[not for review - epic CL] Adding Elastic+Momentum+Layered scrolling to views::ScrollView

ui/views/controls/scrollbar/cocoa_scroll_bar.mm

Index: ui/views/controls/scrollbar/cocoa_scroll_bar.mm
diff --git a/ui/views/controls/scrollbar/cocoa_scroll_bar.mm b/ui/views/controls/scrollbar/cocoa_scroll_bar.mm
index 3c6bb9b607f7881827abd4f587ee52ed03ff39c3..a22610664477005b9f41e6307985679bd35ba1cc 100644
--- a/ui/views/controls/scrollbar/cocoa_scroll_bar.mm
+++ b/ui/views/controls/scrollbar/cocoa_scroll_bar.mm
@@ -168,6 +168,7 @@ CocoaScrollBar::CocoaScrollBar(bool horizontal)
           base::Bind(&CocoaScrollBar::HideScrollbar, base::Unretained(this)),
           false),
       thickness_animation_(this),
+      last_contents_scroll_offset_(0),
       is_expanded_(false),
       did_start_dragging_(false) {
   bridge_.reset([[ViewsScrollbarBridge alloc] initWithDelegate:this]);
@@ -322,10 +323,24 @@ void CocoaScrollBar::OnMouseExited(const ui::MouseEvent& event) {
 }
 
 //////////////////////////////////////////////////////////////////
-// CocoaScrollBar::BaseScrollBar:
+// CocoaScrollBar::ScrollBar:
+
+void CocoaScrollBar::Update(int viewport_size,
+                            int content_size,
+                            int contents_scroll_offset) {
+  // TODO(tapted): Pass in overscroll amounts from the Layer and "Squish" the
+  // scroller thumb accordingly.
+  BaseScrollBar::Update(viewport_size, content_size, contents_scroll_offset);
+
+  // Only reveal the scroller when |contents_scroll_offset| changes. Note this
+  // is different to GetPosition() which can change due to layout. A layout
+  // change can also change the offset; show the scroller in these cases. This
+  // is consistent with WebContents (Cocoa will also show a scroller with any
+  // mouse-initiated layout, put not programmatic size changes).
+  if (contents_scroll_offset == last_contents_scroll_offset_)
+    return;
 
-void CocoaScrollBar::ScrollToPosition(int position) {
-  BaseScrollBar::ScrollToPosition(position);
+  last_contents_scroll_offset_ = contents_scroll_offset;
 
   if (GetCocoaScrollBarThumb()->IsStatePressed())
     did_start_dragging_ = true;