MacViews: Reveal scrollbars when resting on the trackpad.

ui/views/controls/scroll_view.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/scroll_view.h"
 
 #include "base/feature_list.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "ui/events/event.h"
@@ skipping 454 matching lines @@
 
 void ScrollView::OnMouseExited(const ui::MouseEvent& event) {
   if (horiz_sb_)
     horiz_sb_->OnMouseExitedScrollView(event);
   if (vert_sb_)
     vert_sb_->OnMouseExitedScrollView(event);
 }
 
 void ScrollView::OnScrollEvent(ui::ScrollEvent* event) {
 #if defined(OS_MACOSX)
+  if (!contents_)
+    return;
+
   // TODO(tapted): Send |event| to a cc::InputHandler. For now, there's nothing
   // to do because Widget::OnScrollEvent() will automatically process an
   // unhandled ScrollEvent as a MouseWheelEvent.
+
+  // A direction might not be known when the event stream starts, notify both
+  // scrollbars that they may be about scroll, or that they may need to cancel
+  // UI feedback once the scrolling direction is known.
+  if (horiz_sb_)
+    horiz_sb_->ObserveScrollEvent(*event);
+  if (vert_sb_)
+    vert_sb_->ObserveScrollEvent(*event);
 #endif
 }
 
 void ScrollView::OnGestureEvent(ui::GestureEvent* event) {
   // If the event happened on one of the scrollbars, then those events are
   // sent directly to the scrollbars. Otherwise, only scroll events are sent to
   // the scrollbars.
   bool scroll_event = event->type() == ui::ET_GESTURE_SCROLL_UPDATE ||
                       event->type() == ui::ET_GESTURE_SCROLL_BEGIN ||
                       event->type() == ui::ET_GESTURE_SCROLL_END ||
@@ skipping 297 matching lines @@
 
 VariableRowHeightScrollHelper::RowInfo
     FixedRowHeightScrollHelper::GetRowInfo(int y) {
   if (y < top_margin_)
     return RowInfo(0, top_margin_);
   return RowInfo((y - top_margin_) / row_height_ * row_height_ + top_margin_,
                  row_height_);
 }
 
 }  // namespace views