Composite ScrollTimeline scrollSource when appropriate.

This CL makes being the scrollSource of an active (i.e. the scrollSource is
scrollable) and attached (i.e. it is being used by at least one WorkletAnimation
as a timeline) ScrollTimeline a compositing reason. It does not change the
requirement that the scrollSource must be composited for the WorkletAnimation to
be sent to the compositor, nor does it change the case where a scrollSource stops
being composited whilst a WorkletAnimation is running. Those will be addressed in
follow-up CLs.

Bug: 776533
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Icae43e1c9cfe2720cc739401c4766b592571f231
Reviewed-on: https://chromium-review.googlesource.com/1036934
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556541}

third_party/WebKit/LayoutTests/virtual/threaded/fast/animationworklet/animation-worklet-scroll-timeline.html

@@ -16,7 +16,6 @@
   overflow: auto;
   height: 100px;
   width: 100px;
-  will-change: transform; /* force compositing */
 }
 
 #contents {

third_party/blink/renderer/core/BUILD.gn

@@ -1633,6 +1633,7 @@ jumbo_source_set("unit_tests") {
     "animation/keyframe_effect_model_test.cc",
     "animation/keyframe_effect_test.cc",
     "animation/property_handle_test.cc",
+    "animation/scroll_timeline_test.cc",
     "animation/timing_calculations_test.cc",
     "animation/timing_input_test.cc",
     "clipboard/data_object_test.cc",

third_party/blink/renderer/core/animation/scroll_timeline.cc

@@ -6,11 +6,18 @@
 
 #include "third_party/blink/renderer/core/dom/exception_code.h"
 #include "third_party/blink/renderer/core/layout/layout_box.h"
+#include "third_party/blink/renderer/core/layout/layout_view.h"
 #include "third_party/blink/renderer/core/paint/paint_layer_scrollable_area.h"
 
 namespace blink {
 
 namespace {
+using ActiveScrollTimelineSet = PersistentHeapHashCountedSet<WeakMember<Node>>;
+ActiveScrollTimelineSet& GetActiveScrollTimelineSet() {
+  DEFINE_STATIC_LOCAL(ActiveScrollTimelineSet, set, ());
+  return set;
+}
+
 bool StringToScrollDirection(String scroll_direction,
                              ScrollTimeline::ScrollDirection& result) {
   // TODO(smcgruer): Support 'auto' value.
@@ -46,17 +53,17 @@ ScrollTimeline* ScrollTimeline::Create(Document& document,
     return nullptr;
   }
 
-  return new ScrollTimeline(document, scroll_source, orientation,
+  return new ScrollTimeline(scroll_source, orientation,
                             options.timeRange().GetAsDouble());
 }
 
-ScrollTimeline::ScrollTimeline(const Document& document,
-                               Element* scroll_source,
+ScrollTimeline::ScrollTimeline(Element* scroll_source,
                                ScrollDirection orientation,
                                double time_range)
     : scroll_source_(scroll_source),
       orientation_(orientation),
       time_range_(time_range) {
+  DCHECK(scroll_source_);
 }
 
 double ScrollTimeline::currentTime(bool& is_null) {
@@ -140,9 +147,36 @@ void ScrollTimeline::timeRange(DoubleOrScrollTimelineAutoKeyword& result) {
   result.SetDouble(time_range_);
 }
 
+void ScrollTimeline::AttachAnimation() {
+  if (!scroll_source_)
+    return;
+  GetActiveScrollTimelineSet().insert(scroll_source_);
+  scroll_source_->GetDocument()
+      .GetLayoutView()
+      ->Compositor()
+      ->SetNeedsCompositingUpdate(kCompositingUpdateRebuildTree);
+}
+
+void ScrollTimeline::DetachAnimation() {
+  if (!scroll_source_)
+    return;
+  DCHECK(GetActiveScrollTimelineSet().Contains(scroll_source_));
+  GetActiveScrollTimelineSet().erase(scroll_source_);
+  scroll_source_->GetDocument()
+      .GetLayoutView()
+      ->Compositor()
+      ->SetNeedsCompositingUpdate(kCompositingUpdateRebuildTree);
+}
+
 void ScrollTimeline::Trace(blink::Visitor* visitor) {
   visitor->Trace(scroll_source_);
   AnimationTimeline::Trace(visitor);
 }
 
+bool ScrollTimeline::HasActiveScrollTimeline(Node* node) {
+  ActiveScrollTimelineSet& set = GetActiveScrollTimelineSet();
+  auto it = set.find(node);
+  return it != set.end() && it->value > 0;
+}
+
 }  // namespace blink

third_party/blink/renderer/core/animation/scroll_timeline.h

@@ -46,10 +46,22 @@ class CORE_EXPORT ScrollTimeline final : public AnimationTimeline {
 
   ScrollDirection GetOrientation() const { return orientation_; }
 
+  // Must be called when this ScrollTimeline is attached/unattached from an
+  // animation.
+  void AttachAnimation();
+  void DetachAnimation();
+
   void Trace(blink::Visitor*) override;
 
+  // For the AnimationWorklet origin trial, we need to automatically composite
+  // elements that are targets of ScrollTimelines (http://crbug.com/776533). We
+  // expose a static lookup method to enable this.
+  //
+  // TODO(crbug.com/839341): Remove once WorkletAnimations can run on main.
+  static bool HasActiveScrollTimeline(Node* node);
+
  private:
-  ScrollTimeline(const Document&, Element*, ScrollDirection, double);
+  ScrollTimeline(Element*, ScrollDirection, double);
 
