Improve scroll anchoring's interactions with the visual viewport

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

Index: third_party/WebKit/Source/core/frame/RootFrameViewport.cpp
diff --git a/third_party/WebKit/Source/core/frame/RootFrameViewport.cpp b/third_party/WebKit/Source/core/frame/RootFrameViewport.cpp
index 2eed1f5ab8236f91d790f10d6246ae3cf7024062..5b1435d3f041072cba94559554389ac5b4998df8 100644
--- a/third_party/WebKit/Source/core/frame/RootFrameViewport.cpp
+++ b/third_party/WebKit/Source/core/frame/RootFrameViewport.cpp
@@ -23,6 +23,11 @@ void RootFrameViewport::setLayoutViewport(ScrollableArea& newLayoutViewport)
     m_layoutViewport = &newLayoutViewport;
 }
 
+LayoutBox* RootFrameViewport::layoutBox() const
+{
+    return layoutViewport().layoutBox();
+}
+
 void RootFrameViewport::updateScrollAnimator()
 {
     scrollAnimator().setCurrentPosition(toFloatPoint(scrollOffsetFromScrollAnimators()));
@@ -82,8 +87,13 @@ void RootFrameViewport::setScrollPosition(const DoublePoint& position, ScrollTyp
     if (scrollType == ProgrammaticScroll && !layoutViewport().isProgrammaticallyScrollable())
         return;
 
+    if (scrollType == AnchoringScroll) {
+        distributeScrollBetweenViewports(position, scrollType, scrollBehavior, LayoutViewport);
+        return;
+    }
+
     if (scrollBehavior == ScrollBehaviorSmooth) {
-        distributeScrollBetweenViewports(position, scrollType, scrollBehavior);
+        distributeScrollBetweenViewports(position, scrollType, scrollBehavior, VisualViewport);
         return;
     }
 
@@ -124,10 +134,10 @@ LayoutRect RootFrameViewport::scrollIntoView(const LayoutRect& rectInContent, co
 
 void RootFrameViewport::setScrollOffset(const DoublePoint& offset, ScrollType scrollType)
 {
-    distributeScrollBetweenViewports(DoublePoint(offset), scrollType, ScrollBehaviorInstant);
+    distributeScrollBetweenViewports(DoublePoint(offset), scrollType, ScrollBehaviorInstant, VisualViewport);
 }
 
-void RootFrameViewport::distributeScrollBetweenViewports(const DoublePoint& offset, ScrollType scrollType, ScrollBehavior behavior)
+void RootFrameViewport::distributeScrollBetweenViewports(const DoublePoint& offset, ScrollType scrollType, ScrollBehavior behavior, ViewportToScrollFirst scrollFirst)
 {
     // Make sure we use the scroll positions as reported by each viewport's ScrollAnimatorBase, since its
     // ScrollableArea's position may have the fractional part truncated off.
@@ -138,22 +148,25 @@ void RootFrameViewport::distributeScrollBetweenViewports(const DoublePoint& offs
     if (delta.isZero())
         return;
 
-    DoublePoint targetPosition = visualViewport().clampScrollPosition(
-        visualViewport().scrollAnimator().currentPosition() + delta);
+    ScrollableArea& primary = scrollFirst == VisualViewport ? visualViewport() : layoutViewport();
+    ScrollableArea& secondary = scrollFirst == VisualViewport ? layoutViewport() : visualViewport();
+
+    DoublePoint targetPosition = primary.clampScrollPosition(
+        primary.scrollAnimator().currentPosition() + delta);
 
-    visualViewport().setScrollPosition(targetPosition, scrollType, behavior);
+    primary.setScrollPosition(targetPosition, scrollType, behavior);
 
     // Scroll the secondary viewport if all of the scroll was not applied to the
     // primary viewport.
-    DoublePoint updatedPosition = layoutViewport().scrollAnimator().currentPosition() + FloatPoint(targetPosition);
+    DoublePoint updatedPosition = secondary.scrollAnimator().currentPosition() + FloatPoint(targetPosition);
     DoubleSize applied = updatedPosition - oldPosition;
     delta -= applied;
 
     if (delta.isZero())
         return;
 
-    targetPosition = layoutViewport().clampScrollPosition(layoutViewport().scrollAnimator().currentPosition() + delta);
-    layoutViewport().setScrollPosition(targetPosition, scrollType, behavior);
+    targetPosition = secondary.clampScrollPosition(secondary.scrollAnimator().currentPosition() + delta);
+    secondary.setScrollPosition(targetPosition, scrollType, behavior);
 }
 
 IntPoint RootFrameViewport::scrollPosition() const