Viewport resize anchoring should use the current layout viewport.

third_party/WebKit/Source/core/frame/FrameView.cpp

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index 002fc8d1ec0aae8670224e90f8ff3dbf3c913059..c1bcb113968d0c9ecd9e516aba230b5c90b98274 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -810,8 +810,11 @@ void FrameView::performPreLayoutTasks()
         m_viewportScrollableArea = RootFrameViewport::create(visualViewport, *layoutViewport);
     }
 
-    if (!m_scrollAnchor.hasScroller())
-        m_scrollAnchor.setScroller(m_viewportScrollableArea ? m_viewportScrollableArea : this);
+    if (!m_scrollAnchor.hasScroller()) {
+        m_scrollAnchor.setScroller(m_viewportScrollableArea
+            ? static_cast<ScrollableArea*>(m_viewportScrollableArea.get())

[skobes, 2016/08/13 00:16:11]

Can you avoid the cast with a ScrollableArea* temp var, or by using "if" instead
of "?:" ?

[bokan, 2016/08/13 00:38:21]

Replaced with a ScrollableArea* temp

+            : this);
+    }
 
     if (shouldPerformScrollAnchoring())
         m_scrollAnchor.save();
@@ -4082,6 +4085,11 @@ ScrollableArea* FrameView::layoutViewportScrollableArea()
     return layoutViewItem.isNull() ? nullptr : layoutViewItem.getScrollableArea();
 }
 
+RootFrameViewport* FrameView::getRootFrameViewport()
+{
+    return m_viewportScrollableArea.get();
+}
+
 LayoutObject* FrameView::viewportLayoutObject() const
 {
     if (Document* document = frame().document()) {