Don't allow new line chars as preceding char for ::first-letter.

Because CR and LF new line chars belong to C* unicode category,
but allowed unicode categories as preceding chars are P* and Z*.

But in the context of WhiteSpaceCollapse::kCollapse,
space and new line chars are ignored,
so apply the behavior specified above only Blink::ShouldPreserveBreaks() is true.

Firefox 116.0.2 have this behavior,
but Safari 16.6 shows the same behavior with Chrome of before this change.

This change is per https://drafts.csswg.org/css-pseudo/#first-letter-pattern.

Bug: 1477217
Change-Id: I80aa6a14ecd867f864eb49a12e229e5390dd9a30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4825211
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Joonghun Park <pjh0718@gmail.com>
Cr-Commit-Position: refs/heads/main@{#1190634}

third_party/blink/renderer/core/dom/first_letter_pseudo_element.cc

@@ -54,29 +54,57 @@ static inline bool IsPunctuationForFirstLetter(UChar32 c) {
          char_category == WTF::unicode::kPunctuation_Other;
 }
 
-static inline bool IsSpaceForFirstLetter(UChar c) {
-  return IsSpaceOrNewline(c) || c == WTF::unicode::kNoBreakSpaceCharacter;
+static inline bool IsNewLine(UChar c) {
+  if (c == 0xA || c == 0xD) {
+    return true;
+  }
+
+  return false;
 }
 
-unsigned FirstLetterPseudoElement::FirstLetterLength(const String& text) {
+static inline bool IsSpace(UChar c) {
+  if (IsNewLine(c)) {
+    return false;
+  }
+
+  return IsSpaceOrNewline(c);
+}
+
+static inline bool IsSpaceForFirstLetter(UChar c, bool preserve_breaks) {
+  return (RuntimeEnabledFeatures::
+                      CSSFirstLetterNoNewLineAsPrecedingCharEnabled() &&
+                  preserve_breaks
+              ? IsSpace(c)
+              : IsSpaceOrNewline(c)) ||
+         c == WTF::unicode::kNoBreakSpaceCharacter;
+}
+
+unsigned FirstLetterPseudoElement::FirstLetterLength(const String& text,
+                                                     bool preserve_breaks) {
   unsigned length = 0;
   unsigned text_length = text.length();
 
   if (text_length == 0)
     return length;
 
   // Account for leading spaces first.
-  while (length < text_length && IsSpaceForFirstLetter(text[length]))
+  while (length < text_length &&
+         IsSpaceForFirstLetter(text[length], preserve_breaks)) {
     length++;
+  }
+
   // Now account for leading punctuation.
   while (length < text_length &&
-         IsPunctuationForFirstLetter(text.CharacterStartingAt(length)))
+         IsPunctuationForFirstLetter(text.CharacterStartingAt(length))) {
     length += LengthOfGraphemeCluster(text, length);
+  }
 
   // Bail if we didn't find a letter before the end of the text or before a
   // space.
-  if (IsSpaceForFirstLetter(text[length]) || length == text_length)
+  if (IsSpaceForFirstLetter(text[length], preserve_breaks) ||
+      IsNewLine(text[length]) || length == text_length) {
     return 0;
+  }
 
   // Account the next character for first letter.
   length += LengthOfGraphemeCluster(text, length);
@@ -153,7 +181,9 @@ LayoutText* FirstLetterPseudoElement::FirstLetterTextLayoutObject(
                        ? To<LayoutTextFragment>(first_letter_text_layout_object)
                              ->CompleteText()
                        : layout_text->OriginalText();
-      if (FirstLetterLength(str.Impl()) ||
+      bool preserve_breaks = ShouldPreserveBreaks(
+          first_letter_text_layout_object->StyleRef().GetWhiteSpaceCollapse());
+      if (FirstLetterLength(str.Impl(), preserve_breaks) ||
           IsInvalidFirstLetterLayoutObject(first_letter_text_layout_object)) {
         break;
       }
@@ -248,7 +278,10 @@ void FirstLetterPseudoElement::UpdateTextFragments() {
   String old_text(remaining_text_layout_object_->CompleteText());
   DCHECK(old_text.Impl());
 
