Bug 695832 - Perpsective origin offset should be relative to the transform origin. r=roc

layout/base/nsDisplayList.cpp

@@ -2483,13 +2483,6 @@ gfxPoint3D GetDeltaToMozPerspectiveOrigin(const nsIFrame* aFrame,
     }
   }
 
-  /**
-   * An offset of (0,0) results in the perspective-origin being at the centre of the element,
-   * so include a shift of the centre point to (0,0).
-   */
-  result.x -= NSAppUnitsToFloatPixels(boundingRect.width, aFactor)/2;
-  result.y -= NSAppUnitsToFloatPixels(boundingRect.height, aFactor)/2;
-
   return result;
 }
 
@@ -2551,7 +2544,10 @@ nsDisplayTransform::GetResultingTransformMatrix(const nsIFrame* aFrame,
     perspective._34 =
       -1.0 / NSAppUnitsToFloatPixels(parentDisp->mChildPerspective.GetCoordValue(),
                                      aFactor);
-    result = result * nsLayoutUtils::ChangeMatrixBasis(toPerspectiveOrigin, perspective);
+    /* At the point when perspective is applied, we have been translated to the transform origin.
+     * The translation to the perspective origin is the difference between these values.
+     */
+    result = result * nsLayoutUtils::ChangeMatrixBasis(toPerspectiveOrigin - toMozOrigin, perspective);
   }
 
   if (aFrame->Preserves3D() && nsLayoutUtils::Are3DTransformsEnabled()) {