diff --git a/mozilla/layout/base/nsIPresShell.h b/mozilla/layout/base/nsIPresShell.h
index 72011ea894d..dc7e1468d73 100644
--- a/mozilla/layout/base/nsIPresShell.h
+++ b/mozilla/layout/base/nsIPresShell.h
@@ -694,7 +694,7 @@ public:
    * @param aRect is the region to capture into the offscreen buffer, in the
    * root frame's coordinate system (if aIgnoreViewportScrolling is false)
    * or in the root scrolled frame's coordinate system
-   * (if aIgnoreViewportScrolling is true)
+   * (if aIgnoreViewportScrolling is true). The coordinates are in appunits.
    * @param aUntrusted set to PR_TRUE if the contents may be passed to malicious
    * agents. E.g. we might choose not to paint the contents of sensitive widgets
    * such as the file name in a file upload widget, and we might choose not
@@ -702,7 +702,9 @@ public:
    * @param aIgnoreViewportScrolling ignore clipping/scrolling/scrollbar painting
    * due to scrolling in the viewport
    * @param aBackgroundColor a background color to render onto
-   * @param aRenderedContext the gfxContext to render to
+   * @param aRenderedContext the gfxContext to render to. We render so that
+   * one CSS pixel in the source document is rendered to one unit in the current
+   * transform.
    */
   NS_IMETHOD RenderDocument(const nsRect& aRect, PRBool aUntrusted,
                             PRBool aIgnoreViewportScrolling,
diff --git a/mozilla/layout/base/nsPresShell.cpp b/mozilla/layout/base/nsPresShell.cpp
index 191b2edfbb6..66ef465c9ad 100644
--- a/mozilla/layout/base/nsPresShell.cpp
+++ b/mozilla/layout/base/nsPresShell.cpp
@@ -4799,6 +4799,12 @@ PresShell::RenderDocument(const nsRect& aRect, PRBool aUntrusted,
 {
   NS_ENSURE_TRUE(!aUntrusted, NS_ERROR_NOT_IMPLEMENTED);
 
+  gfxRect r(0, 0,
+            nsPresContext::AppUnitsToFloatCSSPixels(aRect.width),
+            nsPresContext::AppUnitsToFloatCSSPixels(aRect.height));
+  aThebesContext->Save();
+  aThebesContext->Clip(r);
+
   aThebesContext->PushGroup(NS_GET_A(aBackgroundColor) == 0xff ?
                             gfxASurface::CONTENT_COLOR :
                             gfxASurface::CONTENT_COLOR_ALPHA);
@@ -4839,10 +4845,13 @@ PresShell::RenderDocument(const nsRect& aRect, PRBool aUntrusted,
     if (NS_SUCCEEDED(rv)) {
       // Ensure that r.x,r.y gets drawn at (0,0)
       aThebesContext->Save();
-      aThebesContext->Translate(gfxPoint(-mPresContext->AppUnitsToGfxUnits(rect.x),
-                                         -mPresContext->AppUnitsToGfxUnits(rect.y)));
+      aThebesContext->Translate(gfxPoint(-nsPresContext::AppUnitsToFloatCSSPixels(rect.x),
+                                         -nsPresContext::AppUnitsToFloatCSSPixels(rect.y)));
 
       nsIDeviceContext* devCtx = mPresContext->DeviceContext();
+      gfxFloat scale = gfxFloat(devCtx->AppUnitsPerDevPixel())/nsPresContext::AppUnitsPerCSSPixel();
+      aThebesContext->Scale(scale, scale);
+      
       nsCOMPtr<nsIRenderingContext> rc;
       devCtx->CreateRenderingContextInstance(*getter_AddRefs(rc));
       rc->Init(devCtx, aThebesContext);
@@ -4861,6 +4870,8 @@ PresShell::RenderDocument(const nsRect& aRect, PRBool aUntrusted,
   aThebesContext->PopGroupToSource();
   aThebesContext->Paint();
 
+  aThebesContext->Restore();
+
   return NS_OK;
 }