Bug 1227210: Ensure the mask and the surface are in the right space when being blended. r=mattwoodrow

gfx/layers/basic/BasicLayerManager.cpp

@@ -128,6 +128,16 @@ BasicLayerManager::PushGroupForLayer(gfxContext* aContext, Layer* aLayer, const
   Matrix maskTransform;
   RefPtr<SourceSurface> maskSurf = GetMaskForLayer(aLayer, &maskTransform);
 
+  if (maskSurf) {
+    // The returned transform will transform the mask to device space on the
+    // destination. Since the User->Device space transform will be applied
+    // to the mask by PopGroupAndBlend we need to adjust the transform to
+    // transform the mask to user space.
+    Matrix currentTransform = ToMatrix(group.mFinalTarget->CurrentMatrix());
+    currentTransform.Invert();
+    maskTransform = maskTransform * currentTransform;
+  }
+
   if (aLayer->CanUseOpaqueSurface() &&
       ((didCompleteClip && aRegion.GetNumRects() == 1) ||
        !aContext->CurrentMatrix().HasNonIntegerTranslation())) {
@@ -172,8 +182,12 @@ BasicLayerManager::PopGroupForLayer(PushedGroup &group)
   RefPtr<SourceSurface> src = sourceDT->Snapshot();
 
   if (group.mMaskSurface) {
-    dt->SetTransform(group.mMaskTransform * Matrix::Translation(-group.mFinalTarget->GetDeviceOffset()));
-    dt->MaskSurface(SurfacePattern(src, ExtendMode::CLAMP, Matrix::Translation(group.mGroupOffset.x, group.mGroupOffset.y)),
+    Point finalOffset = group.mFinalTarget->GetDeviceOffset();
+    dt->SetTransform(group.mMaskTransform * Matrix::Translation(-finalOffset));
+    Matrix surfTransform = group.mMaskTransform;
+    surfTransform.Invert();
+    dt->MaskSurface(SurfacePattern(src, ExtendMode::CLAMP, surfTransform *
+                                                           Matrix::Translation(group.mGroupOffset.x, group.mGroupOffset.y)),
                     group.mMaskSurface, Point(0, 0), DrawOptions(group.mOpacity, group.mOperator));
   } else {
     // For now this is required since our group offset is in device space of the final target,