--- a/dom/base/nsFrameLoader.cpp
+++ b/dom/base/nsFrameLoader.cpp
@@ -1000,16 +1000,38 @@ nsFrameLoader::Hide()
 
   nsCOMPtr<nsIBaseWindow> baseWin = do_QueryInterface(mDocShell);
   NS_ASSERTION(baseWin,
                "Found an nsIDocShell which doesn't implement nsIBaseWindow.");
   baseWin->SetVisibility(false);
   baseWin->SetParentWidget(nullptr);
 }
 
+void
+nsFrameLoader::ForceLayoutIfNecessary()
+{
+  nsIFrame* frame = GetPrimaryFrameOfOwningContent();
+  if (!frame) {
+    return;
+  }
+
+  nsPresContext* presContext = frame->PresContext();
+  if (!presContext) {
+    return;
+  }
+
+  // Only force the layout flush if the frameloader hasn't ever been
+  // run through layout.
+  if (frame->GetStateBits() & NS_FRAME_FIRST_REFLOW) {
+    if (nsCOMPtr<nsIPresShell> shell = presContext->GetPresShell()) {
+      shell->FlushPendingNotifications(FlushType::Layout);
+    }
+  }
+}
+
 nsresult
 nsFrameLoader::SwapWithOtherRemoteLoader(nsFrameLoader* aOther,
                                          nsIFrameLoaderOwner* aThisOwner,
                                          nsIFrameLoaderOwner* aOtherOwner)
 {
   MOZ_ASSERT(NS_IsMainThread());
 
 #ifdef DEBUG

--- a/dom/base/nsFrameLoader.h
+++ b/dom/base/nsFrameLoader.h
@@ -228,16 +228,21 @@ public:
 
   /**
    * Called from the layout frame associated with this frame loader, when
    * the frame is being torn down; this notifies us that out widget and view
    * are going away and we should unhook from them.
    */
   void Hide();
 
+  // Used when content is causing a FrameLoader to be created, and
+  // needs to try forcing layout to flush in order to get accurate
+  // dimensions for the content area.
+  void ForceLayoutIfNecessary();
+
   // The guts of an nsIFrameLoaderOwner::SwapFrameLoader implementation.  A
   // frame loader owner needs to call this, and pass in the two references to
   // nsRefPtrs for frame loaders that need to be swapped.
   nsresult SwapWithOtherLoader(nsFrameLoader* aOther,
                                nsIFrameLoaderOwner* aThisOwner,
                                nsIFrameLoaderOwner* aOtherOwner);
 
   nsresult SwapWithOtherRemoteLoader(nsFrameLoader* aOther,

--- a/dom/ipc/ContentParent.cpp
+++ b/dom/ipc/ContentParent.cpp
@@ -4732,16 +4732,22 @@ ContentParent::CommonCreateWindow(PBrows
                                               nsIBrowserDOMWindow::OPEN_NEW,
                                               aNextTabParentId, aName,
                                               getter_AddRefs(frameLoaderOwner));
     }
     if (NS_SUCCEEDED(aResult) && frameLoaderOwner) {
       RefPtr<nsFrameLoader> frameLoader = frameLoaderOwner->GetFrameLoader();
       if (frameLoader) {
         aNewTabParent = frameLoader->GetTabParent();
+        // At this point, it's possible the inserted frameloader hasn't gone through
+        // layout yet. To ensure that the dimensions that we send down when telling the
+        // frameloader to display will be correct (instead of falling back to a 10x10
+        // default), we force layout if necessary to get the most up-to-date dimensions.
+        // See bug 1358712 for details.
+        frameLoader->ForceLayoutIfNecessary();
       }
     } else if (NS_SUCCEEDED(aResult) && !frameLoaderOwner) {
       // Fall through to the normal window opening code path when there is no
       // window which we can open a new tab in.
       openLocation = nsIBrowserDOMWindow::OPEN_NEWWINDOW;
     } else {
       *aWindowIsNew = false;
     }
@@ -4763,16 +4769,36 @@ ContentParent::CommonCreateWindow(PBrows
                                              aNextTabParentId,
                                              !aSetOpener,
                                              getter_AddRefs(aNewTabParent));
   if (NS_WARN_IF(NS_FAILED(aResult))) {
     return IPC_OK();
   }
 
   MOZ_ASSERT(aNewTabParent);
+
+  // At this point, it's possible the inserted frameloader hasn't gone through
+  // layout yet. To ensure that the dimensions that we send down when telling the
+  // frameloader to display will be correct (instead of falling back to a 10x10
+  // default), we force layout if necessary to get the most up-to-date dimensions.
+  // See bug 1358712 for details.
+  //
+  // This involves doing a bit of gymnastics in order to get at the FrameLoader,
+  // so we scope this to avoid polluting the main function scope.
+  {
+    nsCOMPtr<Element> frameElement =
+      TabParent::GetFrom(aNewTabParent)->GetOwnerElement();
+    MOZ_ASSERT(frameElement);
+    nsCOMPtr<nsIFrameLoaderOwner> frameLoaderOwner = do_QueryInterface(frameElement);
+    MOZ_ASSERT(frameLoaderOwner);
+    RefPtr<nsFrameLoader> frameLoader = frameLoaderOwner->GetFrameLoader();
+    MOZ_ASSERT(frameLoader);
+    frameLoader->ForceLayoutIfNecessary();
+  }
+
   // If we were passed a name for the window which would override the default,
   // we should send it down to the new tab.
   if (nsContentUtils::IsOverridingWindowName(aName)) {
     Unused << TabParent::GetFrom(aNewTabParent)->SendSetWindowName(aName);
   }
 
   // Don't send down the OriginAttributes if the content process is handling
   // setting up the window for us. We only want to send them in the async case.