Remove dummy layers from CompositedLayerMapping

We no longer need scrolling_layer_ after BGPT launched.

For squashing_containment_layer_, we can remove it by ensuring
squashing_layer_ is always the last child of graphics_layer_.

There is still one dummy layer: overflow_controls_host_layer_ which
is needed for convenience of reparenting overflow control layers.

Change-Id: Ibdafef32d918e66a12c35d2a5726c50ea374f7a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2205518
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769984}

third_party/blink/renderer/core/page/scrolling/root_scroller_test.cc

@@ -681,7 +681,6 @@ TEST_F(RootScrollerTest, AlwaysCreateCompositedScrollingLayers) {
 
   ASSERT_TRUE(layer->HasCompositedLayerMapping());
   EXPECT_TRUE(layer->GetCompositedLayerMapping()->ScrollingContentsLayer());
-  EXPECT_TRUE(layer->GetCompositedLayerMapping()->ScrollingLayer());
 
   SetAndSelectRootScroller(*MainFrame()->GetDocument(), nullptr);
 

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

@@ -106,7 +106,9 @@ void BoxPainter::PaintBoxDecorationBackground(
     // composited in PaintArtifactCompositor::UpdateNonFastScrollableRegions.
     if (layout_box_.HasLayer() &&
         layout_box_.Layer()->GetCompositedLayerMapping() &&
-        layout_box_.Layer()->GetCompositedLayerMapping()->HasScrollingLayer()) {
+        layout_box_.Layer()
+            ->GetCompositedLayerMapping()
+            ->ScrollingContentsLayer()) {
       needs_scroll_hit_test = false;
     }
   }

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

@@ -113,8 +113,6 @@ class CORE_EXPORT CompositedLayerMapping final : public GraphicsLayerClient {
     return decoration_outline_layer_.get();
   }
 
-  bool HasScrollingLayer() const { return scrolling_layer_.get(); }
-  GraphicsLayer* ScrollingLayer() const { return scrolling_layer_.get(); }
   GraphicsLayer* ScrollingContentsLayer() const {
     return scrolling_contents_layer_.get();
   }
@@ -122,19 +120,14 @@ class CORE_EXPORT CompositedLayerMapping final : public GraphicsLayerClient {
   GraphicsLayer* MaskLayer() const { return mask_layer_.get(); }
 
   GraphicsLayer* ParentForSublayers() const;
-  GraphicsLayer* ChildForSuperlayers() const;
   void SetSublayers(const GraphicsLayerVector&);
 
-  GraphicsLayer* SquashingContainmentLayer() const {
-    return squashing_containment_layer_.get();
-  }
   GraphicsLayer* SquashingLayer() const { return squashing_layer_.get(); }
   const IntSize& SquashingLayerOffsetFromLayoutObject() const {
     return squashing_layer_offset_from_layout_object_;
   }
 
-  void SetSquashingContentsNeedDisplay();
-  void SetContentsNeedDisplay();
+  void SetAllLayersNeedDisplay();
 
   // Let all DrawsContent GraphicsLayers check raster invalidations after
   // a no-change paint.
@@ -207,9 +200,6 @@ class CORE_EXPORT CompositedLayerMapping final : public GraphicsLayerClient {
   // position.
   GraphicsLayer* DetachLayerForOverflowControls();
 
-  // We may similarly need to reattach the layer for outlines and decorations.
-  GraphicsLayer* DetachLayerForDecorationOutline();
-
   void SetBlendMode(BlendMode);
 
   bool NeedsGraphicsLayerUpdate() {
@@ -314,7 +304,7 @@ class CORE_EXPORT CompositedLayerMapping final : public GraphicsLayerClient {
   void UpdateForegroundLayerGeometry();
   void UpdateDecorationOutlineLayerGeometry(
       const IntSize& relative_compositing_bounds_size);
-  void UpdateScrollingLayerGeometry();
+  void UpdateScrollingContentsLayerGeometry();
 
   void CreatePrimaryGraphicsLayer();
 
