Bug 1672269 - NextWord shouldn't return empty. r=jfkthame

After landing bug 425915, we use NextWord instead of BreakInBetween.
NextWord is possible to return empty string (offset equals to current
position). So it shouldn't return empty string.

Differential Revision: https://phabricator.services.mozilla.com/D94266

intl/lwbrk/WordBreaker.cpp

@@ -200,7 +200,7 @@ int32_t WordBreaker::NextWord(const char16_t* aText, uint32_t aLen,
     AutoTArray<uint8_t, 256> breakBefore;
     breakBefore.SetLength(aLen - aPos);
     NS_GetComplexLineBreaks(aText + aPos, aLen - aPos, breakBefore.Elements());
-    uint32_t i = 0;
+    uint32_t i = 1;
     while (i < cur - aPos && !breakBefore[i]) {
       i++;
     }
@@ -213,5 +213,6 @@ int32_t WordBreaker::NextWord(const char16_t* aText, uint32_t aLen,
     return NS_WORDBREAKER_NEED_MORE_TEXT;
   }
 
+  MOZ_ASSERT(cur != aPos);
   return cur;
 }

intl/lwbrk/gtest/TestLineBreak.cpp

@@ -252,6 +252,20 @@ void TestFindWordBreakFromPosition(uint32_t fragN, uint32_t offset,
       << "FindWordBreakFromPosition(" << fragN << ", " << offset << ")";
 }
 
+void TestNextWordBreakWithComplexLanguage() {
+  RefPtr<mozilla::intl::WordBreaker> wbk = mozilla::intl::WordBreaker::Create();
+  nsString fragText(u"\u0e40\u0e1b\u0e47\u0e19\u0e19\u0e31\u0e01");
+
+  int32_t offset = 0;
+  while (offset != NS_WORDBREAKER_NEED_MORE_TEXT) {
+    int32_t newOffset =
+        wbk->NextWord(fragText.get(), fragText.Length(), offset);
+    ASSERT_NE(offset, newOffset);
+    offset = newOffset;
+  }
+  ASSERT_TRUE(true);
+}
+
 TEST(LineBreak, WordBreakUsage)
 {
   TestPrintWordWithBreak();
@@ -265,4 +279,5 @@ TEST(LineBreak, WordBreakUsage)
   TestFindWordBreakFromPosition(3, 8, "ernationalization");
   TestFindWordBreakFromPosition(4, 6, " ");
   TestFindWordBreakFromPosition(4, 7, "work");
+  TestNextWordBreakWithComplexLanguage();
 }