Limit minimum scale of PaintedScrollbarLayer to 1

This is to avoid too low scale when animating scale above the
scrollbar layer.

This is not ideal. In the future we should move rasterization of
scrollbar layers to impl-side to better handle scales
(crbug.com/1009291).

Bug: 1133997
Change-Id: I3364754f530914816fe8526c627e6cf12e7942ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453436Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814681}

cc/layers/painted_scrollbar_layer.cc

@@ -4,6 +4,10 @@
 
 #include "cc/layers/painted_scrollbar_layer.h"
 
+#include <algorithm>
+#include <memory>
+#include <utility>
+
 #include "base/auto_reset.h"
 #include "cc/layers/painted_scrollbar_layer_impl.h"
 #include "cc/paint/skia_paint_canvas.h"
@@ -140,6 +144,10 @@ bool PaintedScrollbarLayer::UpdateInternalContentScale() {
   gfx::Vector2dF transform_scales = MathUtil::ComputeTransform2dScaleComponents(
       transform, layer_tree_host()->device_scale_factor());
   float scale = std::max(transform_scales.x(), transform_scales.y());
+  // Clamp minimum scale to 1 to avoid too low scale during scale animation.
+  // TODO(crbug.com/1009291): Move rasterization of scrollbars to the impl side
+  // to better handle scale changes.
+  scale = std::max(1.0f, scale);
 
   bool updated = false;
   updated |= UpdateProperty(scale, &internal_contents_scale_);

cc/layers/painted_scrollbar_layer_unittest.cc

@@ -4,9 +4,12 @@
 
 #include "cc/layers/painted_scrollbar_layer.h"
 
+#include <memory>
+
 #include "cc/animation/animation_host.h"
 #include "cc/test/fake_layer_tree_host.h"
 #include "cc/test/fake_layer_tree_host_client.h"
+#include "cc/test/fake_painted_scrollbar_layer.h"
 #include "cc/test/fake_scrollbar.h"
 #include "cc/test/layer_test_common.h"
 #include "cc/test/test_task_graph_runner.h"
@@ -19,6 +22,20 @@ namespace cc {
 
 namespace {
 
+class PaintedScrollbarLayerTest : public testing::Test {
+ protected:
+  void SetUp() override {
+    animation_host_ = AnimationHost::CreateForTesting(ThreadInstance::MAIN);
+    layer_tree_host_ = FakeLayerTreeHost::Create(
+        &fake_client_, &task_graph_runner_, animation_host_.get());
+  }
+
+  FakeLayerTreeHostClient fake_client_;
+  TestTaskGraphRunner task_graph_runner_;
+  std::unique_ptr<AnimationHost> animation_host_;
+  std::unique_ptr<FakeLayerTreeHost> layer_tree_host_;
+};
+
 class MockScrollbar : public FakeScrollbar {
  public:
   MockScrollbar() {
@@ -35,14 +52,7 @@ class MockScrollbar : public FakeScrollbar {
   ~MockScrollbar() override = default;
 };
 
-TEST(PaintedScrollbarLayerTest, NeedsPaint) {
-  FakeLayerTreeHostClient fake_client_;
-  TestTaskGraphRunner task_graph_runner_;
-
-  auto animation_host = AnimationHost::CreateForTesting(ThreadInstance::MAIN);
-  auto layer_tree_host = FakeLayerTreeHost::Create(
-      &fake_client_, &task_graph_runner_, animation_host.get());
-
+TEST_F(PaintedScrollbarLayerTest, NeedsPaint) {
   auto scrollbar = base::MakeRefCounted<MockScrollbar>();
   scoped_refptr<PaintedScrollbarLayer> scrollbar_layer =
       PaintedScrollbarLayer::Create(scrollbar);
@@ -50,10 +60,10 @@ TEST(PaintedScrollbarLayerTest, NeedsPaint) {
   scrollbar_layer->SetIsDrawable(true);
   scrollbar_layer->SetBounds(gfx::Size(100, 100));
 
-  layer_tree_host->SetRootLayer(scrollbar_layer);
-  UpdateDrawProperties(layer_tree_host.get());
+  layer_tree_host_->SetRootLayer(scrollbar_layer);
+  UpdateDrawProperties(layer_tree_host_.get());
 
-  EXPECT_EQ(scrollbar_layer->layer_tree_host(), layer_tree_host.get());
+  EXPECT_EQ(scrollbar_layer->layer_tree_host(), layer_tree_host_.get());
 
   // Request no paint, but expect them to be painted because they have not
   // yet been initialized.
@@ -95,5 +105,32 @@ TEST(PaintedScrollbarLayerTest, NeedsPaint) {
   Mock::VerifyAndClearExpectations(scrollbar.get());
 }
 
+TEST_F(PaintedScrollbarLayerTest, InternalContentBounds) {
+  auto scrollbar = base::MakeRefCounted<FakeScrollbar>();
+  auto scrollbar_layer = PaintedScrollbarLayer::Create(scrollbar);
+  scrollbar_layer->SetIsDrawable(true);
+  scrollbar_layer->SetBounds(gfx::Size(10, 100));
+
+  layer_tree_host_->SetRootLayer(scrollbar_layer);
+  UpdateDrawProperties(layer_tree_host_.get());
+  EXPECT_EQ(scrollbar_layer->layer_tree_host(), layer_tree_host_.get());
+  scrollbar_layer->Update();
+  EXPECT_EQ(gfx::Size(10, 100), scrollbar_layer->internal_content_bounds());
+
+  layer_tree_host_->SetViewportRectAndScale(
+      layer_tree_host_->device_viewport_rect(), 2.0f,
+      layer_tree_host_->local_surface_id_from_parent());
+  UpdateDrawProperties(layer_tree_host_.get());
+  scrollbar_layer->Update();
+  EXPECT_EQ(gfx::Size(20, 200), scrollbar_layer->internal_content_bounds());
+
+  layer_tree_host_->SetViewportRectAndScale(
+      layer_tree_host_->device_viewport_rect(), 0.1f,
+      layer_tree_host_->local_surface_id_from_parent());
+  UpdateDrawProperties(layer_tree_host_.get());
+  scrollbar_layer->Update();
+  EXPECT_EQ(gfx::Size(10, 100), scrollbar_layer->internal_content_bounds());
+}
+
 }  // namespace
 }  // namespace cc