Stop matching scrollbar pseudo element without a scrollbar.

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc.
  * All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
@@ skipping 242 matching lines @@
 SelectorChecker::MatchStatus SelectorChecker::MatchForSubSelector(
     const SelectorCheckingContext& context,
     MatchResult& result) const {
   SelectorCheckingContext next_context = PrepareNextContextForRelation(context);
 
   PseudoId dynamic_pseudo = result.dynamic_pseudo;
   next_context.has_scrollbar_pseudo =
       dynamic_pseudo != kPseudoIdNone &&
       (scrollbar_ || dynamic_pseudo == kPseudoIdScrollbarCorner ||
        dynamic_pseudo == kPseudoIdResizer);
+
+  // Only match pseudo classes following scrollbar pseudo elements while
+  // actually computing style for scrollbar pseudo elements. This is to
+  // avoid incorrectly setting affected-by flags on actual elements for
+  // cases like: div::-webkit-scrollbar-thumb:hover { }
+  if (context.in_rightmost_compound && dynamic_pseudo != kPseudoIdNone &&
+      dynamic_pseudo != kPseudoIdSelection &&
+      !next_context.has_scrollbar_pseudo) {
+    return kSelectorFailsCompletely;
+  }
+
   next_context.has_selection_pseudo = dynamic_pseudo == kPseudoIdSelection;
   next_context.is_sub_selector = true;
   return MatchSelector(next_context, result);
 }
 
 static inline bool IsV0ShadowRoot(const Node* node) {
   return node && node->IsShadowRoot() &&
          ToShadowRoot(node)->GetType() == ShadowRootType::V0;
 }
 
@@ skipping 1088 matching lines @@
 bool SelectorChecker::MatchesFocusPseudoClass(const Element& element) {
   bool force_pseudo_state = false;
   probe::forcePseudoState(const_cast<Element*>(&element),
                           CSSSelector::kPseudoFocus, &force_pseudo_state);
   if (force_pseudo_state)
     return true;
   return element.IsFocused() && IsFrameFocused(element);
 }
 
 }  // namespace blink