Treat trivial 3D transforms as triggers for composited scrolling

Trivial 3D transforms are not considered direct compositing reasons when
the DoNotCompositeTrivial3D experiment is active (see:
CompositingReasonFinder::RequiresCompositingFor3DTransform). This has
caused a performance regression on top sites (google.com and many
others) where we no longer use composited scrolling. This change
continues to use trivial 3D transforms as a reason for composited
scrolling.

Bug: 1012775
Change-Id: Ibef9b9b3de4d57afe30d0fc3dc75da29855b7deb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1907415Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714480}

third_party/blink/renderer/core/paint/paint_layer_scrollable_area.cc

@@ -2353,6 +2353,18 @@ bool PaintLayerScrollableArea::ComputeNeedsCompositedScrolling(
   if (layer_->Size().IsEmpty())
     return false;
 
+  const auto* box = GetLayoutBox();
+
+  // Although trivial 3D transforms are not always a direct compositing reason
+  // (see CompositingReasonFinder::RequiresCompositingFor3DTransform), we treat
+  // them as one for composited scrolling. This is because of the amount of
+  // content that depends on this optimization, and because of the long-term
+  // desire to use composited scrolling whenever possible.
+  if (box->HasTransformRelatedProperty() &&
+      box->StyleRef().Has3DTransformOperation()) {
+    return true;
+  }
+
   if (!force_prefer_compositing_to_lcd_text &&
       !LayerNodeMayNeedCompositedScrolling(layer_)) {
     return false;
@@ -2362,7 +2374,6 @@ bool PaintLayerScrollableArea::ComputeNeedsCompositedScrolling(
 
   // TODO(flackr): Allow integer transforms as long as all of the ancestor
   // transforms are also integer.
-  const LayoutBox* box = GetLayoutBox();
   bool background_supports_lcd_text =
       box->StyleRef().IsStackingContext() &&
       (box->GetBackgroundPaintLocation(

third_party/blink/renderer/core/paint/paint_layer_scrollable_area_test.cc

@@ -4,7 +4,9 @@
 
 #include "third_party/blink/renderer/core/paint/paint_layer_scrollable_area.h"
 
+#include "base/test/scoped_feature_list.h"
 #include "testing/gmock/include/gmock/gmock.h"
+#include "third_party/blink/public/common/features.h"
 #include "third_party/blink/renderer/core/frame/local_frame_view.h"
 #include "third_party/blink/renderer/core/layout/layout_box_model_object.h"
 #include "third_party/blink/renderer/core/paint/paint_layer.h"
@@ -1287,4 +1289,55 @@ TEST_P(PaintLayerScrollableAreaTest, ShowCustomResizerInTextarea) {
   EXPECT_NE(paint_layer->GetScrollableArea()->Resizer(), nullptr);
 }
 
+class PaintLayerScrollableAreaCompositingTest
+    : public PaintLayerScrollableAreaTestBase,
+      public testing::WithParamInterface<unsigned>,
+      private ScopedCompositeAfterPaintForTest {
+ public:
+  PaintLayerScrollableAreaCompositingTest()
+      : ScopedCompositeAfterPaintForTest(GetParam() & kCompositeAfterPaint) {
+    if (GetParam() & kDoNotCompositeTrivial3D) {
+      scoped_feature_list_.InitAndEnableFeature(
+          blink::features::kDoNotCompositeTrivial3D);
+    } else {
+      scoped_feature_list_.InitAndDisableFeature(
+          blink::features::kDoNotCompositeTrivial3D);
+    }
+  }
+
+ private:
+  base::test::ScopedFeatureList scoped_feature_list_;
+};
+
+INSTANTIATE_DO_NOT_COMPOSITE_TRIVIAL_3D_P(
+    PaintLayerScrollableAreaCompositingTest);
+
+// Test that a trivial 3D transform results in composited scrolling.
+TEST_P(PaintLayerScrollableAreaCompositingTest, CompositeWithTrivial3D) {
+  SetBodyInnerHTML(R"HTML(
+    <style>
+      #scroller {
+        width: 100px;
+        height: 100px;
+        overflow: scroll;
+        transform: translateZ(0);
+      }
+      #scrolled {
+        width: 200px;
+        height: 200px;
+      }
+    </style>
+    <div id="scroller">
+      <div id="scrolled"></div>
+    </div>
+  )HTML");
+
+  LayoutBox* scroller = ToLayoutBox(GetLayoutObjectByElementId("scroller"));
+  PaintLayerScrollableArea* scrollable_area = scroller->GetScrollableArea();
+  if (!RuntimeEnabledFeatures::CompositeAfterPaintEnabled())
+    EXPECT_TRUE(scrollable_area->NeedsCompositedScrolling());
+  const auto* properties = scroller->FirstFragment().PaintProperties();
+  EXPECT_TRUE(properties->ScrollTranslation()->HasDirectCompositingReasons());
+}
+
 }  // namespace blink

third_party/blink/renderer/platform/testing/paint_test_configurations.h

@@ -15,6 +15,7 @@ enum {
   kCompositeAfterPaint = 1 << 0,
   kUnderInvalidationChecking = 1 << 1,
   kFastBorderRadius = 1 << 2,
+  kDoNotCompositeTrivial3D = 1 << 3,
 };
 
 class PaintTestConfigurations
@@ -51,6 +52,12 @@ class PaintTestConfigurations
   INSTANTIATE_TEST_SUITE_P(All, test_class,             \
                            ::testing::Values(0, kCompositeAfterPaint))
 
+#define INSTANTIATE_DO_NOT_COMPOSITE_TRIVIAL_3D_P(test_class)              \
+  INSTANTIATE_TEST_SUITE_P(                                                \
+      All, test_class,                                                     \
+      ::testing::Values(0, kCompositeAfterPaint, kDoNotCompositeTrivial3D, \
+                        kCompositeAfterPaint | kDoNotCompositeTrivial3D))
+
 }  // namespace blink
 
 #endif  // THIRD_PARTY_BLINK_RENDERER_PLATFORM_TESTING_PAINT_TEST_CONFIGURATIONS_H_