-  unsigned length = FirstLetterPseudoElement::FirstLetterLength(old_text);
+  bool preserve_breaks = ShouldPreserveBreaks(
+      remaining_text_layout_object_->StyleRef().GetWhiteSpaceCollapse());
+  unsigned length =
+      FirstLetterPseudoElement::FirstLetterLength(old_text, preserve_breaks);
   remaining_text_layout_object_->SetTextFragment(
       old_text.Impl()->Substring(length, old_text.length()), length,
       old_text.length() - length);
@@ -371,7 +404,10 @@ void FirstLetterPseudoElement::AttachFirstLetterTextLayoutObjects(
 
   // FIXME: This would already have been calculated in firstLetterLayoutObject.
   // Can we pass the length through?
-  unsigned length = FirstLetterPseudoElement::FirstLetterLength(old_text);
+  bool preserve_breaks = ShouldPreserveBreaks(
+      first_letter_text->StyleRef().GetWhiteSpaceCollapse());
+  unsigned length =
+      FirstLetterPseudoElement::FirstLetterLength(old_text, preserve_breaks);
 
   // In case of inline level content made of punctuation, we use
   // first_letter_text length instead of FirstLetterLength.

third_party/blink/renderer/core/dom/first_letter_pseudo_element.h

@@ -46,7 +46,8 @@ class CORE_EXPORT FirstLetterPseudoElement final : public PseudoElement {
   void Trace(Visitor*) const override;
 
   static LayoutText* FirstLetterTextLayoutObject(const Element&);
-  static unsigned FirstLetterLength(const String&);
+  static unsigned FirstLetterLength(const String&,
+                                    bool preserve_breaks = false);
 
   void ClearRemainingTextLayoutObject();
   LayoutTextFragment* RemainingTextLayoutObject() const {

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -872,6 +872,12 @@
       name: "CSSExponentialFunctions",
       status: "experimental",
     },
+    {
+      // Don't allow new line as preceding character for ::first-letter.
+      // See crbug.com/1477217
+      name: "CSSFirstLetterNoNewLineAsPrecedingChar",
+      status: "stable",
+    },
     {
       name: "CSSFocusVisible",
       status: "stable",

third_party/blink/web_tests/external/wpt/css/css-pseudo/first-letter-with-preceding-new-line.html

@@ -0,0 +1,31 @@
+<!doctype html>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSS Test: ::first-letter with preceding new line</title>
+  <link rel="author" title="Joonghun Park" href="mailto:pjh0718@gmail.com">
+  <link rel="match" href="reference/first-letter-with-preceding-new-line-ref.html">
+  <link rel="help" href="https://drafts.csswg.org/css-pseudo/#first-letter-pattern">
+  <meta name="assert" content="Test checks if preceding new line character is allowed or not">
+  <style>
+    body { font-size: 30px; }
+    #sample {
+      margin: 20px;
+      white-space: pre;
+    }
+    #sample::first-line {
+      color: red;
+    }
+    #sample::first-letter {
+      color: green;
+      border: solid 1px blue;
+    }
+  </style>
+</head>
+<body>
+  <div id="sample">
+  The second line.
+  The third line.
+  </div>
+</body>
+</html>

third_party/blink/web_tests/external/wpt/css/css-pseudo/reference/first-letter-with-preceding-new-line-ref.html

@@ -0,0 +1,20 @@
+<!doctype html>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>CSS Test: ::first-letter with preceding new line</title>
+  <style>
+    body { font-size: 30px; }
+    #sample {
+      margin: 20px;
+      white-space: pre;
+    }
+  </style>
+</head>
+<body>
+  <div id="sample">
+  The second line.
+  The third line.
+  </div>
+</body>
+</html>