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

ui/views/controls/scrollbar/cocoa_scroll_bar.mm

 // 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.
 
 #import "ui/views/controls/scrollbar/cocoa_scroll_bar.h"
 
 #import "base/mac/sdk_forward_declarations.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "third_party/skia/include/effects/SkGradientShader.h"
 #include "ui/compositor/layer.h"
@@ skipping 150 matching lines @@
 // CocoaScrollBar class
 
 CocoaScrollBar::CocoaScrollBar(bool horizontal)
     : BaseScrollBar(horizontal, new CocoaScrollBarThumb(this)),
       hide_scrollbar_timer_(
           FROM_HERE,
           base::TimeDelta::FromMilliseconds(kScrollbarHideTimeoutMs),
           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]);
   scroller_style_ = [ViewsScrollbarBridge getPreferredScrollerStyle];
 
   thickness_animation_.SetSlideDuration(kExpandDurationMs);
 
   SetPaintToLayer(true);
   has_scrolltrack_ = scroller_style_ == NSScrollerStyleLegacy;
   layer()->SetOpacity(scroller_style_ == NSScrollerStyleOverlay ? 0.0f : 1.0f);
@@ skipping 134 matching lines @@
   }
 
   hide_scrollbar_timer_.Reset();
 }
 
 void CocoaScrollBar::OnMouseExited(const ui::MouseEvent& event) {
   ResetOverlayScrollbar();
 }
 
 //////////////////////////////////////////////////////////////////
-// CocoaScrollBar::BaseScrollBar:
+// CocoaScrollBar::ScrollBar:
 
-void CocoaScrollBar::ScrollToPosition(int position) {
-  BaseScrollBar::ScrollToPosition(position);
+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;
+
+  last_contents_scroll_offset_ = contents_scroll_offset;
 
   if (GetCocoaScrollBarThumb()->IsStatePressed())
     did_start_dragging_ = true;
 
   if (scroller_style_ == NSScrollerStyleOverlay) {
     ShowScrollbar();
     hide_scrollbar_timer_.Reset();
   }
 }
 
@@ skipping 140 matching lines @@
 void CocoaScrollBar::SetScrolltrackVisible(bool visible) {
   has_scrolltrack_ = visible;
   SchedulePaint();
 }
 
 CocoaScrollBarThumb* CocoaScrollBar::GetCocoaScrollBarThumb() const {
   return static_cast<CocoaScrollBarThumb*>(GetThumb());
 }
 
 }  // namespace views