Bug 1736805: Register written normalised length. r=platform-i18n-reviewers,gregtatum

Differential Revision: https://phabricator.services.mozilla.com/D129428

intl/components/gtest/TestString.cpp

@@ -121,6 +121,13 @@ TEST(IntlString, NormalizeNFD)
   alreadyNormalized = String::Normalize(NormalizationForm::NFD, u"½"sv, buf);
   ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::Yes);
   ASSERT_EQ(buf.get_string_view(), u"");
+
+  // Test with inline capacity.
+  TestBuffer<char16_t, 2> buf2;
+
+  alreadyNormalized = String::Normalize(NormalizationForm::NFD, u" ç"sv, buf2);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::No);
+  ASSERT_EQ(buf2.get_string_view(), u" c\u0327");
 }
 
 TEST(IntlString, NormalizeNFKC)

intl/components/src/String.h

@@ -150,6 +150,8 @@ class String final {
     // Copy the already normalized prefix.
     if (spanLength > 0) {
       PodCopy(aBuffer.data(), aString.data(), spanLength);
+
+      aBuffer.written(spanLength);
     }
 
     MOZ_TRY(FillBufferWithICUCall(