Uses |AXPosition| to enable navigation by character, word and line in content editables on the Mac.

ui/accessibility/ax_position.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_ACCESSIBILITY_AX_POSITION_H_
 #define UI_ACCESSIBILITY_AX_POSITION_H_
 
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 132 matching lines @@
               " anchor_id=" + base::IntToString(anchor_id_) + " text_offset=" +
               str_text_offset + " affinity=" +
               ui::ToString(static_cast<AXTextAffinity>(affinity_));
       }
     }
 
     if (!IsTextPosition() || text_offset() > MaxTextOffset())
       return str;
 
     std::string text = base::UTF16ToUTF8(GetInnerText());
-    DCHECK(!text.empty());
+    DCHECK_GE(text_offset_, 0);
+    DCHECK_LE(text_offset_, static_cast<int>(text.length()));
     std::string annotated_text;
     if (text_offset() == MaxTextOffset()) {
       annotated_text = text + "<>";
     } else {
       annotated_text = text.substr(0, text_offset()) + "<" +
                        text[text_offset()] + ">" +
                        text.substr(text_offset() + 1);
     }
 
     return str + " annotated_text=" + annotated_text;
@@ skipping 97 matching lines @@
   AXPositionInstance AsTreePosition() const {
     if (IsNullPosition() || IsTreePosition())
       return Clone();
 
     AXPositionInstance copy = Clone();
     DCHECK(copy);
     DCHECK_NE(copy->text_offset_, INVALID_OFFSET);
     // If the anchor node has no text inside it then the child index should be
     // set to |BEFORE_TEXT|, hence the check for the text's length.
     if (copy->MaxTextOffset() > 0 &&
-        copy->text_offset_ == copy->MaxTextOffset()) {
+        copy->text_offset_ >= copy->MaxTextOffset()) {
       copy->child_index_ = copy->AnchorChildCount();
     } else {
       DCHECK_GE(copy->text_offset_, 0);
-      DCHECK_LT(copy->text_offset_, copy->MaxTextOffset());
       copy->child_index_ = BEFORE_TEXT;
 
       int current_offset = 0;
       for (int i = 0; i < copy->AnchorChildCount(); ++i) {
         AXPositionInstance child = copy->CreateChildPositionAt(i);
         DCHECK(child);
         if (copy->text_offset_ >= current_offset &&
             copy->text_offset_ < (current_offset + child->MaxTextOffset())) {
           copy->child_index_ = i;
           break;
@@ skipping 849 matching lines @@
 
 template <class AXPositionType, class AXNodeType>
 bool operator>=(const AXPosition<AXPositionType, AXNodeType>& first,
                 const AXPosition<AXPositionType, AXNodeType>& second) {
   return first == second || first > second;
 }
 
 }  // namespace ui
 
 #endif  // UI_ACCESSIBILITY_AX_POSITION_H_