@@ -343,7 +333,7 @@ class CORE_EXPORT CompositedLayerMapping final : public GraphicsLayerClient {
   bool RequiresHorizontalScrollbarLayer() const;
   bool RequiresVerticalScrollbarLayer() const;
   bool RequiresScrollCornerLayer() const;
-  bool UpdateScrollingLayers(bool scrolling_layers);
+  bool UpdateScrollingContentsLayer(bool needs_scrolling_contents_layer);
   bool UpdateSquashingLayers(bool needs_squashing_layers);
   void UpdateDrawsContentAndPaintsHitTest();
   void UpdateCompositedBounds();
@@ -395,29 +385,40 @@ class CORE_EXPORT CompositedLayerMapping final : public GraphicsLayerClient {
   // that layer does not appear earlier in the set of layers for this object.
   bool InvalidateLayerIfNoPrecedingEntry(wtf_size_t);
 
+  void SetContentsNeedDisplay();
+
   PaintLayer& owning_layer_;
 
   // The hierarchy of layers that is maintained by the CompositedLayerMapping
   // looks like this:
   //
   //    + graphics_layer_
-  //      + (scrolling_layer_ + scrolling_contents_layer_) [OPTIONAL]
-  //      | + overflow_controls_host_layer_ [OPTIONAL]
-  //      |   + layer_for_vertical_scrollbar_ [OPTIONAL]
-  //      |   + layer_for_horizontal_scrollbar_ [OPTIONAL]
-  //      |   + layer_for_scroll_corner_ [OPTIONAL]
+  //      + contents layers (or contents layers under scrolling_contents_layer_)
+  //      + overflow_controls_host_layer_ [OPTIONAL]
+  //      | + layer_for_vertical_scrollbar_ [OPTIONAL]
+  //      | + layer_for_horizontal_scrollbar_ [OPTIONAL]
+  //      | + layer_for_scroll_corner_ [OPTIONAL]
   //      + decoration_outline_layer_ [OPTIONAL]
   //      + mask_layer_ [ OPTIONAL ]
+  //      + squashing_layer_ [ OPTIONAL ]
+  //
   // The overflow controls may need to be repositioned in the graphics layer
   // tree by the RLC to ensure that they stack above scrolling content.
+  //
+  // Contents layers are directly under |graphics_layer_|, or under
+  // |scrolling_contents_layer_| when the layer is using composited scrolling.
+  // If owning_layer_ is a stacking context, contents layers include:
+  //   - negative z-index children
+  //   - foreground_layer_
+  //   - normal flow and positive z-index children
+  // If owning_layer_ is not a stacking context, contents layers are normal
+  // flow children.
 
   std::unique_ptr<GraphicsLayer> graphics_layer_;
 
-  // Only used if the layer is using composited scrolling.
-  std::unique_ptr<GraphicsLayer> scrolling_layer_;
-
   // Only used if the layer is using composited scrolling.
   std::unique_ptr<GraphicsLayer> scrolling_contents_layer_;
+  IntSize previous_scroll_container_size_;
 
   // Only used if we have a mask.
   std::unique_ptr<GraphicsLayer> mask_layer_;
@@ -447,20 +448,6 @@ class CORE_EXPORT CompositedLayerMapping final : public GraphicsLayerClient {
   // DecorationLayer which paints outline.
   std::unique_ptr<GraphicsLayer> decoration_outline_layer_;
 
-  // A squashing CLM has the following structure:
-  // squashing_containment_layer_
-  //   + graphics_layer_
-  //   + squashing_layer_
-  //
-  // Stacking children of a squashed layer receive graphics layers that are
-  // parented to the composited ancestor of the squashed layer (i.e. nearest
-  // enclosing composited layer that is not
-  // squashed).
-
-  // Only used if any squashed layers exist, this contains the squashed layers
-  // as siblings to the rest of the GraphicsLayer tree chunk.
-  std::unique_ptr<GraphicsLayer> squashing_containment_layer_;
-
   // Only used if any squashed layers exist, this is the backing that squashed
   // layers paint into.
   std::unique_ptr<GraphicsLayer> squashing_layer_;

