diff --git a/content/canvas/public/nsICanvasElement.h b/content/canvas/public/nsICanvasElement.h
index b61c741806b0..c8e61bb70cd1 100644
--- a/content/canvas/public/nsICanvasElement.h
+++ b/content/canvas/public/nsICanvasElement.h
@@ -39,10 +39,10 @@
 #define nsICanvasElement_h___
 
 #include "nsISupports.h"
-#include "nsRect.h"
 
 class gfxContext;
 class nsIFrame;
+struct gfxRect;
 
 // {C234660C-BD06-493e-8583-939A5A158B37}
 #define NS_ICANVASELEMENT_IID \
@@ -91,9 +91,9 @@ public:
 
   /*
    * Ask the canvas frame to invalidate a portion of the frame; damageRect
-   * is relative to the origin of the canvas frame.
+   * is relative to the origin of the canvas frame in CSS pixels.
    */
-  NS_IMETHOD InvalidateFrameSubrect (const nsRect& damageRect) = 0;
+  NS_IMETHOD InvalidateFrameSubrect (const gfxRect& damageRect) = 0;
 
   /*
    * Get the number of contexts in this canvas, and request a context at
diff --git a/content/canvas/src/nsCanvasRenderingContext2D.cpp b/content/canvas/src/nsCanvasRenderingContext2D.cpp
index e73f6f461f1e..f865ef28a91f 100644
--- a/content/canvas/src/nsCanvasRenderingContext2D.cpp
+++ b/content/canvas/src/nsCanvasRenderingContext2D.cpp
@@ -318,6 +318,8 @@ public:
     virtual ~nsCanvasRenderingContext2D();
 
     nsresult Redraw();
+    // this rect is in CSS pixels
+    nsresult Redraw(const gfxRect& r);
 
     // nsICanvasRenderingContextInternal
     NS_IMETHOD SetCanvasElement(nsICanvasElement* aParentCanvas);
@@ -889,6 +891,20 @@ nsCanvasRenderingContext2D::Redraw()
     return NS_OK;
 }
 
+nsresult
+nsCanvasRenderingContext2D::Redraw(const gfxRect& r)
+{
+    if (!mCanvasElement)
+        return NS_OK;
+
+    if (!mIsFrameInvalid) {
+        mIsFrameInvalid = PR_TRUE;
+        return mCanvasElement->InvalidateFrameSubrect(r);
+    }
+
+    return NS_OK;
+}
+
 NS_IMETHODIMP
 nsCanvasRenderingContext2D::SetDimensions(PRInt32 width, PRInt32 height)
 {
@@ -3433,7 +3449,13 @@ nsCanvasRenderingContext2D::DrawWindow(nsIDOMWindow* aWindow, float aX, float aY
     mThebes->SetColor(gfxRGBA(1,1,1,1));
     DirtyAllStyles();
 
-    Redraw();
+    // note that aX and aY are coordinates in the document that
+    // we're drawing; aX and aY are drawn to 0,0 in current user
+    // space.
+    gfxRect damageRect = mThebes->UserToDevice(gfxRect(0, 0, aW, aH));
+    damageRect.RoundOut();
+
+    Redraw(damageRect);
 
     return rv;
 }
diff --git a/content/html/content/src/nsHTMLCanvasElement.cpp b/content/html/content/src/nsHTMLCanvasElement.cpp
index 2971d962593f..1db2e90301cd 100644
--- a/content/html/content/src/nsHTMLCanvasElement.cpp
+++ b/content/html/content/src/nsHTMLCanvasElement.cpp
@@ -92,7 +92,7 @@ public:
   virtual PRBool IsWriteOnly();
   virtual void SetWriteOnly();
   NS_IMETHOD InvalidateFrame ();
-  NS_IMETHOD InvalidateFrameSubrect (const nsRect& damageRect);
+  NS_IMETHOD InvalidateFrameSubrect (const gfxRect& damageRect);
   virtual PRInt32 CountContexts();
   virtual nsICanvasRenderingContextInternal *GetContextAtIndex (PRInt32 index);
   virtual PRBool GetIsOpaque();
@@ -555,11 +555,29 @@ nsHTMLCanvasElement::InvalidateFrame()
 }
 
