Support Style Sharing for Shadow DOM V1

third_party/WebKit/Source/core/css/SelectorChecker.cpp

Index: third_party/WebKit/Source/core/css/SelectorChecker.cpp
diff --git a/third_party/WebKit/Source/core/css/SelectorChecker.cpp b/third_party/WebKit/Source/core/css/SelectorChecker.cpp
index bb729dca535bbf0d9c7c63f0bfcffcfaedc17c3a..b8c76c9a1b81511e35f06dd45457ac3abc1e5edd 100644
--- a/third_party/WebKit/Source/core/css/SelectorChecker.cpp
+++ b/third_party/WebKit/Source/core/css/SelectorChecker.cpp
@@ -114,16 +114,17 @@ static Element* parentElement(
   return context.element->parentElement();
 }
 
+// If context has scope, return slot that matches the scope, otherwise return
+// the assigned slot for scope-less matching of ::slotted pseudo element.
 static const HTMLSlotElement* findSlotElementInScope(
     const SelectorChecker::SelectorCheckingContext& context) {
   if (!context.scope)
-    return nullptr;
+    return context.element->assignedSlot();

[rune, 2017/01/11 11:34:27]

Not sure I understand this one. This is fixing a known bug? Test?

[kochi, 2017/01/13 10:46:45]

ElemenetRuleCollector::hasAnyMatchingRules() does scope-less
checking for rules, and SharedStyleFinder uses it for
m_siblingRuleSet and m_uncommonAttributeRuleSet.

A "::slotted([attr])" style rule is put to m_uncommonAttributeRuleSet
but fails to match because of this scope checking.
Therefore such element that should match the rule fails to match,
and falsely shares style with some sibling if exists.

 
-  const HTMLSlotElement* slot = context.element->assignedSlot();
-  while (slot) {
+  for (const HTMLSlotElement* slot = context.element->assignedSlot(); slot;
+       slot = slot->assignedSlot()) {
     if (slot->treeScope() == context.scope->treeScope())
       return slot;
-    slot = slot->assignedSlot();
   }
   return nullptr;
 }