SelectorChecker should update scope and behaviorAtBoundary for /content/ in SharingRules mode

Source/core/css/SelectorChecker.cpp

Index: Source/core/css/SelectorChecker.cpp
diff --git a/Source/core/css/SelectorChecker.cpp b/Source/core/css/SelectorChecker.cpp
index 36d9f5f8c2ddee0301070505f5bd4d84b48fe9e4..48febf898bf217051ffcd11ad298892fcd5af2e7 100644
--- a/Source/core/css/SelectorChecker.cpp
+++ b/Source/core/css/SelectorChecker.cpp
@@ -356,7 +356,10 @@ SelectorChecker::Match SelectorChecker::matchForShadowDistributed(const Element*
 
         // If a given scope is a shadow host of an insertion point but behaviorAtBoundary doesn't have ScopeIsShadowHost,
         // we need to update behaviorAtBoundary to make selectors like ":host > ::content" work correctly.
-        if (scope == insertionPoints[i]->containingShadowRoot()->shadowHost() && !(behaviorAtBoundary & ScopeIsShadowHost))
+        if (m_mode == SharingRules) {
+            nextContext.behaviorAtBoundary = static_cast<BehaviorAtBoundary>(behaviorAtBoundary | ScopeIsShadowHost);
+            nextContext.scope = insertionPoints[i]->containingShadowRoot()->shadowHost();
+        } else if (scope == insertionPoints[i]->containingShadowRoot()->shadowHost() && !(behaviorAtBoundary & ScopeIsShadowHost))
             nextContext.behaviorAtBoundary = static_cast<BehaviorAtBoundary>(behaviorAtBoundary | ScopeIsShadowHost);
         else
             nextContext.behaviorAtBoundary = behaviorAtBoundary;
@@ -366,7 +369,7 @@ SelectorChecker::Match SelectorChecker::matchForShadowDistributed(const Element*
         if (match(nextContext, siblingTraversalStrategy, result) == SelectorMatches)
             return SelectorMatches;
     }
-    return SelectorFailsCompletely;
+    return SelectorFailsLocally;
 }
 
 static inline bool containsHTMLSpace(const AtomicString& string)
@@ -521,6 +524,10 @@ bool SelectorChecker::checkOne(const SelectorCheckingContext& context, const Sib
                 // We select between :visited and :link when applying. We don't know which one applied (or not) yet.
                 if (subContext.selector->pseudoType() == CSSSelector::PseudoVisited || (subContext.selector->pseudoType() == CSSSelector::PseudoLink && subContext.visitedMatchType == VisitedMatchEnabled))
                     return true;
+                // context.scope is not available if m_mode == SharingRules.
+                // We cannot determine whether :host or :scope matches a given element or not.
+                if (m_mode == SharingRules && (subContext.selector->isHostPseudoClass() || subContext.selector->pseudoType() == CSSSelector::PseudoScope))
+                    return true;
                 if (!checkOne(subContext, DOMSiblingTraversalStrategy()))
                     return true;
             }
@@ -860,6 +867,8 @@ bool SelectorChecker::checkOne(const SelectorCheckingContext& context, const Sib
 
         case CSSSelector::PseudoScope:
             {
+                if (m_mode == SharingRules)
+                    return true;
                 const Node* contextualReferenceNode = !context.scope ? element.document().documentElement() : context.scope;
                 if (element == contextualReferenceNode)
                     return true;
@@ -874,6 +883,8 @@ bool SelectorChecker::checkOne(const SelectorCheckingContext& context, const Sib
         case CSSSelector::PseudoHost:
         case CSSSelector::PseudoAncestor:
             {
+                if (m_mode == SharingRules)
+                    return true;
                 // :host only matches a shadow host when :host is in a shadow tree of the shadow host.
                 if (!context.scope)
                     return false;