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

@@ -1157,7 +1157,7 @@ TEST_F(CompositedLayerMappingTest,
   // contents layer we don't have a scrolling contents layer to paint into in
   // this case.
   const auto* mapping = container->Layer()->GetCompositedLayerMapping();
-  EXPECT_FALSE(mapping->HasScrollingLayer());
+  EXPECT_FALSE(mapping->ScrollingContentsLayer());
   EXPECT_EQ(kBackgroundPaintInGraphicsLayer,
             container->GetBackgroundPaintLocation());
 }
@@ -1490,9 +1490,9 @@ TEST_F(CompositedLayerMappingTest, ScrollingLayerBackgroundColor) {
 
 TEST_F(CompositedLayerMappingTest, ScrollLayerSizingSubpixelAccumulation) {
   // This test verifies that when subpixel accumulation causes snapping it
-  // applies to both the scrolling and scrolling contents layers. Verify that
-  // the mapping doesn't have any vertical scrolling introduced as a result of
-  // the snapping behavior. https://crbug.com/801381.
+  // applies to the scrolling contents layer. Verify that the mapping doesn't
+  // have any vertical scrolling introduced as a result of the snapping
+  // behavior. https://crbug.com/801381.
   GetDocument().GetFrame()->GetSettings()->SetPreferCompositingToLCDTextEnabled(
       true);
 
@@ -1525,10 +1525,9 @@ TEST_F(CompositedLayerMappingTest, ScrollLayerSizingSubpixelAccumulation) {
                       ->Layer()
                       ->GetCompositedLayerMapping();
   ASSERT_TRUE(mapping);
-  ASSERT_TRUE(mapping->ScrollingLayer());
   ASSERT_TRUE(mapping->ScrollingContentsLayer());
-  EXPECT_EQ(mapping->ScrollingLayer()->Size().height(),
-            mapping->ScrollingContentsLayer()->Size().height());
+  EXPECT_EQ(gfx::Size(200, 200), mapping->MainGraphicsLayer()->Size());
+  EXPECT_EQ(gfx::Size(1000, 200), mapping->ScrollingContentsLayer()->Size());
 }
 
 TEST_F(CompositedLayerMappingTest, SquashingScrollInterestRect) {

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

@@ -153,7 +153,7 @@ void GraphicsLayerTreeBuilder::RebuildRecursive(
 
     if (ShouldAppendLayer(layer)) {
       child_layers.push_back(
-          current_composited_layer_mapping->ChildForSuperlayers());
+          current_composited_layer_mapping->MainGraphicsLayer());
     }
   }
 

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

@@ -122,13 +122,13 @@ void GraphicsLayerUpdater::UpdateRecursive(
     CompositedLayerMapping* mapping = layer.GetCompositedLayerMapping();
 
     if (update_type == kForceUpdate || mapping->NeedsGraphicsLayerUpdate()) {
-      bool had_scrolling_layer = mapping->ScrollingLayer();
+      bool had_scrolling_layer = mapping->ScrollingContentsLayer();
       const auto* compositing_container = context.CompositingContainer(layer);
       if (mapping->UpdateGraphicsLayerConfiguration(compositing_container)) {
         needs_rebuild_tree_ = true;
         // Change of existence of scrolling layer affects visual rect offsets of
         // descendants via LayoutObject::ScrollAdjustmentForPaintInvalidation().
-        if (had_scrolling_layer != !!mapping->ScrollingLayer())
+        if (had_scrolling_layer != !!mapping->ScrollingContentsLayer())
           layers_needing_paint_invalidation.push_back(&layer);
       }
       mapping->UpdateGraphicsLayerGeometry(compositing_container,

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

@@ -426,9 +426,8 @@ void PaintLayerCompositor::UpdateIfNeeded(
   // Save off our current parent. We need this in subframes, because our
   // parent attached us to itself via AttachFrameContentLayersToIframeLayer().
   if (!IsMainFrame() && update_root->GetCompositedLayerMapping()) {
-    current_parent = update_root->GetCompositedLayerMapping()
-                         ->ChildForSuperlayers()
-                         ->Parent();
+    current_parent =
+        update_root->GetCompositedLayerMapping()->MainGraphicsLayer()->Parent();
   }
 
