Use preTranslate when applying offset to matrix (#21648)

Author: knopp

flow/layers/opacity_layer.cc

@@ -17,7 +17,7 @@ void OpacityLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
   FML_DCHECK(!GetChildContainer()->layers().empty());  // We can't be a leaf.
 
   SkMatrix child_matrix = matrix;
-  child_matrix.postTranslate(offset_.fX, offset_.fY);
+  child_matrix.preTranslate(offset_.fX, offset_.fY);
 
   // Similar to what's done in TransformLayer::Preroll, we have to apply the
   // reverse transformation to the cull rect to properly cull child layers.

flow/layers/opacity_layer_unittests.cc

@@ -54,6 +54,23 @@ TEST_F(OpacityLayerTest, PaintBeforePreollDies) {
 }
 #endif
 
+TEST_F(OpacityLayerTest, TranslateChildren) {
+  SkPath child_path1;
+  child_path1.addRect(10.0f, 10.0f, 20.0f, 20.f);
+  SkPaint child_paint1(SkColors::kGray);
+  auto layer = std::make_shared<OpacityLayer>(0.5, SkPoint::Make(10, 10));
+  auto mock_layer1 = std::make_shared<MockLayer>(child_path1, child_paint1);
+  layer->Add(mock_layer1);
+
+  auto initial_transform = SkMatrix::Scale(2.0, 2.0);
+  layer->Preroll(preroll_context(), initial_transform);
+
+  SkRect layer_bounds = mock_layer1->paint_bounds();
+  mock_layer1->parent_matrix().mapRect(&layer_bounds);
+
+  EXPECT_EQ(layer_bounds, SkRect::MakeXYWH(40, 40, 20, 20));
+}
+
 TEST_F(OpacityLayerTest, ChildIsCached) {
   const SkAlpha alpha_half = 255 / 2;
   auto initial_transform = SkMatrix::Translate(50.0, 25.5);

flow/layers/picture_layer.cc

@@ -30,7 +30,7 @@ void PictureLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
     TRACE_EVENT0("flutter", "PictureLayer::RasterCache (Preroll)");
 
     SkMatrix ctm = matrix;
-    ctm.postTranslate(offset_.x(), offset_.y());
+    ctm.preTranslate(offset_.x(), offset_.y());
 #ifndef SUPPORT_FRACTIONAL_TRANSLATION
     ctm = RasterCache::GetIntegralTransCTM(ctm);
 #endif