Styling slot fallback content with ::slotted()

third_party/WebKit/Source/core/html/HTMLSlotElement.cpp

Index: third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp b/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
index bf89d354ebb0401e3a1e7986bb1eb47a4c570fc8..6e6938883b290df59194b3a37710dd8167f0fb01 100644
--- a/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
@@ -51,6 +51,10 @@ inline HTMLSlotElement::HTMLSlotElement(Document& document)
     : HTMLElement(slotTag, document) {
   UseCounter::Count(document, UseCounter::kHTMLSlotElement);
   SetHasCustomStyleCallbacks();
+  // TODO(kochi): This is required for slot fallback contents to be matched
+  // against ::slotted() pseudo element in a document tree. Remove this once
+  // Shadow DOM V0 code is removed.
+  document.SetShadowCascadeOrder(ShadowCascadeOrder::kShadowCascadeV1);
 }
 
 DEFINE_NODE_FACTORY(HTMLSlotElement);
@@ -60,6 +64,35 @@ AtomicString HTMLSlotElement::NormalizeSlotName(const AtomicString& name) {
   return (name.IsNull() || name.IsEmpty()) ? g_empty_atom : name;
 }
 
+// static
+HTMLSlotElement* HTMLSlotElement::FindFallbackSlotElementFor(
+    const Element& element) {
+  // Check if |element| is a fallback content.
+  Element* parent = element.parentElement();
+  if (!parent || !isHTMLSlotElement(parent) ||
+      !toHTMLSlotElement(parent)->AssignedNodes().IsEmpty())
+    return nullptr;
+
+  // At this point, |parent| is an immediate parent of element, which is
+  // a <slot> with empty assigned nodes. This slot is one of the following:
+  // (1) doesn't have any slot in ancestor.
+  // (2) have a slot in ancestor which is assigned to another slot.
+  // (3) have a slot in ancestor which has assigned nodes.
+  // In cases (1) and (2), this slot is a fallback content.
+  // In case (3), this slot is not a fallback content.
+  HTMLSlotElement* slot = toHTMLSlotElement(parent);
+  while ((parent = parent->parentElement())) {
+    if (!isHTMLSlotElement(parent))
+      continue;
+    if (!toHTMLSlotElement(parent)->AssignedNodes().IsEmpty())
+      return nullptr;
+    slot = toHTMLSlotElement(parent);
+    if (slot->AssignedSlot())
+      break;
+  }
+  return slot;
+}
+
 const HeapVector<Member<Node>>& HTMLSlotElement::AssignedNodes() {
   DCHECK(!NeedsDistributionRecalc());
   DCHECK(IsInShadowTree() || assigned_nodes_.IsEmpty());