared_ptr<minikin::FontFamily>& GetFontFamilyForTypeface(
       const sk_sp<SkTypeface>& typeface);