Scrollbar no longer inherits from FrameViewBase.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@gmail.com>
@@ skipping 1973 matching lines @@
   } else {
     ScrollbarControlSize scrollbar_size = kRegularScrollbar;
     if (style_source.StyleRef().HasAppearance()) {
       scrollbar_size = LayoutTheme::GetTheme().ScrollbarControlSizeForPart(
           style_source.StyleRef().Appearance());
     }
     scrollbar = Scrollbar::Create(
         ScrollableArea(), orientation, scrollbar_size,
         &ScrollableArea()->Box().GetFrame()->GetPage()->GetChromeClient());
   }
-  ScrollableArea()->Box().GetDocument().View()->AddChild(scrollbar);
+  ScrollableArea()->Box().GetDocument().View()->AddScrollbar(scrollbar);
   return scrollbar;
 }
 
 void PaintLayerScrollableArea::ScrollbarManager::DestroyScrollbar(
     ScrollbarOrientation orientation) {
   Member<Scrollbar>& scrollbar =
       orientation == kHorizontalScrollbar ? h_bar_ : v_bar_;
   DCHECK(orientation == kHorizontalScrollbar ? !h_bar_is_attached_
                                              : !v_bar_is_attached_);
   if (!scrollbar)
     return;
 
   ScrollableArea()->SetScrollbarNeedsPaintInvalidation(orientation);
   if (orientation == kHorizontalScrollbar)
     ScrollableArea()->rebuild_horizontal_scrollbar_layer_ = true;
   else
     ScrollableArea()->rebuild_vertical_scrollbar_layer_ = true;
 
   if (!scrollbar->IsCustomScrollbar())
     ScrollableArea()->WillRemoveScrollbar(*scrollbar, orientation);
 
-  ToFrameView(scrollbar->Parent())->RemoveChild(scrollbar.Get());
+  ScrollableArea()->Box().GetDocument().View()->RemoveScrollbar(scrollbar);

[szager1, 2017/04/21 08:37:52]

Can't this still be accessed via scrollbar->Parent() ?  That would be simpler
and easier to understand.

[joelhockey, 2017/04/21 09:56:10]

No.  The visibility between core and platform is at the heart of what makes this
change quite tricky.

scrollbar->Parent is of type FrameViewBase.  The RemoveScrollbar method belongs
to FrameView.  I want to get away from the downcasting.

[szager1, 2017/04/21 10:22:36]

I think the proper fix is to define Scrollbar ownership semantics in
FrameViewBase rather than FrameView.  Ideally, this would look something like:

- Move ScrollbarManager from paint/ to platform/
- Make sure ScrollbarManger has all methods that Scrollbar needs
- Make Scrollbar::parent_ a ScrollbarManager

With that in place, all you need to do is move all remaining methods from
FrameViewBase to FrameView, and declare victory.

[szager1, 2017/04/21 10:42:30]

Just to be totally clear, this line of code would become:

scrollbar->Manager()->RemoveScrollbar(scrollbar)

... or even:

scrollbar->RemoveFromManager()

[joelhockey, 2017/04/21 12:22:05]

Yes, nice idea.  I'll have a go at that.  I still need to get
https://codereview.chromium.org/2832883003 landed first when you have some time
to look at it.

   scrollbar->DisconnectFromScrollableArea();
   scrollbar = nullptr;
 }
 
 uint64_t PaintLayerScrollableArea::Id() const {
   return DOMNodeIds::IdForNode(Box().GetNode());
 }
 
 int PaintLayerScrollableArea::PreventRelayoutScope::count_ = 0;
 SubtreeLayoutScope*
@@ skipping 97 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     ClampScrollableAreas() {
   for (auto& scrollable_area : *needs_clamp_)
     scrollable_area->ClampScrollOffsetAfterOverflowChange();
   delete needs_clamp_;
   needs_clamp_ = nullptr;
 }
 
 }  // namespace blink