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 a6c28a74e72c4c6e712e2d07c185f1504890f323..64ba632205eca7573aff4d51029a6918687e1b41 100644
--- a/Source/web/WebViewImpl.cpp
+++ b/Source/web/WebViewImpl.cpp
@@ -1689,8 +1689,10 @@ 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.
@@ -1710,15 +1712,33 @@ 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);

[aelias_OOO_until_Jul13, 2014/09/17 05:16:36]

nit: second FloatSize() shouldn't be necessary, please delete it. 
initialization "=" is shorthand for constructor invocation, if I recall my C++
correctly.

[timav, 2014/09/17 17:54:45]

The cast is required to convert WebSize -> IntSize -> FloatSize, I guess C++
does implicit conversions only if there is a direct path from A to B.
IntSize() works here too, but compiler complains if there is no cast at all.

+            pinchViewportSize.scale(1 / newPageScaleFactor);
+
+            IntPoint mainFrameOrigin;
+            FloatPoint pinchViewportOrigin;
+            viewportAnchor.computeOrigins(*view, 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())
@@ -2918,6 +2938,39 @@ WebFloatPoint WebViewImpl::pinchViewportOffset() const
     return page()->frameHost().pinchViewport().visibleRect().location();
 }
 
+void WebViewImpl::scrollAndRescaleViewports(float scaleFactor,
+    const IntPoint & mainFrameOrigin,

[aelias_OOO_until_Jul13, 2014/09/17 05:16:36]

nit: "IntPoint& mainFrameOrigin" (no space before)

[timav, 2014/09/17 17:54:45]

Done.

+    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.

[aelias_OOO_until_Jul13, 2014/09/17 05:16:36]

Please delete the three comments starting here.  But it's fine to keep the
"Order is important" comment.

[timav, 2014/09/17 17:54:45]

Done.

+    view->setScrollOffset(mainFrameOrigin);
+
+    // Set the scale.
+    setPageScaleFactor(scaleFactor);
+
+    // Set inner viewport position.
+    page()->frameHost().pinchViewport().setLocation(pinchViewportOrigin);
+}
+
 void WebViewImpl::setPageScaleFactor(float scaleFactor)
 {
     ASSERT(page());
@@ -2960,7 +3013,6 @@ void WebViewImpl::setPageScaleFactor(float scaleFactor, const WebPoint& origin)
         page()->setPageScaleFactor(scaleFactor, newScrollOffset);
 }
 
-

[aelias_OOO_until_Jul13, 2014/09/17 05:16:36]

nit: unnecessary newline

[timav, 2014/09/17 17:54:45]

Newline removed in the new version.

 float WebViewImpl::deviceScaleFactor() const
 {
     if (!page())