Fix codepoint iteration in ReplaceControlCharactersWithSymbols

The CL is fixing a bug when the rewriting loop iterate through
the input text. If the codepoint is updated with a larger codepoint,
the next_offset was not updated properly and was pointing in the middle
of the surrogate pair. This was not happening since only the PUA
codepoints were producing larger characters.

Change-Id: I4c6eea9a1a02374d04a0271bd4d6140e123069ba
Bug: 4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1903808Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713491}

ui/gfx/render_text.cc

@@ -316,20 +316,19 @@ void ReplaceControlCharactersWithSymbols(bool multiline, base::string16* text) {
   // Control Pictures block (see: https://unicode.org/charts/PDF/U2400.pdf).
   constexpr base::char16 kSymbolsCodepoint = 0x2400;
 
-  size_t next_offset = 0;
-  while (next_offset < text->length()) {
-    size_t offset = next_offset;
+  size_t offset = 0;
+  while (offset < text->length()) {
     UChar32 codepoint;
-    U16_NEXT(text->c_str(), next_offset, text->length(), codepoint);
+    U16_GET(text->c_str(), 0, offset, text->length(), codepoint);
 
     if (codepoint >= 0 && codepoint <= 0x1F) {
       // The newline character should be kept as-is when rendertext is
       // multiline.
-      if (codepoint == '\n' && multiline)
-        continue;
+      if (codepoint != '\n' || !multiline) {
         // Replace codepoints with their visual symbols, which are at the same
         // offset from kSymbolsCodepoint.
         (*text)[offset] = kSymbolsCodepoint + codepoint;
+      }
     } else if (codepoint == 0x7F) {
       // Replace the 'del' codepoint by its symbol (u2421).
       (*text)[offset] = kSymbolsCodepoint + 0x21;
@@ -349,6 +348,10 @@ void ReplaceControlCharactersWithSymbols(bool multiline, base::string16* text) {
         ReplaceCodepointAtIndex(offset, kReplacementCodepoint, text);
       }
     }
+
+    // Move offset to the index of the next codepoint. This must be computed
+    // after any rewriting steps above since codepoint size may differ.
+    U16_NEXT(text->c_str(), offset, text->length(), codepoint);
   }
 }
 

ui/gfx/render_text_unittest.cc

@@ -4792,6 +4792,11 @@ TEST_F(RenderTextTest, PrivateUseCharacterReplacement) {
   // a surrogate pair, it needs to be replaced by two characters.
   EXPECT_EQ(WideToUTF16(L"xx\ufffd\ufffda\ufffdz"),
             render_text->GetDisplayText());
+
+  // The private use characters from Area-B must be replaced. The rewrite step
+  // replaced 2 characters by 1 character.
+  render_text->SetText(WideToUTF16(L"x\U00100000\U00100001\U00100002"));
+  EXPECT_EQ(WideToUTF16(L"x\ufffd\ufffd\ufffd"), render_text->GetDisplayText());
 }
 
 TEST_F(RenderTextTest, InvalidSurrogateCharacterReplacement) {