--- a/gfx/layers/ThebesLayerBuffer.cpp
+++ b/gfx/layers/ThebesLayerBuffer.cpp
@@ -143,16 +143,36 @@ ThebesLayerBuffer::DrawBufferWithRotatio
   // Draw four quadrants. We could use REPEAT_, but it's probably better
   // not to, to be performance-safe.
   DrawBufferQuadrant(aTarget, LEFT, TOP, aOpacity, aXRes, aYRes);
   DrawBufferQuadrant(aTarget, RIGHT, TOP, aOpacity, aXRes, aYRes);
   DrawBufferQuadrant(aTarget, LEFT, BOTTOM, aOpacity, aXRes, aYRes);
   DrawBufferQuadrant(aTarget, RIGHT, BOTTOM, aOpacity, aXRes, aYRes);
 }
 
+already_AddRefed<gfxContext>
+ThebesLayerBuffer::GetContextForQuadrantUpdate(const nsIntRect& aBounds,
+                                               float aXResolution,
+                                               float aYResolution)
+{
+  nsRefPtr<gfxContext> ctx = new gfxContext(mBuffer);
+
+  // Figure out which quadrant to draw in
+  PRInt32 xBoundary = mBufferRect.XMost() - mBufferRotation.x;
+  PRInt32 yBoundary = mBufferRect.YMost() - mBufferRotation.y;
+  XSide sideX = aBounds.XMost() <= xBoundary ? RIGHT : LEFT;
+  YSide sideY = aBounds.YMost() <= yBoundary ? BOTTOM : TOP;
+  nsIntRect quadrantRect = GetQuadrantRectangle(sideX, sideY);
+  NS_ASSERTION(quadrantRect.Contains(aBounds), "Messed up quadrants");
+  ctx->Scale(aXResolution, aYResolution);
+  ctx->Translate(-gfxPoint(quadrantRect.x, quadrantRect.y));
+
+  return ctx.forget();
+}
+
 static void
 WrapRotationAxis(PRInt32* aRotationPoint, PRInt32 aSize)
 {
   if (*aRotationPoint < 0) {
     *aRotationPoint += aSize;
   } else if (*aRotationPoint >= aSize) {
     *aRotationPoint -= aSize;
   }
@@ -285,27 +305,18 @@ ThebesLayerBuffer::BeginPaint(ThebesLaye
   if (bufferDimsChanged) {
     mBufferDims = destBufferDims;
   }
 
   nsIntRegion invalidate;
   invalidate.Sub(aLayer->GetValidRegion(), destBufferRect);
   result.mRegionToInvalidate.Or(result.mRegionToInvalidate, invalidate);
 
-  result.mContext = new gfxContext(mBuffer);
-
-  // Figure out which quadrant to draw in
-  PRInt32 xBoundary = mBufferRect.XMost() - mBufferRotation.x;
-  PRInt32 yBoundary = mBufferRect.YMost() - mBufferRotation.y;
-  XSide sideX = drawBounds.XMost() <= xBoundary ? RIGHT : LEFT;
-  YSide sideY = drawBounds.YMost() <= yBoundary ? BOTTOM : TOP;
-  nsIntRect quadrantRect = GetQuadrantRectangle(sideX, sideY);
-  NS_ASSERTION(quadrantRect.Contains(drawBounds), "Messed up quadrants");
-  result.mContext->Scale(aXResolution, aYResolution);
-  result.mContext->Translate(-gfxPoint(quadrantRect.x, quadrantRect.y));
+  result.mContext = GetContextForQuadrantUpdate(drawBounds,
+                                                aXResolution, aYResolution);
 
   gfxUtils::ClipToRegion(result.mContext, result.mRegionToDraw);
   if (aContentType == gfxASurface::CONTENT_COLOR_ALPHA && !isClear) {
     result.mContext->SetOperator(gfxContext::OPERATOR_CLEAR);
     result.mContext->Paint();
     result.mContext->SetOperator(gfxContext::OPERATOR_OVER);
   }
   return result;

--- a/gfx/layers/ThebesLayerBuffer.h
+++ b/gfx/layers/ThebesLayerBuffer.h
@@ -177,16 +177,24 @@ protected:
     nsRefPtr<gfxASurface> tmp = mBuffer.forget();
     mBuffer = aBuffer;
     mBufferDims = aBufferDims;
     mBufferRect = aBufferRect;
     mBufferRotation = aBufferRotation;
     return tmp.forget();
   }
 
+  /**
+   * Get a context at the specified resolution for updating |aBounds|,
+   * which must be contained within a single quadrant.
+   */
+  already_AddRefed<gfxContext>
+  GetContextForQuadrantUpdate(const nsIntRect& aBounds,
+                              float aXResolution, float aYResolution);
+
 private:
   PRBool BufferSizeOkFor(const nsIntSize& aSize)
   {
     return (aSize == mBufferDims ||
             (SizedToVisibleBounds != mBufferSizePolicy &&
              aSize < mBufferDims));
   }