Record non-stacking-context as main thread scrolling reasons

There is another common main thread scrolling reason that needs
to be added. A scrollable area with an opaque background
is not a stacking context therefore we don't composite it for
now.

BUG=660907
TEST=ScrollingCoordinatorTest.StackingContextTest
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2841603003
Cr-Commit-Position: refs/heads/master@{#468122}

cc/input/main_thread_scrolling_reason.h

@@ -44,7 +44,8 @@ struct MainThreadScrollingReason {
     kHasBorderRadius = 1 << 19,
     kHasClipRelatedProperty = 1 << 20,
     kHasBoxShadowFromNonRootLayer = 1 << 21,
-    kNonCompositedReasonsLast = 21,
+    kIsNotStackingContextAndLCDText = 1 << 22,
+    kNonCompositedReasonsLast = 22,
 
     // Transient scrolling reasons. These are computed for each scroll begin.
     kNonFastScrollableRegion = 1 << 5,
@@ -59,13 +60,14 @@ struct MainThreadScrollingReason {
     // New flags should increment this number but it should never be decremented
     // because the values are used in UMA histograms. It should also be noted
     // that it excludes the kNotScrollingOnMain value.
-    kMainThreadScrollingReasonCount = 22,
+    kMainThreadScrollingReasonCount = 23,
   };
 
   static const uint32_t kNonCompositedReasons =
       kHasOpacityAndLCDText | kHasTransformAndLCDText |
       kBackgroundNotOpaqueInRectAndLCDText | kHasBorderRadius |
-      kHasClipRelatedProperty | kHasBoxShadowFromNonRootLayer;
+      kHasClipRelatedProperty | kHasBoxShadowFromNonRootLayer |
+      kIsNotStackingContextAndLCDText;
 
   // Returns true if the given MainThreadScrollingReason can be set by the main
   // thread.
@@ -140,6 +142,8 @@ struct MainThreadScrollingReason {
       tracedValue->AppendString("Has clip related property");
     if (reasons & MainThreadScrollingReason::kHasBoxShadowFromNonRootLayer)
       tracedValue->AppendString("Has box shadow from non-root layer");
+    if (reasons & MainThreadScrollingReason::kIsNotStackingContextAndLCDText)
+      tracedValue->AppendString("Is not stacking context and LCD text");
 
     // Transient scrolling reasons.
     if (reasons & MainThreadScrollingReason::kNonFastScrollableRegion)

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

@@ -1876,6 +1876,10 @@ bool PaintLayerScrollableArea::ComputeNeedsCompositedScrolling(
       non_composited_main_thread_scrolling_reasons_ |=
           MainThreadScrollingReason::kBackgroundNotOpaqueInRectAndLCDText;
     }
+    if (!layer->GetLayoutObject().Style()->IsStackingContext()) {
+      non_composited_main_thread_scrolling_reasons_ |=
+          MainThreadScrollingReason::kIsNotStackingContextAndLCDText;
+    }
 
     needs_composited_scrolling = false;
   }

third_party/WebKit/Source/web/tests/ScrollMetricsTest.cpp

@@ -347,9 +347,10 @@ TEST_F(NonCompositedMainThreadScrollingReasonRecordTest, NestedScrollersTest) {
   // areas from the chain have corresponding reasons.
   EXPECT_WHEEL_BUCKET(kHasOpacityAndLCDText, 1);
   EXPECT_WHEEL_BUCKET(kBackgroundNotOpaqueInRectAndLCDText, 1);
+  EXPECT_WHEEL_BUCKET(kIsNotStackingContextAndLCDText, 1);
   EXPECT_WHEEL_BUCKET(kHasBorderRadius, 1);
   EXPECT_WHEEL_BUCKET(kHasTransformAndLCDText, 0);
-  EXPECT_WHEEL_TOTAL(3);
+  EXPECT_WHEEL_TOTAL(4);
 }
 
 }  // namespace

third_party/WebKit/Source/web/tests/ScrollingCoordinatorTest.cpp

@@ -1052,7 +1052,8 @@ class NonCompositedMainThreadScrollingReasonTest
   static const uint32_t kLCDTextRelatedReasons =
       MainThreadScrollingReason::kHasOpacityAndLCDText |
       MainThreadScrollingReason::kHasTransformAndLCDText |
-      MainThreadScrollingReason::kBackgroundNotOpaqueInRectAndLCDText;
+      MainThreadScrollingReason::kBackgroundNotOpaqueInRectAndLCDText |
+      MainThreadScrollingReason::kIsNotStackingContextAndLCDText;
 
  protected:
   NonCompositedMainThreadScrollingReasonTest() {
@@ -1207,6 +1208,35 @@ TEST_P(NonCompositedMainThreadScrollingReasonTest, BoxShadowTest) {
       "box-shadow", MainThreadScrollingReason::kHasBoxShadowFromNonRootLayer);
 }
 
+TEST_P(NonCompositedMainThreadScrollingReasonTest, StackingContextTest) {
+  GetWebViewImpl()->GetSettings()->SetPreferCompositingToLCDTextEnabled(false);
+
+  Document* document = GetFrame()->GetDocument();
+  Element* container = document->getElementById("scroller1");
+  ASSERT_TRUE(container);
+
+  ForceFullCompositingUpdate();
+
+  // If a scroller contains all its children, it's not a stacking context.
+  PaintLayerScrollableArea* scrollable_area =
+      ToLayoutBoxModelObject(container->GetLayoutObject())->GetScrollableArea();
+  ASSERT_TRUE(scrollable_area);
+  EXPECT_TRUE(scrollable_area->GetNonCompositedMainThreadScrollingReasons() &
+              MainThreadScrollingReason::kIsNotStackingContextAndLCDText);
+
+  GetWebViewImpl()->GetSettings()->SetPreferCompositingToLCDTextEnabled(true);
+  ForceFullCompositingUpdate();
+  EXPECT_FALSE(scrollable_area->GetNonCompositedMainThreadScrollingReasons() &
+               MainThreadScrollingReason::kIsNotStackingContextAndLCDText);
+  GetWebViewImpl()->GetSettings()->SetPreferCompositingToLCDTextEnabled(false);
+
+  // Adding "contain: paint" to force a stacking context leads to promotion.
+  container->setAttribute("style", "contain: paint", ASSERT_NO_EXCEPTION);
+  ForceFullCompositingUpdate();
+
+  EXPECT_FALSE(scrollable_area->GetNonCompositedMainThreadScrollingReasons());
+}
+
 TEST_P(NonCompositedMainThreadScrollingReasonTest,
        CompositedWithLCDTextRelatedReasonsTest) {
   // With "will-change:transform" we composite elements with

tools/metrics/histograms/histograms.xml

@@ -104547,6 +104547,7 @@ from previous Chrome versions.
   <int value="20" label="Has border radius"/>
   <int value="21" label="Has clip related property"/>
   <int value="22" label="Has box shadow from non-root layer"/>
+  <int value="23" label="Is not stacking context"/>
 </enum>
 
 <enum name="MakeChromeDefaultResult" type="int">