Avoid unnecessary scrollable area layer's main thread scrolling repaint.

CompositedLayerMapping::NeedsRepaint always returned true if the target graphics
layer is a scrollable area layer. This can be optimized because we can get paint
invalidation flag from the overflow controls.

Change-Id: Ie2612794403fabd8dcb8dad7aca41a60dfbb0c69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1943501Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720456}

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

@@ -2103,9 +2103,8 @@ LayoutSize CompositedLayerMapping::SubpixelAccumulation() const {
 
 bool CompositedLayerMapping::NeedsRepaint(
     const GraphicsLayer& graphics_layer) const {
-  return IsScrollableAreaLayer(&graphics_layer)
-             ? true
-             : owning_layer_.SelfOrDescendantNeedsRepaint();
+  return IsScrollableAreaLayerWhichNeedsRepaint(&graphics_layer) ||
+         owning_layer_.SelfOrDescendantNeedsRepaint();
 }
 
 bool CompositedLayerMapping::AdjustForCompositedScrolling(
@@ -2242,6 +2241,23 @@ bool CompositedLayerMapping::IsScrollableAreaLayer(
          graphics_layer == LayerForScrollCorner();
 }
 
+bool CompositedLayerMapping::IsScrollableAreaLayerWhichNeedsRepaint(
+    const GraphicsLayer* graphics_layer) const {
+  if (PaintLayerScrollableArea* scrollable_area =
+          owning_layer_.GetScrollableArea()) {
+    if (graphics_layer == LayerForHorizontalScrollbar())
+      return scrollable_area->HorizontalScrollbarNeedsPaintInvalidation();
+
+    if (graphics_layer == LayerForVerticalScrollbar())
+      return scrollable_area->VerticalScrollbarNeedsPaintInvalidation();
+
+    if (graphics_layer == LayerForScrollCorner())
+      return scrollable_area->ScrollCornerNeedsPaintInvalidation();
+  }
+
+  return false;
+}
+
 bool CompositedLayerMapping::ShouldThrottleRendering() const {
   return GetLayoutObject().GetFrame()->ShouldThrottleRendering();
 }

third_party/blink/renderer/core/paint/compositing/composited_layer_mapping.h

@@ -299,6 +299,10 @@ class CORE_EXPORT CompositedLayerMapping final : public GraphicsLayerClient {
   // associated with this mapping.
   bool IsScrollableAreaLayer(const GraphicsLayer*) const;
 
+  // Returns whether the given layer is a repaint needed part of the scrollable
+  // area, if any, associated with this mapping.
+  bool IsScrollableAreaLayerWhichNeedsRepaint(const GraphicsLayer*) const;
+
   // Helper methods to updateGraphicsLayerGeometry:
   void ComputeGraphicsLayerParentLocation(
       const PaintLayer* compositing_container,

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

@@ -1721,4 +1721,67 @@ TEST_F(CompositedLayerMappingTest, CompositedHiddenAnimatingLayer) {
               CompositingReason::kActiveTransformAnimation);
 }
 
+TEST_F(CompositedLayerMappingTest,
+       RepaintScrollableAreaLayersInMainThreadScrolling) {
+  SetHtmlInnerHTML(R"HTML(
+    <style>
+      #scroller {
+        width: 200px;
+        height: 100px;
+        overflow: scroll;
+        opacity: 0.8; /*MainThreadScrollingReason::kHasOpacityAndLCDText*/
+      }
+
+      #child {
+        width: 100px;
+        height: 200px;
+        transform: translate3d(0, 0, 0);
+        overflow: hidden;
+      }
+      #uncorrelated {
+        transform: translate3d(0, 0, 0);
+        height: 100px;
+        width: 100px;
+        background-color: red;
+      }
+    </style>
+
+    <div id="scroller">
+      <div id="child">
+      </div>
+    </div>
+    <div id="uncorrelated"></div>
+  )HTML");
+
+  PaintLayer* scroller =
+      ToLayoutBoxModelObject(GetLayoutObjectByElementId("scroller"))->Layer();
+
+  PaintLayerScrollableArea* scrollable_area = scroller->GetScrollableArea();
+  ASSERT_TRUE(scrollable_area);
+
+  GraphicsLayer* vertical_scrollbar =
+      scrollable_area->GraphicsLayerForVerticalScrollbar();
+  ASSERT_TRUE(vertical_scrollbar);
+
+  CompositedLayerMapping* mapping = scroller->GetCompositedLayerMapping();
+  ASSERT_TRUE(mapping);
+
+  // Input events, animations and DOM changes, etc, can trigger cc::ProxyMain::
+  // BeginMainFrame, which may check if all graphics layers need repaint.
+  //
+  // We shouldn't repaint scrollable area layer which has no paint invalidation
+  // in many uncorrelated BeginMainFrame scenes, such as moving mouse over the
+  // non-scrollbar area, animating or DOM changes in another composited layer.
+  GetDocument()
+      .getElementById("uncorrelated")
+      ->setAttribute(html_names::kStyleAttr, "width: 200px");
+  GetDocument().View()->UpdateAllLifecyclePhasesExceptPaint();
+  EXPECT_FALSE(mapping->NeedsRepaint(*vertical_scrollbar));
+
+  GetDocument().getElementById("child")->setAttribute(html_names::kStyleAttr,
+                                                      "height: 300px");
+  GetDocument().View()->UpdateAllLifecyclePhasesExceptPaint();
+  EXPECT_TRUE(mapping->NeedsRepaint(*vertical_scrollbar));
+}
+
 }  // namespace blink

third_party/blink/renderer/core/scroll/scrollable_area.h

@@ -385,6 +385,16 @@ class CORE_EXPORT ScrollableArea : public GarbageCollectedMixin {
   bool HasLayerForVerticalScrollbar() const;
   bool HasLayerForScrollCorner() const;
 
+  bool HorizontalScrollbarNeedsPaintInvalidation() const {
+    return horizontal_scrollbar_needs_paint_invalidation_;
+  }
+  bool VerticalScrollbarNeedsPaintInvalidation() const {
+    return vertical_scrollbar_needs_paint_invalidation_;
+  }
+  bool ScrollCornerNeedsPaintInvalidation() const {
+    return scroll_corner_needs_paint_invalidation_;
+  }
+
   void LayerForScrollingDidChange(CompositorAnimationTimeline*);
   bool NeedsShowScrollbarLayers() const { return needs_show_scrollbar_layers_; }
   void DidShowScrollbarLayers() { needs_show_scrollbar_layers_ = false; }
@@ -493,15 +503,6 @@ class CORE_EXPORT ScrollableArea : public GarbageCollectedMixin {
   friend class ScrollAnimatorCompositorCoordinator;
   void ScrollOffsetChanged(const ScrollOffset&, ScrollType);
 
-  bool HorizontalScrollbarNeedsPaintInvalidation() const {
-    return horizontal_scrollbar_needs_paint_invalidation_;
-  }
-  bool VerticalScrollbarNeedsPaintInvalidation() const {
-    return vertical_scrollbar_needs_paint_invalidation_;
-  }
-  bool ScrollCornerNeedsPaintInvalidation() const {
-    return scroll_corner_needs_paint_invalidation_;
-  }
   void ClearNeedsPaintInvalidationForScrollControls() {
     horizontal_scrollbar_needs_paint_invalidation_ = false;
     vertical_scrollbar_needs_paint_invalidation_ = false;