diff --git a/layout/svg/base/src/nsISVGChildFrame.h b/layout/svg/base/src/nsISVGChildFrame.h
index 0be28de9cce..0e01c82f6e2 100644
--- a/layout/svg/base/src/nsISVGChildFrame.h
+++ b/layout/svg/base/src/nsISVGChildFrame.h
@@ -76,7 +76,13 @@ public:
 
   NS_IMETHOD_(nsRect) GetCoveredRegion()=0;
   NS_IMETHOD UpdateCoveredRegion()=0;
+
+  // Called once on all SVG child frames, either when their nsSVGOuterSVGFrame
+  // receives its initial reflow (i.e. once the SVG viewport dimensions are
+  // known), or else when they're inserted into the frame tree (if they're
+  // inserted after the initial reflow).
   NS_IMETHOD InitialUpdate()=0;
+
   NS_IMETHOD NotifyCanvasTMChanged(PRBool suppressInvalidation)=0;
   NS_IMETHOD NotifyRedrawSuspended()=0;
   NS_IMETHOD NotifyRedrawUnsuspended()=0;
diff --git a/layout/svg/base/src/nsSVGContainerFrame.cpp b/layout/svg/base/src/nsSVGContainerFrame.cpp
index b1dcb8154e0..212e0a5f4b4 100644
--- a/layout/svg/base/src/nsSVGContainerFrame.cpp
+++ b/layout/svg/base/src/nsSVGContainerFrame.cpp
@@ -125,20 +125,23 @@ nsSVGDisplayContainerFrame::InsertFrames(nsIAtom* aListName,
   // Insert the new frames
   nsSVGContainerFrame::InsertFrames(aListName, aPrevFrame, aFrameList);
 
-  // call InitialUpdate() on all new frames:
-  nsIFrame* end = nsnull;
-  if (lastNewFrame)
-    end = lastNewFrame->GetNextSibling();
-  
-  for (nsIFrame* kid = aFrameList; kid != end;
-       kid = kid->GetNextSibling()) {
-    nsISVGChildFrame* SVGFrame=nsnull;
-    CallQueryInterface(kid, &SVGFrame);
-    if (SVGFrame) {
-      SVGFrame->InitialUpdate(); 
+  // Call InitialUpdate on the new frames ONLY if our nsSVGOuterSVGFrame has had
+  // its initial reflow (our NS_FRAME_FIRST_REFLOW bit is clear) - bug 399863.
+  if (!(GetStateBits() & NS_FRAME_FIRST_REFLOW)) {
+    nsIFrame* end = nsnull;
+    if (lastNewFrame)
+      end = lastNewFrame->GetNextSibling();
+
+    for (nsIFrame* kid = aFrameList; kid != end;
+         kid = kid->GetNextSibling()) {
+      nsISVGChildFrame* SVGFrame = nsnull;
+      CallQueryInterface(kid, &SVGFrame);
+      if (SVGFrame) {
+        SVGFrame->InitialUpdate(); 
+      }
     }
   }
-  
+
   return NS_OK;
 }
 
@@ -214,6 +217,10 @@ nsSVGDisplayContainerFrame::UpdateCoveredRegion()
 NS_IMETHODIMP
 nsSVGDisplayContainerFrame::InitialUpdate()
 {
+  NS_ASSERTION(GetStateBits() & NS_FRAME_FIRST_REFLOW,
+               "Yikes! We've been called already! Hopefully we weren't called "
+               "before our nsSVGOuterSVGFrame's initial Reflow()!!!");
+
   for (nsIFrame* kid = mFrames.FirstChild(); kid;
        kid = kid->GetNextSibling()) {
     nsISVGChildFrame* SVGFrame = nsnull;
diff --git a/layout/svg/base/src/nsSVGForeignObjectFrame.cpp b/layout/svg/base/src/nsSVGForeignObjectFrame.cpp
index bb7d221836f..0b492914f6d 100644
--- a/layout/svg/base/src/nsSVGForeignObjectFrame.cpp
+++ b/layout/svg/base/src/nsSVGForeignObjectFrame.cpp
@@ -365,6 +365,10 @@ nsSVGForeignObjectFrame::UpdateCoveredRegion()
 NS_IMETHODIMP
 nsSVGForeignObjectFrame::InitialUpdate()
 {
+  NS_ASSERTION(GetStateBits() & NS_FRAME_FIRST_REFLOW,
+               "Yikes! We've been called already! Hopefully we weren't called "
+               "before our nsSVGOuterSVGFrame's initial Reflow()!!!");
+
   UpdateCoveredRegion();
   DoReflow();
 
@@ -585,6 +589,10 @@ nsSVGForeignObjectFrame::DoReflow()
   printf("**nsSVGForeignObjectFrame::DoReflow()\n");
 #endif
 
+  NS_ASSERTION(!(nsSVGUtils::GetOuterSVGFrame(this)->
+                             GetStateBits() & NS_FRAME_FIRST_REFLOW),
+               "Calling InitialUpdate too early - must not call DoReflow!!!");
+
   if (IsDisabled())
     return;
 
diff --git a/layout/svg/base/src/nsSVGGlyphFrame.cpp b/layout/svg/base/src/nsSVGGlyphFrame.cpp
index 0ac8d7e75aa..3e9d1cd1e0b 100644
--- a/layout/svg/base/src/nsSVGGlyphFrame.cpp
+++ b/layout/svg/base/src/nsSVGGlyphFrame.cpp
@@ -461,6 +461,10 @@ nsSVGGlyphFrame::UpdateCoveredRegion()
 NS_IMETHODIMP
 nsSVGGlyphFrame::InitialUpdate()
 {
+  NS_ASSERTION(GetStateBits() & NS_FRAME_FIRST_REFLOW,
+               "Yikes! We've been called already! Hopefully we weren't called "
+               "before our nsSVGOuterSVGFrame's initial Reflow()!!!");
+
   NS_ASSERTION(!(mState & NS_FRAME_IN_REFLOW),
                "We don't actually participate in reflow");
   
diff --git a/layout/svg/base/src/nsSVGPathGeometryFrame.cpp b/layout/svg/base/src/nsSVGPathGeometryFrame.cpp
index 1a39fdc0898..d651dbd4279 100644
--- a/layout/svg/base/src/nsSVGPathGeometryFrame.cpp
+++ b/layout/svg/base/src/nsSVGPathGeometryFrame.cpp
@@ -444,6 +444,10 @@ nsSVGPathGeometryFrame::UpdateCoveredRegion()
 NS_IMETHODIMP
 nsSVGPathGeometryFrame::InitialUpdate()
 {
+  NS_ASSERTION(GetStateBits() & NS_FRAME_FIRST_REFLOW,
+               "Yikes! We've been called already! Hopefully we weren't called "
+               "before our nsSVGOuterSVGFrame's initial Reflow()!!!");
+
   UpdateGraphic();
 
   NS_ASSERTION(!(mState & NS_FRAME_IN_REFLOW),