--- a/dom/base/nsFrameLoader.cpp
+++ b/dom/base/nsFrameLoader.cpp
@@ -2332,16 +2332,17 @@ nsFrameLoader::GetWindowDimensions(nsInt
 NS_IMETHODIMP
 nsFrameLoader::UpdatePositionAndSize(nsSubDocumentFrame *aIFrame)
 {
   if (IsRemoteFrame()) {
     if (mRemoteBrowser) {
       ScreenIntSize size = aIFrame->GetSubdocumentSize();
       nsIntRect dimensions;
       NS_ENSURE_SUCCESS(GetWindowDimensions(dimensions), NS_ERROR_FAILURE);
+      mLazySize = size;
       mRemoteBrowser->UpdateDimensions(dimensions, size);
     }
     return NS_OK;
   }
   UpdateBaseWindowPositionAndSize(aIFrame);
   return NS_OK;
 }
 
@@ -2362,23 +2363,50 @@ nsFrameLoader::UpdateBaseWindowPositionA
     baseWindow->GetPosition(&x, &y);
 
     if (!weakFrame.IsAlive()) {
       // GetPosition() killed us
       return;
     }
 
     ScreenIntSize size = aIFrame->GetSubdocumentSize();
+    mLazySize = size;
 
     baseWindow->SetPositionAndSize(x, y, size.width, size.height,
                                    nsIBaseWindow::eDelayResize);
   }
 }
 
 NS_IMETHODIMP
+nsFrameLoader::GetLazyWidth(uint32_t* aLazyWidth)
+{
+  *aLazyWidth = mLazySize.width;
+
+  nsIFrame* frame = GetPrimaryFrameOfOwningContent();
+  if (frame) {
+    *aLazyWidth = frame->PresContext()->DevPixelsToIntCSSPixels(*aLazyWidth);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsFrameLoader::GetLazyHeight(uint32_t* aLazyHeight)
+{
+  *aLazyHeight = mLazySize.height;
+
+  nsIFrame* frame = GetPrimaryFrameOfOwningContent();
+  if (frame) {
+    *aLazyHeight = frame->PresContext()->DevPixelsToIntCSSPixels(*aLazyHeight);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
 nsFrameLoader::GetEventMode(uint32_t* aEventMode)
 {
   *aEventMode = mEventMode;
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsFrameLoader::SetEventMode(uint32_t aEventMode)

--- a/dom/base/nsFrameLoader.h
+++ b/dom/base/nsFrameLoader.h
@@ -370,16 +370,19 @@ private:
 
   TabParent* mRemoteBrowser;
   uint64_t mChildID;
 
   // See nsIFrameLoader.idl. EVENT_MODE_NORMAL_DISPATCH automatically
   // forwards some input events to out-of-process content.
   uint32_t mEventMode;
 
+  // Holds the last known size of the frame.
+  mozilla::ScreenIntSize mLazySize;
+
   bool mIsPrerendered : 1;
   bool mDepthTooGreat : 1;
   bool mIsTopLevelContent : 1;
   bool mDestroyCalled : 1;
   bool mNeedsAsyncDestroy : 1;
   bool mInSwap : 1;
   bool mInShow : 1;
   bool mHideCalled : 1;

--- a/dom/base/nsIFrameLoader.idl
+++ b/dom/base/nsIFrameLoader.idl
@@ -222,16 +222,31 @@ interface nsIFrameLoader : nsISupports
   readonly attribute boolean ownerIsMozBrowserOrAppFrame;
 
   /**
    * Find out whether the owner content really is a widget. If this attribute
    * returns true, |ownerIsMozBrowserOrAppFrame| must return true.
    */
   readonly attribute boolean ownerIsWidget;
 
+  /**
+   * The last known width of the frame. Reading this property will not trigger
+   * a reflow, and therefore may not reflect the current state of things. It
+   * should only be used in asynchronous APIs where values are not guaranteed
+   * to be up-to-date when received.
+   */
+  readonly attribute unsigned long lazyWidth;
+
+  /**
+   * The last known height of the frame. Reading this property will not trigger
+   * a reflow, and therefore may not reflect the current state of things. It
+   * should only be used in asynchronous APIs where values are not guaranteed
+   * to be up-to-date when received.
+   */
+  readonly attribute unsigned long lazyHeight;
 };
 
 %{C++
 class nsFrameLoader;
 %}
 
 native alreadyAddRefed_nsFrameLoader(already_AddRefed<nsFrameLoader>);