 NS_IMETHODIMP
-nsHTMLCanvasElement::InvalidateFrameSubrect(const nsRect& damageRect)
+nsHTMLCanvasElement::InvalidateFrameSubrect(const gfxRect& damageRect)
 {
   nsIFrame *frame = GetPrimaryFrame(Flush_Frames);
   if (frame) {
-    frame->Invalidate(damageRect);
+    nsRect contentArea(frame->GetContentRect());
+    nsIntSize size = GetWidthHeight();
+
+    // damageRect and size are in CSS pixels; contentArea is in appunits
+    // We want a rect in appunits; so avoid doing pixels-to-appunits and
+    // vice versa conversion here.
+    gfxRect realRect(damageRect);
+    realRect.Scale(contentArea.width / gfxFloat(size.width),
+                   contentArea.height / gfxFloat(size.height));
+    realRect.RoundOut();
+
+    // then make it a nsRect
+    nsRect invalRect(realRect.X(), realRect.Y(),
+                     realRect.Width(), realRect.Height());
+
+    // account for border/padding
+    invalRect.MoveBy(contentArea.TopLeft() - frame->GetPosition());
+
+    frame->Invalidate(invalRect);
   }
 
   return NS_OK;
diff --git a/layout/generic/nsHTMLCanvasFrame.cpp b/layout/generic/nsHTMLCanvasFrame.cpp
index 17427c56589e..6af9bb91e9d9 100644
--- a/layout/generic/nsHTMLCanvasFrame.cpp
+++ b/layout/generic/nsHTMLCanvasFrame.cpp
@@ -49,6 +49,7 @@
 
 #include "nsTransform2D.h"
 
+#include "gfxContext.h"
 
 class nsDisplayItemCanvas : public nsDisplayItem {
 public:
@@ -224,6 +225,7 @@ void
 nsHTMLCanvasFrame::PaintCanvas(nsIRenderingContext& aRenderingContext,
                                const nsRect& aDirtyRect, nsPoint aPt) 
 {
+  nsPresContext *presContext = PresContext();
   nsRect inner = GetInnerArea() + aPt;
 
   nsCOMPtr<nsICanvasElement> canvas(do_QueryInterface(GetContent()));
@@ -234,34 +236,24 @@ nsHTMLCanvasFrame::PaintCanvas(nsIRenderingContext& aRenderingContext,
   if (inner.width == 0 || inner.height == 0)
     return;
 
-  nsIntSize canvasSize = GetCanvasSize();
-  nsSize sizeAppUnits(PresContext()->DevPixelsToAppUnits(canvasSize.width),
-                      PresContext()->DevPixelsToAppUnits(canvasSize.height));
+  nsIntSize sizeCSSPixels = GetCanvasSize();
+  nsSize sizeAppUnits(nsPresContext::CSSPixelsToAppUnits(sizeCSSPixels.width),
+                      nsPresContext::CSSPixelsToAppUnits(sizeCSSPixels.height));
 
-  // XXXvlad clip to aDirtyRect!
+  gfxContext *ctx = aRenderingContext.ThebesContext();
 
-  if (inner.Size() != sizeAppUnits)
-  {
-    float sx = inner.width / (float) sizeAppUnits.width;
-    float sy = inner.height / (float) sizeAppUnits.height;
+  gfxFloat sx = inner.width / (gfxFloat) sizeAppUnits.width;
+  gfxFloat sy = inner.height / (gfxFloat) sizeAppUnits.height;
 
-    aRenderingContext.PushState();
-    aRenderingContext.Translate(inner.x, inner.y);
-    aRenderingContext.Scale(sx, sy);
+  ctx->Save();
 
-    canvas->RenderContexts(aRenderingContext.ThebesContext());
+  ctx->Translate(gfxPoint(presContext->AppUnitsToGfxUnits(inner.x),
+                          presContext->AppUnitsToGfxUnits(inner.y)));
+  ctx->Scale(sx, sy);
 
-    aRenderingContext.PopState();
-  } else {
-    //nsIRenderingContext::AutoPushTranslation(&aRenderingContext, px, py);
+  canvas->RenderContexts(ctx);
 
-    aRenderingContext.PushState();
-    aRenderingContext.Translate(inner.x, inner.y);
-
-    canvas->RenderContexts(aRenderingContext.ThebesContext());
-
-    aRenderingContext.PopState();
-  }
+  ctx->Restore();
 }
 
 NS_IMETHODIMP