 #if DCHECK_IS_ON()
@@ -612,10 +611,8 @@ bool PaintLayerCompositor::AttachFrameContentLayersToIframeLayer(
 }
 
 static void FullyInvalidatePaintRecursive(PaintLayer* layer) {
-  if (layer->GetCompositingState() == kPaintsIntoOwnBacking) {
-    layer->GetCompositedLayerMapping()->SetContentsNeedDisplay();
-    layer->GetCompositedLayerMapping()->SetSquashingContentsNeedDisplay();
-  }
+  if (layer->GetCompositingState() == kPaintsIntoOwnBacking)
+    layer->GetCompositedLayerMapping()->SetAllLayersNeedDisplay();
 
   for (PaintLayer* child = layer->FirstChild(); child;
        child = child->NextSibling())
@@ -637,7 +634,7 @@ PaintLayer* PaintLayerCompositor::RootLayer() const {
 
 GraphicsLayer* PaintLayerCompositor::RootGraphicsLayer() const {
   if (CompositedLayerMapping* clm = RootLayer()->GetCompositedLayerMapping())
-    return clm->ChildForSuperlayers();
+    return clm->MainGraphicsLayer();
   return nullptr;
 }
 

third_party/blink/renderer/core/paint/ng/ng_box_fragment_painter.cc

@@ -899,7 +899,7 @@ void NGBoxFragmentPainter::PaintBoxDecorationBackground(
     // composited in PaintArtifactCompositor::UpdateNonFastScrollableRegions.
     const auto* layer = PhysicalFragment().Layer();
     if (layer && layer->GetCompositedLayerMapping() &&
-        layer->GetCompositedLayerMapping()->HasScrollingLayer()) {
+        layer->GetCompositedLayerMapping()->ScrollingContentsLayer()) {
       needs_scroll_hit_test = false;
     }
   }

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

@@ -88,7 +88,7 @@ void ViewPainter::PaintBoxDecorationBackground(const PaintInfo& paint_info) {
         layout_view_.Layer()->GetCompositedLayerMapping() &&
         layout_view_.Layer()
             ->GetCompositedLayerMapping()
-            ->HasScrollingLayer()) {
+            ->ScrollingContentsLayer()) {
       paints_scroll_hit_test = false;
     }
   }

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

@@ -121,16 +121,8 @@ constexpr CompositingReasonStringMap kCompositingReasonsStringMap[] = {
      "Secondary layer, the scroll corner layer"},
     {CompositingReason::kLayerForScrollingContents, "layerForScrollingContents",
      "Secondary layer, to house contents that can be scrolled"},
-    {CompositingReason::kLayerForScrollingContainer,
-     "layerForScrollingContainer",
-     "Secondary layer, used to position the scrolling contents while "
-     "scrolling"},
     {CompositingReason::kLayerForSquashingContents, "layerForSquashingContents",
      "Secondary layer, home for a group of squashable content"},
-    {CompositingReason::kLayerForSquashingContainer,
-     "layerForSquashingContainer",
-     "Secondary layer, no-op layer to place the squashing layer correctly in "
-     "the composited layer tree"},
     {CompositingReason::kLayerForForeground, "layerForForeground",
      "Secondary layer, to contain any normal flow and positive z-index "
      "contents on top of a negative z-index layer"},

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

@@ -74,9 +74,7 @@ using CompositingReasons = uint64_t;
   V(LayerForOverflowControlsHost)                                             \
   V(LayerForScrollCorner)                                                     \
   V(LayerForScrollingContents)                                                \
-  V(LayerForScrollingContainer)                                               \
   V(LayerForSquashingContents)                                                \
-  V(LayerForSquashingContainer)                                               \
   V(LayerForForeground)                                                       \
   V(LayerForMask)                                                             \
   /* Composited layer painted on top of all other layers as decoration. */    \