Viewport resize anchoring should use the current layout viewport.

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

 // Copyright 2015 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 "core/frame/RootFrameViewport.h"
 
 #include "core/frame/FrameView.h"
 #include "core/layout/ScrollAlignment.h"
 #include "platform/geometry/DoubleRect.h"
 #include "platform/geometry/FloatRect.h"
 #include "platform/geometry/LayoutRect.h"
 
 namespace blink {
 
 RootFrameViewport::RootFrameViewport(ScrollableArea& visualViewport, ScrollableArea& layoutViewport)
     : m_visualViewport(visualViewport)
     , m_layoutViewport(layoutViewport)
 {
 }
 
 void RootFrameViewport::setLayoutViewport(ScrollableArea& newLayoutViewport)
 {
     m_layoutViewport = &newLayoutViewport;
 }
 
+void RootFrameViewport::restoreToAnchor(const DoublePoint& targetPosition)
+{
+    // Clamp the scroll offset of each viewport now so that we force any invalid
+    // offsets to become valid so we can compute the correct deltas.
+    visualViewport().setScrollPosition(
+        visualViewport().scrollPositionDouble(), ProgrammaticScroll);
+    layoutViewport().setScrollPosition(
+        layoutViewport().scrollPositionDouble(), ProgrammaticScroll);
+
+    DoubleSize delta = targetPosition - scrollPositionDouble();
+
+    visualViewport().setScrollPosition(
+        visualViewport().scrollPositionDouble() + delta, ProgrammaticScroll);
+
+    delta = targetPosition - scrollPositionDouble();
+
+    // Since the main thread FrameView has integer scroll offsets, scroll it to
+    // the next pixel and then we'll scroll the visual viewport again to
+    // compensate for the sub-pixel offset. We need this "overscroll" to ensure
+    // the pixel of which we want to be partially in appears fully inside the
+    // FrameView since the VisualViewport is bounded by the FrameView.
+    IntSize layoutDelta = IntSize(
+        delta.width() < 0 ? floor(delta.width()) : ceil(delta.width()),
+        delta.height() < 0 ? floor(delta.height()) : ceil(delta.height()));
+
+    layoutViewport().setScrollPosition(
+        layoutViewport().scrollPosition() + layoutDelta, ProgrammaticScroll);
+
+    delta = targetPosition - scrollPositionDouble();
+    visualViewport().setScrollPosition(
+        visualViewport().scrollPositionDouble() + delta, ProgrammaticScroll);
+}
+
 LayoutBox* RootFrameViewport::layoutBox() const
 {
     return layoutViewport().layoutBox();
 }
 
 void RootFrameViewport::updateScrollAnimator()
 {
     scrollAnimator().setCurrentPosition(toFloatPoint(scrollOffsetFromScrollAnimators()));
 }
 
@@ skipping 324 matching lines @@
 }
 
 DEFINE_TRACE(RootFrameViewport)
 {
     visitor->trace(m_visualViewport);
     visitor->trace(m_layoutViewport);
     ScrollableArea::trace(visitor);
 }
 
 } // namespace blink