Initial viewport is not the same as FrameView rect.

third_party/WebKit/Source/web/WebViewImpl.cpp

Index: third_party/WebKit/Source/web/WebViewImpl.cpp
diff --git a/third_party/WebKit/Source/web/WebViewImpl.cpp b/third_party/WebKit/Source/web/WebViewImpl.cpp
index e7656d4599529f41e05797222ecb259d8cabcb79..494aba995760a8a85db9bc0175d235d8ff24eae9 100644
--- a/third_party/WebKit/Source/web/WebViewImpl.cpp
+++ b/third_party/WebKit/Source/web/WebViewImpl.cpp
@@ -1906,6 +1906,7 @@ void WebViewImpl::performResize() {
   page()->frameHost().visualViewport().setSize(m_size);
 
   if (mainFrameImpl()->frameView()) {
+    mainFrameImpl()->frameView()->setInitialViewportSize(ICBSize);
     if (!mainFrameImpl()->frameView()->needsLayout())
       postLayoutResize(mainFrameImpl());
   }
@@ -4009,6 +4010,7 @@ void WebViewImpl::layoutUpdated(WebLocalFrameImpl* webframe) {
 
       page()->frameHost().visualViewport().setSize(m_size);
       pageScaleConstraintsSet().didChangeInitialContainingBlockSize(m_size);
+      frame->view()->setInitialViewportSize(m_size);
 
       m_client->didAutoResize(m_size);
       sendResizeEventAndRepaint();