diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index bde55ee5857..2fee5e9c18e 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -379,6 +379,23 @@ static nsresult
 DeletingFrameSubtree(nsFrameManager* aFrameManager,
                      nsIFrame*       aFrame);
 
+void nsFocusEventSuppressor::Suppress(nsIPresShell *aPresShell)
+{
+  NS_ASSERTION(aPresShell, "Need non-null nsIPresShell!");
+  if (!mViewManager) {
+    nsFrameManager *frameManager = aPresShell->FrameManager();
+    mViewManager = frameManager->GetPresContext()->GetViewManager();
+    NS_ASSERTION(mViewManager, "We must have an mViewManager here");
+  }
+  mViewManager->SuppressFocusEvents();
+}
+
+void nsFocusEventSuppressor::Unsuppress()
+{
+  NS_ASSERTION(mViewManager, "We must have an mViewManager here");
+  mViewManager->UnsuppressFocusEvents();
+}
+
 #ifdef  MOZ_SVG
 
 static nsIFrame *
@@ -10233,6 +10250,12 @@ nsCSSFrameConstructor::AttributeChanged(nsIContent* aContent,
   return result;
 }
 
+void
+nsCSSFrameConstructor::BeginUpdate() {
+  mFocusSuppressor.Suppress(mPresShell);
+  ++mUpdateCount;
+}
+
 void
 nsCSSFrameConstructor::EndUpdate()
 {
@@ -10243,7 +10266,7 @@ nsCSSFrameConstructor::EndUpdate()
     RecalcQuotesAndCounters();
     NS_ASSERTION(mUpdateCount == 1, "Odd update count");
   }
-
+  mFocusSuppressor.Unsuppress();
   --mUpdateCount;
 }
 
diff --git a/layout/base/nsCSSFrameConstructor.h b/layout/base/nsCSSFrameConstructor.h
index 0a025e2bf75..1f8e874d76d 100644
--- a/layout/base/nsCSSFrameConstructor.h
+++ b/layout/base/nsCSSFrameConstructor.h
@@ -52,6 +52,7 @@
 #include "nsHashKeys.h"
 #include "nsThreadUtils.h"
 #include "nsPageContentFrame.h"
+#include "nsIViewManager.h"
 
 class nsIDocument;
 struct nsFrameItems;
@@ -73,6 +74,19 @@ struct nsFindFrameHint
   nsFindFrameHint() : mPrimaryFrameForPrevSibling(nsnull) { }
 };
 
+// Class which makes an nsIPresShell's ViewManager supress
+// focus/blur events. This prevents the frame tree from being changed
+// by focus handlers etc while *we* are trying to change it.
+// Fix for bug 399852.
+class nsFocusEventSuppressor
+{
+public:
+  void Suppress(nsIPresShell *aPresShell);
+  void Unsuppress();
+private:
+  nsCOMPtr<nsIViewManager> mViewManager;
+};
+
 typedef void (PR_CALLBACK nsLazyFrameConstructionCallback)
              (nsIContent* aContent, nsIFrame* aFrame, void* aArg);
 
@@ -148,7 +162,7 @@ public:
                             PRInt32     aModType,
                             PRUint32    aStateMask);
 
-  void BeginUpdate() { ++mUpdateCount; }
+  void BeginUpdate();
   void EndUpdate();
   void RecalcQuotesAndCounters();
 
@@ -162,6 +176,9 @@ public:
   nsresult ProcessRestyledFrames(nsStyleChangeList& aRestyleArray);
 
 private:
+
+  nsFocusEventSuppressor mFocusSuppressor;
+
   // Note: It's the caller's responsibility to make sure to wrap a
   // ProcessOneRestyle call in a view update batch.
   // This function does not call ProcessAttachedQueue() on the binding manager.
diff --git a/view/public/nsIViewManager.h b/view/public/nsIViewManager.h
index 14f5d65d508..16eae946f60 100644
--- a/view/public/nsIViewManager.h
+++ b/view/public/nsIViewManager.h
@@ -481,6 +481,31 @@ public:
    * (aFromScroll is false) or scrolled (aFromScroll is true).
    */
   NS_IMETHOD SynthesizeMouseMove(PRBool aFromScroll)=0;
+   
+  /**
+   * Enables focus/blur event suppression. This stops focus/blur
+   * events from reaching the widgets. This should be enabled 
+   * when we're messing with the frame tree, so focus/blur handlers
+   * don't mess with stuff while we are. See Bug 399852.   
+   */
+  virtual void SuppressFocusEvents()=0;
+  
+  /**
+   * Disables focus/blur event suppression. This "reboots" the focus
+   * by sending a blur to what was focused before suppression began,
+   * and by sending a focus event to what should be currently focused.
+   * Note this can run arbitrary code, and could even destroy the view
+   * manager. The suppression should be enabled when we're messing with
+   * the frame tree, so focus/blur handlers don't mess with stuff while
+   * we are. See Bug 399852.
+   */
+  virtual void UnsuppressFocusEvents()=0;
+
+  /**
+   * Returns true when focus suppression is on.
+   */
+  virtual PRBool IsFocusSuppressed()=0;
+
 };
 
 NS_DEFINE_STATIC_IID_ACCESSOR(nsIViewManager, NS_IVIEWMANAGER_IID)
diff --git a/view/src/nsView.cpp b/view/src/nsView.cpp
index f2bcbd31ab4..3311a365583 100644
--- a/view/src/nsView.cpp
+++ b/view/src/nsView.cpp
@@ -200,6 +200,13 @@ nsView::~nsView()
 {
   MOZ_COUNT_DTOR(nsView);
 
