Fix crash in BackdropFilterLayer::Diff

flow/layers/backdrop_filter_layer.cc

@@ -25,15 +25,11 @@ void BackdropFilterLayer::Diff(DiffContext* context, const Layer* old_layer) {
   context->AddLayerBounds(paint_bounds);
 
   if (filter_) {
-    // convert paint bounds and filter to screen coordinates
     context->GetTransform().mapRect(&paint_bounds);
     auto input_filter_bounds = paint_bounds.roundOut();
-    auto filter = filter_->makeWithLocalMatrix(context->GetTransform());
-
     auto filter_bounds =  // in screen coordinates
-        filter->filterBounds(input_filter_bounds, SkMatrix::I(),
-                             SkImageFilter::kReverse_MapDirection);
-
+        filter_->filterBounds(input_filter_bounds, context->GetTransform(),
+                              SkImageFilter::kReverse_MapDirection);
     context->AddReadbackRegion(filter_bounds);
   }
 

flow/layers/backdrop_filter_layer_unittests.cc

@@ -327,5 +327,29 @@ TEST_F(BackdropLayerDiffTest, BackdropLayer) {
   EXPECT_EQ(damage.frame_damage, SkIRect::MakeLTRB(0, 0, 190, 190));
 }
 
+TEST_F(BackdropLayerDiffTest, BackdropLayerInvalidTransform) {
+  auto filter = SkImageFilters::Blur(10, 10, SkTileMode::kClamp, nullptr);
+
+  {
+    // tests later assume 30px readback area, fail early if that's not the case
+    auto readback = filter->filterBounds(SkIRect::MakeWH(10, 10), SkMatrix::I(),
+                                         SkImageFilter::kReverse_MapDirection);
+    EXPECT_EQ(readback, SkIRect::MakeLTRB(-30, -30, 40, 40));
+  }
+
+  MockLayerTree l1(SkISize::Make(100, 100));
+  SkMatrix transform;
+  transform.setPerspX(0.1);
+  transform.setPerspY(0.1);
+
+  auto transform_layer = std::make_shared<TransformLayer>(transform);
+  l1.root()->Add(transform_layer);
+  transform_layer->Add(
+      std::make_shared<BackdropFilterLayer>(filter, SkBlendMode::kSrcOver));
+
+  auto damage = DiffLayerTree(l1, MockLayerTree(SkISize::Make(100, 100)));
+  EXPECT_EQ(damage.frame_damage, SkIRect::MakeWH(15, 15));
+}
+
 }  // namespace testing
 }  // namespace flutter