diff --git a/content/canvas/src/nsCanvasRenderingContext2D.cpp b/content/canvas/src/nsCanvasRenderingContext2D.cpp
--- a/content/canvas/src/nsCanvasRenderingContext2D.cpp
+++ b/content/canvas/src/nsCanvasRenderingContext2D.cpp
@@ -318,6 +318,7 @@ public:
     virtual ~nsCanvasRenderingContext2D();
 
     nsresult Redraw();
+    nsresult Redraw(const nsRect& r);
 
     // nsICanvasRenderingContextInternal
     NS_IMETHOD SetCanvasElement(nsICanvasElement* aParentCanvas);
@@ -870,6 +871,20 @@ nsCanvasRenderingContext2D::Redraw()
     if (!mIsFrameInvalid) {
         mIsFrameInvalid = PR_TRUE;
         return mCanvasElement->InvalidateFrame();
+    }
+
+    return NS_OK;
+}
+
+nsresult
+nsCanvasRenderingContext2D::Redraw(const nsRect& r)
+{
+    if (!mCanvasElement)
+        return NS_OK;
+
+    if (!mIsFrameInvalid) {
+        mIsFrameInvalid = PR_TRUE;
+        return mCanvasElement->InvalidateFrameSubrect(r);
     }
 
     return NS_OK;
@@ -3419,7 +3434,17 @@ nsCanvasRenderingContext2D::DrawWindow(n
     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();
+
+    nsRect redrawRect(nsPresContext::CSSPixelsToAppUnits((PRInt32) damageRect.pos.x),
+                      nsPresContext::CSSPixelsToAppUnits((PRInt32) damageRect.pos.y),
+                      nsPresContext::CSSPixelsToAppUnits((PRInt32) damageRect.size.width),
+                      nsPresContext::CSSPixelsToAppUnits((PRInt32) damageRect.size.height));
+    Redraw(redrawRect);
 
     return rv;
 }
diff --git a/content/html/content/src/nsHTMLCanvasElement.cpp b/content/html/content/src/nsHTMLCanvasElement.cpp
--- a/content/html/content/src/nsHTMLCanvasElement.cpp
+++ b/content/html/content/src/nsHTMLCanvasElement.cpp
@@ -559,7 +559,28 @@ nsHTMLCanvasElement::InvalidateFrameSubr
 {
   nsIFrame *frame = GetPrimaryFrame(Flush_Frames);
   if (frame) {
-    frame->Invalidate(damageRect);
+    nsRect contentArea(frame->GetContentRect());
+    nsRect realRect(damageRect);
+
+    nsIntSize sz = GetWidthHeight();
+    nsSize sizeAppUnits(nsPresContext::CSSPixelsToAppUnits(sz.width),
+                        nsPresContext::CSSPixelsToAppUnits(sz.height));
+
+    if (contentArea.Size() != sizeAppUnits) {
+      // the canvas is CSS-scaled, so we need to scale the damage region
+      float sx = contentArea.width / (float) sizeAppUnits.width;
+      float sy = contentArea.height / (float) sizeAppUnits.height;
+
+      realRect.x = NSToIntFloor(realRect.x * sx);
+      realRect.y = NSToIntFloor(realRect.y * sy);
+      realRect.width = NSToIntCeil(realRect.width * sx);
+      realRect.height = NSToIntCeil(realRect.height * sy);
+    }
+
+    // account for border/padding
+    realRect.MoveBy(contentArea.TopLeft() - frame->GetPosition());
+
+    frame->Invalidate(realRect);
   }
 
   return NS_OK;
diff --git a/layout/generic/nsHTMLCanvasFrame.cpp b/layout/generic/nsHTMLCanvasFrame.cpp
--- 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 @@ nsHTMLCanvasFrame::PaintCanvas(nsIRender
 nsHTMLCanvasFrame::PaintCanvas(nsIRenderingContext& aRenderingContext,
                                const nsRect& aDirtyRect, nsPoint aPt) 
 {
+  nsPresContext *presContext = PresContext();
   nsRect inner = GetInnerArea() + aPt;
 
   nsCOMPtr<nsICanvasElement> canvas(do_QueryInterface(GetContent()));
@@ -234,34 +236,39 @@ nsHTMLCanvasFrame::PaintCanvas(nsIRender
   if (inner.width == 0 || inner.height == 0)
     return;
 
-  nsIntSize canvasSize = GetCanvasSize();
-  nsSize sizeAppUnits(PresContext()->DevPixelsToAppUnits(canvasSize.width),
-                      PresContext()->DevPixelsToAppUnits(canvasSize.height));
+  nsIntSize sz = GetCanvasSize();
+  nsSize sizeAppUnits(nsPresContext::CSSPixelsToAppUnits(sz.width),
+                      nsPresContext::CSSPixelsToAppUnits(sz.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 = 1.0;
+  gfxFloat sy = 1.0;
 
-    aRenderingContext.PushState();
-    aRenderingContext.Translate(inner.x, inner.y);
-    aRenderingContext.Scale(sx, sy);
+  if (inner.Size() != sizeAppUnits) {
+    sx = inner.width / (gfxFloat) sizeAppUnits.width;
+    sy = inner.height / (gfxFloat) sizeAppUnits.height;
+  }
 
-    canvas->RenderContexts(aRenderingContext.ThebesContext());
+  gfxRect dirty(presContext->AppUnitsToGfxUnits(aDirtyRect.x),
+                presContext->AppUnitsToGfxUnits(aDirtyRect.y),
+                presContext->AppUnitsToGfxUnits(aDirtyRect.width),
+                presContext->AppUnitsToGfxUnits(aDirtyRect.height));
+  dirty.RoundOut();
 
-    aRenderingContext.PopState();
-  } else {
-    //nsIRenderingContext::AutoPushTranslation(&aRenderingContext, px, py);
+  ctx->Save();
 
-    aRenderingContext.PushState();
-    aRenderingContext.Translate(inner.x, inner.y);
+  ctx->NewPath();
+  ctx->Rectangle(dirty);
+  ctx->Clip();
 
-    canvas->RenderContexts(aRenderingContext.ThebesContext());
+  ctx->Translate(gfxPoint(presContext->AppUnitsToGfxUnits(inner.x),
+                          presContext->AppUnitsToGfxUnits(inner.y)));
+  ctx->Scale(sx, sy);
 
-    aRenderingContext.PopState();
-  }
+  canvas->RenderContexts(ctx);
+
+  ctx->Restore();
 }
 
 NS_IMETHODIMP