[LayoutNG] Fix line breaking when `font-size: 0`

This patch fixes line breaking when break opportunities have
`font-size: 0`.

`font-size: 0` makes all characters at position 0. This
confuses when |NGLineBreaker| needs to rewind, and tries to
find the break opportunity before the current one.

This patch fixes it by using |PreviousBreakOpportunity| in
such case.

Bug: 1006127
Change-Id: I449d6f8aaba4deccd458c5e077920b1a8bf79305
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1843991Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703753}

third_party/blink/renderer/core/layout/ng/inline/ng_line_breaker.cc

@@ -748,6 +748,31 @@ NGLineBreaker::BreakResult NGLineBreaker::BreakText(
   return inline_size <= available_width ? kSuccess : kOverflow;
 }
 
+// Breaks the text item at the previous break opportunity from
+// |item_result->end_offset|. Returns false if there were no previous break
+// opportunities.
+bool NGLineBreaker::BreakTextAtPreviousBreakOpportunity(
+    NGInlineItemResult* item_result) {
+  DCHECK(item_result->item);
+  DCHECK(item_result->may_break_inside);
+  const NGInlineItem& item = *item_result->item;
+  DCHECK_EQ(item.Type(), NGInlineItem::kText);
+  DCHECK(item.Style() && item.Style()->AutoWrap());
+
+  unsigned break_opportunity = break_iterator_.PreviousBreakOpportunity(
+      item_result->end_offset - 1, item_result->start_offset);
+  if (break_opportunity <= item_result->start_offset)
+    return false;
+  item_result->end_offset = break_opportunity;
+  item_result->shape_result =
+      ShapeResultView::Create(item.TextShapeResult(), item_result->start_offset,
+                              item_result->end_offset);
+  item_result->inline_size =
+      item_result->shape_result->SnappedWidth().ClampNegativeToZero();
+  item_result->can_break_after = true;
+  return true;
+}
+
 // This function handles text item for min-content. The specialized logic is
 // because min-content is very expensive by breaking at every break opportunity
 // and producing as many lines as the number of break opportunities.
@@ -1596,10 +1621,24 @@ void NGLineBreaker::HandleOverflow(NGLineInfo* line_info) {
       // If space is available, and if this text is breakable, part of the text
       // may fit. Try to break this item.
       if (width_to_rewind < 0 && item_result->may_break_inside) {
-        // When the text fits but its right margin does not, the break point
-        // must not be at the end.
-        LayoutUnit item_available_width =
-            std::min(-width_to_rewind, item_result->inline_size - 1);
+        LayoutUnit item_available_width = -width_to_rewind;
+        // Make sure the available width is smaller than the current width. The
+        // break point must not be at the end when e.g., the text fits but its
+        // right margin does not or following items do not.
+        const LayoutUnit min_available_width = item_result->inline_size - 1;
+        if (item_available_width > min_available_width) {
+          item_available_width = min_available_width;
+          // If |inline_size| is zero (e.g., `font-size: 0`), |BreakText| cannot
+          // make it shorter. Take the previous break opportunity.
+          if (UNLIKELY(item_available_width <= 0)) {
+            if (BreakTextAtPreviousBreakOpportunity(item_result)) {
+              state_ = LineBreakState::kTrailing;
+              Rewind(i + 1, line_info);
+              return;
+            }
+            continue;
+          }
+        }
         auto was_current_style = current_style_;
         SetCurrentStyle(*item.Style());
         BreakResult break_result =

third_party/blink/renderer/core/layout/ng/inline/ng_line_breaker.h

@@ -138,6 +138,7 @@ class CORE_EXPORT NGLineBreaker {
                         const ShapeResult&,
                         LayoutUnit available_width,
                         NGLineInfo*);
+  bool BreakTextAtPreviousBreakOpportunity(NGInlineItemResult* item_result);
   bool ShouldHangTraillingSpaces(const NGInlineItem&);
   bool HandleTextForFastMinContent(NGInlineItemResult*,
                                    const NGInlineItem&,

third_party/blink/web_tests/external/wpt/css/CSS2/linebox/line-breaking-font-size-zero-001-ref.html

+<!DOCTYPE html>
+<style>
+div {
+  width: 100px;
+  background: blue;
+}
+
+inline-block {
+  display: inline-block;
+  width: 80px;
+  height: 1em;
+  background: orange;
+}
+</style>
+</head>
+<body>
+  <div>
+    <inline-block></inline-block><br>
+    <inline-block></inline-block><br>
+    <inline-block></inline-block><br>
+    <inline-block></inline-block><br>
+    <inline-block></inline-block>
+  </div>
+</body>

third_party/blink/web_tests/external/wpt/css/CSS2/linebox/line-breaking-font-size-zero-001.html

+<!DOCTYPE html>
+<title>CSS Test: Check line breaks when break opportunities have font-size: 0</title>
+<link rel="match" href="line-breaking-font-size-zero-001-ref.html">
+<link rel="help" href="https://drafts.csswg.org/css2/text.html#propdef-white-space" />
+<link rel="author" href="mailto:kojii@chromium.org">
+<style>
+div {
+  width: 100px;
+  background: blue;
+}
+
+inline-block {
+  display: inline-block;
+  width: 80px;
+  height: 1em;
+  background: orange;
+}
+
+sep {
+  font-size: 0;
+}
+</style>
+</head>
+<body>
+  <div>
+    <inline-block></inline-block><sep> </sep>
+    <inline-block></inline-block><sep>, </sep>
+    <inline-block></inline-block><sep>) (</sep>
+    <inline-block></inline-block><sep>a</sep>
+    <inline-block></inline-block>
+  </div>
+</body>