Fix main thread top controls scrolling to mirror CC.

third_party/WebKit/Source/web/ResizeViewportAnchor.cpp

Index: third_party/WebKit/Source/web/ResizeViewportAnchor.cpp
diff --git a/third_party/WebKit/Source/web/ResizeViewportAnchor.cpp b/third_party/WebKit/Source/web/ResizeViewportAnchor.cpp
index 8c79f083d325094e0a910dd469f64936fe9b733a..d262e08301a275144238bcc9960ea802665ebfba 100644
--- a/third_party/WebKit/Source/web/ResizeViewportAnchor.cpp
+++ b/third_party/WebKit/Source/web/ResizeViewportAnchor.cpp
@@ -14,13 +14,54 @@ namespace blink {
 
 ResizeViewportAnchor::ResizeViewportAnchor(FrameView& rootFrameView, VisualViewport& visualViewport)
     : ViewportAnchor(rootFrameView, visualViewport)
-    , m_visualViewportInDocument(rootFrameView.getScrollableArea()->visibleContentRectDouble().location())
+    , m_visualViewportInDocument(rootFrameView.getScrollableArea()->scrollPositionDouble())
 {
 }
 
 ResizeViewportAnchor::~ResizeViewportAnchor()
 {
-    m_rootFrameView->getScrollableArea()->setScrollPosition(m_visualViewportInDocument, ProgrammaticScroll);
+    // TODO(bokan): Don't use RootFrameViewport::setScrollPosition since it
+    // assumes we can just set a sub-pixel precision offset on the FrameView.
+    // While we "can" do this, the offset that will be shipped to CC will be the
+    // truncated number and this class is used to handle TopControl movement
+    // which needs the two threads to match exactly pixel-for-pixel. We can
+    // replace this with RFV::setScrollPosition once Blink is sub-pixel scroll
+    // offset aware. crbug.com/414283.
+
+    ScrollableArea* rootViewport = m_rootFrameView->getScrollableArea();
+    ScrollableArea* layoutViewport =
+        m_rootFrameView->layoutViewportScrollableArea();
+
+    // 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.
+    m_visualViewport->clampToBoundaries();
+    layoutViewport->setScrollPosition(
+        layoutViewport->scrollPositionDouble(), ProgrammaticScroll);
+
+    DoubleSize delta = m_visualViewportInDocument
+        - rootViewport->scrollPositionDouble();
+
+    m_visualViewport->move(toFloatSize(delta));
+
+    delta = m_visualViewportInDocument
+        - rootViewport->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 = m_visualViewportInDocument
+        - rootViewport->scrollPositionDouble();
+    m_visualViewport->move(toFloatSize(delta));
 }
 
 } // namespace blink