+  if (this == nsViewManager::GetViewFocusedBeforeSuppression()) {
+    nsViewManager::SetViewFocusedBeforeSuppression(nsnull);
+  }
+  if (this == nsViewManager::GetCurrentlyFocusedView()) {
+    nsViewManager::SetCurrentlyFocusedView(nsnull);
+  }
+
   while (GetFirstChild())
   {
     nsView* child = GetFirstChild();
diff --git a/view/src/nsViewManager.cpp b/view/src/nsViewManager.cpp
index 8746a94d718..56d46fc0269 100644
--- a/view/src/nsViewManager.cpp
+++ b/view/src/nsViewManager.cpp
@@ -935,6 +935,59 @@ void nsViewManager::UpdateViews(nsView *aView, PRUint32 aUpdateFlags)
   }
 }
 
+nsView *nsViewManager::sCurrentlyFocusView = nsnull;
+nsView *nsViewManager::sViewFocusedBeforeSuppression = nsnull;
+PRInt32 nsViewManager::sSuppressCount = 0;
+
+void nsViewManager::SuppressFocusEvents()
+{
+  sSuppressCount++;
+  if (sSuppressCount == 1) {
+    // We're turning on focus/blur suppression, remember what had
+    // the focus.
+    SetViewFocusedBeforeSuppression(GetCurrentlyFocusedView());
+  }  
+}
+
+void nsViewManager::UnsuppressFocusEvents()
+{
+  sSuppressCount--;
+  if (sSuppressCount > 0 ||
+      GetCurrentlyFocusedView() == GetViewFocusedBeforeSuppression())
+  return;
+
+  // We're turning off suppression, synthesize LOSTFOCUS/GOTFOCUS.
+  nsIWidget *widget = nsnull;
+  nsEventStatus status;
+
+  // Backup what is focused before we send the blur. If the
+  // blur causes a focus change, keep that new focus change,
+  // don't overwrite with the old "currently focused view".
+  nsIView *currentFocusBeforeBlur = GetCurrentlyFocusedView();
+
+  // Send NS_LOSTFOCUS to widget that was focused before
+  // focus/blur suppression.
+  if (GetViewFocusedBeforeSuppression()) {
+    widget = GetViewFocusedBeforeSuppression()->GetWidget();
+    if (widget) {
+      nsGUIEvent event(PR_TRUE, NS_LOSTFOCUS, widget);
+      widget->DispatchEvent(&event, status);
+    }
+  }
+
+  // Send NS_GOTFOCUS to the widget that we think should be focused.
+  if (GetCurrentlyFocusedView() &&
+      currentFocusBeforeBlur == GetCurrentlyFocusedView())
+  {
+    widget = GetCurrentlyFocusedView()->GetWidget();
+    if (widget) {
+      nsGUIEvent event(PR_TRUE, NS_GOTFOCUS, widget);
+      widget->DispatchEvent(&event, status); 
+    }
+  }
+
+}
+
 NS_IMETHODIMP nsViewManager::DispatchEvent(nsGUIEvent *aEvent, nsEventStatus *aStatus)
 {
   *aStatus = nsEventStatus_eIgnore;
@@ -1138,6 +1191,13 @@ NS_IMETHODIMP nsViewManager::DispatchEvent(nsGUIEvent *aEvent, nsEventStatus *aS
 
     default:
       {
+        if (aEvent->message == NS_GOTFOCUS) {
+          SetCurrentlyFocusedView(nsView::GetViewFor(aEvent->widget));
+        }
+        if ((aEvent->message == NS_GOTFOCUS || aEvent->message == NS_LOSTFOCUS) &&
+             nsViewManager::IsFocusSuppressed()) 
+          break;
+        
         if ((NS_IS_MOUSE_EVENT(aEvent) &&
              // Ignore moves that we synthesize.
              static_cast<nsMouseEvent*>(aEvent)->reason ==
diff --git a/view/src/nsViewManager.h b/view/src/nsViewManager.h
index d0c69b50584..eae248ef08d 100644
--- a/view/src/nsViewManager.h
+++ b/view/src/nsViewManager.h
@@ -201,10 +201,43 @@ public:
   /* Update the cached RootViewManager pointer on this view manager. */
   void InvalidateHierarchy();
 
+  virtual void SuppressFocusEvents();
+  virtual void UnsuppressFocusEvents();
+
+  virtual PRBool IsFocusSuppressed()
+  {
+    return sSuppressCount > 0;
+  }
+
+  static void SetCurrentlyFocusedView(nsView *aView)
+  {
+    sCurrentlyFocusView = aView;
+  }
+  
+  static nsView* GetCurrentlyFocusedView()
+  {
+    return sCurrentlyFocusView;
+  }
+
+  static void SetViewFocusedBeforeSuppression(nsView *aView)
+  {
+    sViewFocusedBeforeSuppression = aView;
+  }
+
+  static nsView* GetViewFocusedBeforeSuppression()
+  {
+    return sViewFocusedBeforeSuppression;
+  }
+
 protected:
   virtual ~nsViewManager();
 
 private:
+
+  static nsView *sCurrentlyFocusView;
+  static nsView *sViewFocusedBeforeSuppression;
+  static PRInt32 sSuppressCount;
+
   void FlushPendingInvalidates();
   void ProcessPendingUpdates(nsView *aView, PRBool aDoInvalidate);
   void ReparentChildWidgets(nsIView* aView, nsIWidget *aNewWidget);