Reset squashing state when encountering composited iframe contents

Previously, <iframe> elements were always composited, and
CompositingLayerAssigner specifically disallows squashing them. Due
to a recent change (see bug), <iframe> elements are not always
composited; but we cannot squash layers across an iframe with
composited contents.

With this change, an iframe with composited contents will end the
current squashing context in the embedding document.

BUG=1095935

Change-Id: Ie60becde8b99d204395a2c8f35bc1ce1076334a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2309141
Commit-Queue: Stefan Zager <szager@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790514}

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

@@ -28,6 +28,7 @@
 
 #include "third_party/blink/renderer/core/animation/scroll_timeline.h"
 #include "third_party/blink/renderer/core/animation/worklet_animation_controller.h"
+#include "third_party/blink/renderer/core/layout/layout_embedded_content.h"
 #include "third_party/blink/renderer/core/layout/layout_video.h"
 #include "third_party/blink/renderer/core/layout/layout_view.h"
 #include "third_party/blink/renderer/core/page/page.h"
@@ -386,6 +387,16 @@ void CompositingLayerAssigner::AssignLayersToBackingsInternal(
           true;
     }
   }
+
+  // If this is an iframe whose content document is composited, then we can't
+  // squash layers painted after the iframe with layers painted before it.
+  if (layer->GetLayoutObject().IsLayoutEmbeddedContent() &&
+      ToLayoutEmbeddedContent(layer->GetLayoutObject())
+          .ContentDocumentIsCompositing()) {
+    squashing_state.have_assigned_backings_to_entire_squashing_layer_subtree =
+        false;
+  }
+
   layer->ClearNeedsCompositingLayerAssignment();
 }
 

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

@@ -11,6 +11,10 @@
 namespace blink {
 
 class CompositedLayerAssignerTest : public RenderingTest {
+ public:
+  CompositedLayerAssignerTest()
+      : RenderingTest(MakeGarbageCollected<SingleChildLocalFrameClient>()) {}
+
  private:
   void SetUp() override {
     EnableCompositing();
@@ -97,4 +101,23 @@ TEST_F(CompositedLayerAssignerTest,
   EXPECT_EQ(kPaintsIntoOwnBacking, squashed->GetCompositingState());
 }
 
+TEST_F(CompositedLayerAssignerTest,
+       SquashingAcrossCompositedInnerDocumentDisallowed) {
+  SetBodyInnerHTML(R"HTML(
+    <div id="bottom" style="position: absolute; will-change: transform">Bottom</div>
+    <div id="middle" style="position: absolute">
+      <iframe style="border: 10px solid magenta"></iframe>
+    </div>
+    <div id="top" style="position: absolute; width: 200px; height: 200px; background: green;">Top</div>
+  )HTML");
+  SetChildFrameHTML(R"HTML(
+    <style>body {will-change: transform; background: blue}</style>
+  )HTML");
+  LocalFrameView* frame_view = GetDocument().View();
+  frame_view->UpdateAllLifecyclePhases(DocumentUpdateReason::kTest);
+  PaintLayer* top =
+      ToLayoutBoxModelObject(GetLayoutObjectByElementId("top"))->Layer();
+  EXPECT_EQ(kPaintsIntoOwnBacking, top->GetCompositingState());
+}
+
 }  // namespace blink

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

@@ -603,8 +603,14 @@ void CompositingRequirementsUpdater::UpdateRecursive(
   // Layer assignment is needed for allocating or removing composited
   // layers related to this PaintLayer; hence the below conditions.
   if (reasons_to_composite || layer->GetCompositingState() != kNotComposited ||
-      layer->LostGroupedMapping())
+      layer->LostGroupedMapping()) {
     layer->SetNeedsCompositingLayerAssignment();
+  } else if (contains_composited_layer) {
+    // If this is an iframe whose content document is composited, then we need
+    // CompositedLayerAssigner to process this layer, to ensure that we don't
+    // squash layers painted before the iframe with layers painted after it.
+    layer->PropagateDescendantNeedsCompositingLayerAssignment();
+  }
 
   // At this point we have finished collecting all reasons to composite this
   // layer.

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

@@ -2360,7 +2360,10 @@ bool PaintLayer::IsReplacedNormalFlowStacking() const {
 
 void PaintLayer::SetNeedsCompositingLayerAssignment() {
   needs_compositing_layer_assignment_ = true;
+  PropagateDescendantNeedsCompositingLayerAssignment();
+}
 
+void PaintLayer::PropagateDescendantNeedsCompositingLayerAssignment() {
   for (PaintLayer* curr = CompositingContainer();
        curr && !curr->StackingDescendantNeedsCompositingLayerAssignment();
        curr = curr->CompositingContainer()) {

third_party/blink/renderer/core/paint/paint_layer.h

@@ -1106,6 +1106,7 @@ class CORE_EXPORT PaintLayer : public DisplayItemClient {
 
   void SetNeedsCompositingLayerAssignment();
   void ClearNeedsCompositingLayerAssignment();
+  void PropagateDescendantNeedsCompositingLayerAssignment();
 
   bool NeedsCompositingLayerAssignment() const {
     return needs_compositing_layer_assignment_;