--- a/dom/canvas/CanvasRenderingContext2D.cpp
+++ b/dom/canvas/CanvasRenderingContext2D.cpp
@@ -1869,20 +1869,19 @@ CanvasRenderingContext2D::GetHeight() co
 {
   return mHeight;
 }
 
 NS_IMETHODIMP
 CanvasRenderingContext2D::SetDimensions(int32_t aWidth, int32_t aHeight)
 {
   bool retainBuffer = false;
-  // See bug 1318283 as to why we are disabling this optimization.
-  // Based on the results of the investigation, this may go away
-  // completely or come back.
-  // retainBuffer = (aWidth == mWidth && aHeight == mHeight);
+  if (aWidth == mWidth && aHeight == mHeight) {
+    retainBuffer = true;
+  }
   ClearTarget(retainBuffer);
 
   // Zero sized surfaces can cause problems.
   mZero = false;
   if (aHeight == 0) {
     aHeight = 1;
     mZero = true;
   }

--- a/dom/canvas/CanvasRenderingContext2D.h
+++ b/dom/canvas/CanvasRenderingContext2D.h
@@ -994,16 +994,17 @@ protected:
   /**
    * Calls UpdateFilter if the canvas's WriteOnly state has changed between the
    * last call to UpdateFilter and now.
    */
   const gfx::FilterDescription& EnsureUpdatedFilter() {
     bool isWriteOnly = mCanvasElement && mCanvasElement->IsWriteOnly();
     if (CurrentState().filterSourceGraphicTainted != isWriteOnly) {
       UpdateFilter();
+      EnsureTarget();
     }
     MOZ_ASSERT(CurrentState().filterSourceGraphicTainted == isWriteOnly);
     return CurrentState().filter;
   }
 
   bool NeedToCalculateBounds()
   {
     return NeedToDrawShadow() || NeedToApplyFilter();