   Member<Element> scroll_source_;
   ScrollDirection orientation_;

third_party/blink/renderer/core/animation/scroll_timeline_test.cc

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "third_party/blink/renderer/core/animation/scroll_timeline.h"
+
+#include "testing/gtest/include/gtest/gtest.h"
+#include "third_party/blink/renderer/bindings/core/v8/exception_state.h"
+#include "third_party/blink/renderer/core/paint/paint_layer.h"
+#include "third_party/blink/renderer/core/testing/core_unit_test_helper.h"
+#include "third_party/blink/renderer/core/testing/dummy_page_holder.h"
+
+namespace blink {
+
+using ScrollTimelineTest = RenderingTest;
+
+TEST_F(ScrollTimelineTest,
+       AttachingAndDetachingAnimationCausesCompositingUpdate) {
+  EnableCompositing();
+
+  SetBodyInnerHTML(R"HTML(
+    <style>#scroller { overflow: scroll; width: 100px; height: 100px; }</style>
+    <div id='scroller'></div>
+  )HTML");
+
+  LayoutBoxModelObject* scroller =
+      ToLayoutBoxModelObject(GetLayoutObjectByElementId("scroller"));
+  ASSERT_TRUE(scroller);
+
+  // Invariant: the scroller is not composited by default.
+  EXPECT_EQ(DocumentLifecycle::kPaintClean,
+            GetDocument().Lifecycle().GetState());
+  EXPECT_EQ(kNotComposited, scroller->Layer()->GetCompositingState());
+
+  // Create the ScrollTimeline. This shouldn't cause the scrollSource to need
+  // compositing, as it isn't attached to any animation yet.
+  ScrollTimelineOptions options;
+  DoubleOrScrollTimelineAutoKeyword time_range =
+      DoubleOrScrollTimelineAutoKeyword::FromDouble(100);
+  options.setTimeRange(time_range);
+  options.setScrollSource(GetElementById("scroller"));
+  ScrollTimeline* scroll_timeline =
+      ScrollTimeline::Create(GetDocument(), options, ASSERT_NO_EXCEPTION);
+  EXPECT_EQ(DocumentLifecycle::kPaintClean,
+            GetDocument().Lifecycle().GetState());
+  EXPECT_EQ(kNotComposited, scroller->Layer()->GetCompositingState());
+
+  // Now attach an animation. This should require a compositing update.
+  scroll_timeline->AttachAnimation();
+
+  UpdateAllLifecyclePhases();
+  EXPECT_NE(scroller->Layer()->GetCompositingState(), kNotComposited);
+
+  // Now detach an animation. This should again require a compositing update.
+  scroll_timeline->DetachAnimation();
+
+  UpdateAllLifecyclePhases();
+  EXPECT_EQ(scroller->Layer()->GetCompositingState(), kNotComposited);
+}
+
+}  //  namespace blink

third_party/blink/renderer/core/paint/compositing/compositing_reason_finder.cc

@@ -4,6 +4,7 @@
 
 #include "third_party/blink/renderer/core/paint/compositing/compositing_reason_finder.h"
 
+#include "third_party/blink/renderer/core/animation/scroll_timeline.h"
 #include "third_party/blink/renderer/core/css_property_names.h"
 #include "third_party/blink/renderer/core/dom/document.h"
 #include "third_party/blink/renderer/core/frame/local_frame_view.h"
@@ -177,6 +178,14 @@ CompositingReasons CompositingReasonFinder::NonStyleDeterminedDirectReasons(
   if (RequiresCompositingForScrollDependentPosition(layer, ignore_lcd_text))
     direct_reasons |= CompositingReason::kScrollDependentPosition;
 
+  // TODO(crbug.com/839341): Remove once we support main-thread AnimationWorklet
+  // and don't need to promote the scroll-source.
+  if (layer->GetScrollableArea() && layer->GetLayoutObject().GetNode() &&
+      ScrollTimeline::HasActiveScrollTimeline(
+          layer->GetLayoutObject().GetNode())) {
+    direct_reasons |= CompositingReason::kScrollTimelineTarget;
+  }
+
   direct_reasons |= layout_object.AdditionalCompositingReasons();
 
   DCHECK(

third_party/blink/renderer/modules/animationworklet/worklet_animation.cc

@@ -231,6 +231,9 @@ WorkletAnimation::WorkletAnimation(
 
   AnimationEffect* target_effect = effects_.at(0);
   target_effect->Attach(this);
+
+  if (timeline_.IsScrollTimeline())
+    timeline.GetAsScrollTimeline()->AttachAnimation();
 }
 
 String WorkletAnimation::playState() {
@@ -431,6 +434,8 @@ KeyframeEffect* WorkletAnimation::GetEffect() const {
 
 void WorkletAnimation::Dispose() {
   DCHECK(IsMainThread());
+  if (timeline_.IsScrollTimeline())
+    timeline_.GetAsScrollTimeline()->DetachAnimation();
   DestroyCompositorAnimation();
 }
 

third_party/blink/renderer/platform/graphics/compositing_reasons.h

@@ -35,6 +35,7 @@ using CompositingReasons = uint64_t;
   V(WillChangeCompositingHint)                                                \
   V(BackdropFilter)                                                           \
   V(RootScroller)                                                             \
+  V(ScrollTimelineTarget)                                                     \
                                                                               \
   /* Overlap reasons that require knowing what's behind you in paint-order    \
      before knowing the answer. */                                            \