Fix invalid inherited color scheme fallback

src/cascadia/TerminalSettingsModel/CascadiaSettings.cpp

@@ -468,14 +468,31 @@ void CascadiaSettings::_validateAllSchemesExist()
         {
             if (appearance && !colorSchemes.HasKey(appearance.DarkColorSchemeName()))
             {
-                // Clear the user set dark color scheme. We'll just fallback instead.
-                appearance.ClearDarkColorSchemeName();
+                // If the leaf owns the bad name, clear it so the system default
+                // takes effect. If the name was inherited from a parent (fragment,
+                // defaults.json, etc.), the clear would be a no-op because the
+                // leaf's optional is already nullopt. In that case we must
+                // explicitly shadow the parent's value with the system default.
+                if (appearance.HasDarkColorSchemeName())
+                {
+                    appearance.ClearDarkColorSchemeName();
+                }
+                else
+                {
+                    appearance.DarkColorSchemeName(L"Campbell");
+                }
                 foundInvalidDarkScheme = true;
             }
             if (appearance && !colorSchemes.HasKey(appearance.LightColorSchemeName()))
             {
-                // Clear the user set light color scheme. We'll just fallback instead.
-                appearance.ClearLightColorSchemeName();
+                if (appearance.HasLightColorSchemeName())
+                {
+                    appearance.ClearLightColorSchemeName();
+                }
+                else
+                {
+                    appearance.LightColorSchemeName(L"Campbell");
+                }
                 foundInvalidLightScheme = true;
             }
         }

src/cascadia/UnitTests_SettingsModel/DeserializationTests.cpp

@@ -35,6 +35,7 @@ namespace SettingsModelUnitTests
         TEST_METHOD(TestLayeringNameOnlyProfiles);
         TEST_METHOD(TestHideAllProfiles);
         TEST_METHOD(TestInvalidColorSchemeName);
+        TEST_METHOD(TestInvalidColorSchemeNameFromFragmentFallsBack);
         TEST_METHOD(TestHelperFunctions);
         TEST_METHOD(TestCloseOnExitParsing);
         TEST_METHOD(TestCloseOnExitCompatibilityShim);
@@ -743,6 +744,43 @@ namespace SettingsModelUnitTests
         }
     }
 
+    void DeserializationTests::TestInvalidColorSchemeNameFromFragmentFallsBack()
+    {
+        Log::Comment(NoThrowString().Format(
+            L"Ensure that setting a profile's scheme to a nonexistent scheme via a fragment causes a warning and falls back correctly."));
+
+        static constexpr std::string_view userJson{ R"({
+            "profiles": [
+                {
+                    "name" : "profile0"
+                }
+            ]
+        })" };
+
+        static constexpr std::string_view fragmentJson{ R"({
+            "profiles": [
+                {
+                    "name": "profile0",
+                    "colorScheme": "InvalidSchemeName"
+                }
+            ]
+        })" };
+
+        implementation::SettingsLoader loader{ userJson, implementation::LoadStringResource(IDR_DEFAULTS) };
+        loader.MergeInboxIntoUserSettings();
+        loader.MergeFragmentIntoUserSettings(L"fragment", {}, fragmentJson);
+        loader.FinalizeLayering();
+
+        auto settings = winrt::make_self<implementation::CascadiaSettings>(std::move(loader));
+        auto profile = settings->AllProfiles().GetAt(0);
+
+        VERIFY_ARE_EQUAL(1u, settings->Warnings().Size());
+        VERIFY_ARE_EQUAL(SettingsLoadWarnings::UnknownColorScheme, settings->Warnings().GetAt(0));
+
+        VERIFY_ARE_EQUAL(L"Campbell", profile.DefaultAppearance().DarkColorSchemeName());
+        VERIFY_ARE_EQUAL(L"Campbell", profile.DefaultAppearance().LightColorSchemeName());
+    }
+
     void DeserializationTests::ValidateColorSchemeInCommands()
     {
         Log::Comment(NoThrowString().Format(