Initial draft - modify ViewportAnchor to know about both inner and outer viewports.

Source/web/WebViewImpl.cpp

Index: Source/web/WebViewImpl.cpp
diff --git a/Source/web/WebViewImpl.cpp b/Source/web/WebViewImpl.cpp
index b22e36738a6d56b8fc5494ae126db85ef990a710..79db9f375f58571864a0eb20632129b98b5c9fe9 100644
--- a/Source/web/WebViewImpl.cpp
+++ b/Source/web/WebViewImpl.cpp
@@ -1688,8 +1688,10 @@ void WebViewImpl::resize(const WebSize& newSize)
 
     ViewportAnchor viewportAnchor(&localFrameRootTemporary()->frame()->eventHandler());
     if (shouldAnchorAndRescaleViewport) {
-        viewportAnchor.setAnchor(view->visibleContentRect(),
-                                 FloatSize(viewportAnchorXCoord, viewportAnchorYCoord));
+        viewportAnchor.setAnchor(
+            view->visibleContentRect(),
+            visibleRectInDocument(),
+            FloatSize(viewportAnchorXCoord, viewportAnchorYCoord));
     }
 
     // FIXME: TextAutosizer does not yet support out-of-process frames.
@@ -1709,15 +1711,35 @@ void WebViewImpl::resize(const WebSize& newSize)
 
         if (shouldAnchorAndRescaleViewport) {
             float newPageScaleFactor = oldPageScaleFactor / oldMinimumPageScaleFactor * minimumPageScaleFactor();
-            IntSize scaledViewportSize = newSize;
-            scaledViewportSize.scale(1 / newPageScaleFactor);
-            setPageScaleFactor(newPageScaleFactor, viewportAnchor.computeOrigin(scaledViewportSize));
+            newPageScaleFactor = clampPageScaleFactorToLimits(newPageScaleFactor);
+
+            FloatSize pinchViewportSize = FloatSize(newSize);
+            pinchViewportSize.scale(1 / newPageScaleFactor);
+
+            IntSize mainFrameSize = view->visibleContentRect().size();
+
+            IntPoint mainFrameOrigin;
+            FloatPoint pinchViewportOrigin;
+            viewportAnchor.computeOrigins(view, mainFrameSize, pinchViewportSize,
+                &mainFrameOrigin, &pinchViewportOrigin);
+            scrollAndRescaleViewports(newPageScaleFactor, mainFrameOrigin, pinchViewportOrigin);
         }
     }
 
     sendResizeEventAndRepaint();
 }
 
+IntRect WebViewImpl::visibleRectInDocument() const
+{
+    if (pinchVirtualViewportEnabled()) {
+        // Inner viewport in the document coordinates
+        return enclosedIntRect(page()->frameHost().pinchViewport().visibleRectInDocument());
+    }
+
+    // Outer viewport in the document coordinates
+    return localFrameRootTemporary()->frameView()->visibleContentRect();
+}
+
 void WebViewImpl::willEndLiveResize()
 {
     if (mainFrameImpl() && mainFrameImpl()->frameView())
@@ -2913,6 +2935,39 @@ WebFloatPoint WebViewImpl::pinchViewportOffset() const
     return page()->frameHost().pinchViewport().visibleRect().location();
 }
 
+void WebViewImpl::scrollAndRescaleViewports(float scaleFactor,
+    const IntPoint & mainFrameOrigin,
+    const FloatPoint & pinchViewportOrigin)
+{
+    // Old way
+    if (!pinchVirtualViewportEnabled()) {
+        setPageScaleFactor(scaleFactor, mainFrameOrigin);
+        return;
+    }
+
+    if (!page())
+        return;
+
+    if (!mainFrameImpl())
+        return;
+
+    FrameView * view = mainFrameImpl()->frameView();
+    if (!view)
+        return;
+
+    // Order is important: pinch viewport location is clamped based on
+    // main frame scroll position and pinch viewport scale.
+
+    // Set outer viewport position.
+    view->setScrollOffset(mainFrameOrigin);
+
+    // Set the scale.
+    setPageScaleFactor(scaleFactor);
+
+    // Set inner viewport position.
+    page()->frameHost().pinchViewport().setLocation(pinchViewportOrigin);
+}
+
 void WebViewImpl::setPageScaleFactor(float scaleFactor)
 {
     ASSERT(page());
@@ -2955,7 +3010,6 @@ void WebViewImpl::setPageScaleFactor(float scaleFactor, const WebPoint& origin)
         page()->setPageScaleFactor(scaleFactor, newScrollOffset);
 }
 
-
 float WebViewImpl::deviceScaleFactor() const
 {
     if (!page())