Fix ShapeResult::OffsetForPosition edge behavior

third_party/WebKit/Source/platform/fonts/shaping/ShapeResult.cpp

Index: third_party/WebKit/Source/platform/fonts/shaping/ShapeResult.cpp
diff --git a/third_party/WebKit/Source/platform/fonts/shaping/ShapeResult.cpp b/third_party/WebKit/Source/platform/fonts/shaping/ShapeResult.cpp
index 9e1d3f5a1436298880b19f5ee96f9472a8696c9d..43f9dd0af0399f70e2bb1ccc996c57e7b678f9c3 100644
--- a/third_party/WebKit/Source/platform/fonts/shaping/ShapeResult.cpp
+++ b/third_party/WebKit/Source/platform/fonts/shaping/ShapeResult.cpp
@@ -104,6 +104,8 @@ int ShapeResult::RunInfo::CharacterIndexForXPosition(
     float target_x,
     bool include_partial_glyphs) const {
   DCHECK(target_x >= 0 && target_x <= width_);
+  if (target_x <= 0)
+    return !Rtl() ? 0 : num_characters_;
   const unsigned num_glyphs = glyph_data_.size();
   float current_x = 0;
   float current_advance = 0;
@@ -125,12 +127,22 @@ int ShapeResult::RunInfo::CharacterIndexForXPosition(
       // character.
       current_advance = current_advance / 2.0;
       next_x = current_x + prev_advance + current_advance;
+      // When include_partial_glyphs, "<=" or "<" is not a big deal because
+      // |next_x| is not at the character boundary.
+      if (target_x <= next_x)
+        return Rtl() ? prev_character_index : current_character_index;
     } else {
       next_x = current_x + current_advance;
+      if (!Rtl()) {

[eae, 2017/06/06 00:03:54]

How about

static bool targetPastEdge(bool rtl, float target_x, float next_x) {
  // In RTL, the edge belongs to the character on left.
  if (rtl)
    return target_x <= next_x;
  
  // In LTR, the edge belongs to the character on right.
  return target_x < next_x; 
}

if (targetPastEdge(Rtl(), target_x, next_x)
 return current_character_index;

+        // In LTR, the edge belongs to the character on right.
+        if (target_x < next_x)
+          return current_character_index;
+      } else {
+        // In RTL, the edge belongs to the character on left.
+        if (target_x <= next_x)
+          return current_character_index;
+      }
     }
-    if (current_x <= target_x && target_x <= next_x)
-      return include_partial_glyphs && Rtl() ? prev_character_index
-                                             : current_character_index;
     current_x = next_x;
     prev_character_index = current_character_index;
     ++glyph_index;