Incrementally build main thread scrolling reasons [spv2]

third_party/WebKit/Source/core/frame/FrameView.cpp

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index d7af7ba56a5426ac3ae365a376c9ffdab5dac2b4..1a40cfdc81db760a50a60d1fbe68916fc563ab7e 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -1479,8 +1479,12 @@ void FrameView::addBackgroundAttachmentFixedObject(LayoutObject* object) {
     scrollingCoordinator->frameViewHasBackgroundAttachmentFixedObjectsDidChange(
         this);
 
-  // TODO(pdr): When slimming paint v2 is enabled, invalidate the scroll paint
-  // property subtree for this so main thread scroll reasons are recomputed.
+  // Ensure main thread scrolling reasons are recomputed.
+  if (RuntimeEnabledFeatures::slimmingPaintV2Enabled()) {
+    setNeedsPaintPropertyUpdate();
+    // The object's scroll properties are not affected by its own background.
+    object->setAllAncestorsNeedPaintPropertyUpdate();
+  }
 }
 
 void FrameView::removeBackgroundAttachmentFixedObject(LayoutObject* object) {
@@ -1491,8 +1495,12 @@ void FrameView::removeBackgroundAttachmentFixedObject(LayoutObject* object) {
     scrollingCoordinator->frameViewHasBackgroundAttachmentFixedObjectsDidChange(
         this);
 
-  // TODO(pdr): When slimming paint v2 is enabled, invalidate the scroll paint
-  // property subtree for this so main thread scroll reasons are recomputed.
+  // Ensure main thread scrolling reasons are recomputed.
+  if (RuntimeEnabledFeatures::slimmingPaintV2Enabled()) {
+    setNeedsPaintPropertyUpdate();
+    // The object's scroll properties are not affected by its own background.
+    object->setAllAncestorsNeedPaintPropertyUpdate();
+  }
 }
 
 void FrameView::addViewportConstrainedObject(LayoutObject* object) {
@@ -1598,6 +1606,17 @@ void FrameView::invalidateBackgroundAttachmentFixedObjects() {
     layoutObject->setShouldDoFullPaintInvalidation();
 }
 
+bool FrameView::hasBackgroundAttachmentFixedDescendants(
+    const LayoutObject& object) const {
+  for (const auto* potentialDescendant : m_backgroundAttachmentFixedObjects) {
+    if (potentialDescendant == &object)
+      continue;
+    if (potentialDescendant->isDescendantOf(&object))
+      return true;
+  }
+  return false;
+}
+
 bool FrameView::invalidateViewportConstrainedObjects() {
   bool fastPathAllowed = true;
   for (const auto& viewportConstrainedObject : *m_viewportConstrainedObjects) {