Fix event targeting for overlay scrollbar thumbs (in native UI).

ui/views/controls/scrollbar/base_scroll_bar.cc

 // Copyright (c) 2012 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.
 
 #include "ui/views/controls/scrollbar/base_scroll_bar.h"
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/callback.h"
 #include "base/compiler_specific.h"
@@ skipping 14 matching lines @@
 #include "ui/views/widget/widget.h"
 
 #undef min
 #undef max
 
 namespace views {
 
 ///////////////////////////////////////////////////////////////////////////////
 // BaseScrollBar, public:
 
-BaseScrollBar::BaseScrollBar(bool horizontal, BaseScrollBarThumb* thumb)
+BaseScrollBar::BaseScrollBar(bool horizontal)
     : ScrollBar(horizontal),
-      thumb_(thumb),
+      thumb_(nullptr),
       contents_size_(0),
       contents_scroll_offset_(0),
       viewport_size_(0),
       last_scroll_amount_(SCROLL_NONE),
       repeater_(base::Bind(&BaseScrollBar::TrackClicked,
                            base::Unretained(this))),
       context_menu_mouse_position_(0) {
-  AddChildView(thumb_);
+  set_context_menu_controller(this);
+}
 
-  set_context_menu_controller(this);
-  thumb_->set_context_menu_controller(this);
+BaseScrollBar::~BaseScrollBar() {}
+
+void BaseScrollBar::SetThumb(BaseScrollBarThumb* thumb) {
+  DCHECK(!thumb_);
+  thumb_ = thumb;
+  AddChildView(thumb);
+  thumb->set_context_menu_controller(this);
 }
 
 void BaseScrollBar::ScrollByAmount(ScrollAmount amount) {
   int offset = contents_scroll_offset_;
   switch (amount) {
     case SCROLL_START:
       offset = GetMinPosition();
       break;
     case SCROLL_END:
       offset = GetMaxPosition();
@@ skipping 14 matching lines @@
       offset += GetScrollIncrement(true, true);
       offset = std::min(GetMaxPosition(), offset);
       break;
     default:
       break;
   }
   contents_scroll_offset_ = offset;
   ScrollContentsToOffset();
 }
 
-BaseScrollBar::~BaseScrollBar() {
-}
-
 void BaseScrollBar::ScrollToThumbPosition(int thumb_position,
                                           bool scroll_to_middle) {
   contents_scroll_offset_ =
       CalculateContentsOffset(thumb_position, scroll_to_middle);
   if (contents_scroll_offset_ < GetMinPosition()) {
     contents_scroll_offset_ = GetMinPosition();
   } else if (contents_scroll_offset_ > GetMaxPosition()) {
     contents_scroll_offset_ = GetMaxPosition();
   }
   ScrollContentsToOffset();
@@ skipping 381 matching lines @@
   int track_size = GetTrackSize();
   if (track_size == thumb_size)
     return 0;
   if (scroll_to_middle)
     thumb_position = thumb_position - (thumb_size / 2);
   return (thumb_position * (contents_size_ - viewport_size_)) /
          (track_size - thumb_size);
 }
 
 }  // namespace views