Return ActiveSheetsChanged when rulesets change in common prefix.

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/css/ActiveStyleSheets.h"
 
 #include "core/css/CSSStyleSheet.h"
 #include "core/css/MediaQueryEvaluator.h"
 #include "core/css/StyleSheetContents.h"
 #include "core/css/StyleSheetList.h"
@@ skipping 310 matching lines @@
   oldSheets.push_back(std::make_pair(sheet2, &sheet2->contents()->ruleSet()));
 
   newSheets.push_back(std::make_pair(sheet2, &sheet2->contents()->ruleSet()));
   newSheets.push_back(std::make_pair(sheet1, &sheet1->contents()->ruleSet()));
 
   EXPECT_EQ(NoActiveSheetsChanged,
             compareActiveStyleSheets(oldSheets, newSheets, changedRuleSets));
   EXPECT_EQ(0u, changedRuleSets.size());
 }
 
+TEST_F(ActiveStyleSheetsTest, CompareActiveStyleSheets_DisableAndAppend) {
+  ActiveStyleSheetVector oldSheets;
+  ActiveStyleSheetVector newSheets;
+  HeapHashSet<Member<RuleSet>> changedRuleSets;
+
+  CSSStyleSheet* sheet1 = createSheet();
+  CSSStyleSheet* sheet2 = createSheet();
+
+  oldSheets.push_back(std::make_pair(sheet1, &sheet1->contents()->ruleSet()));
+  newSheets.push_back(std::make_pair(sheet1, nullptr));
+  newSheets.push_back(std::make_pair(sheet2, &sheet2->contents()->ruleSet()));
+
+  EXPECT_EQ(ActiveSheetsChanged,
+            compareActiveStyleSheets(oldSheets, newSheets, changedRuleSets));
+  EXPECT_EQ(2u, changedRuleSets.size());
+}
+
 TEST_F(ApplyRulesetsTest, AddUniversalRuleToDocument) {
   document().view()->updateAllLifecyclePhases();
 
   CSSStyleSheet* sheet = createSheet("body * { color:red }");
 
   ActiveStyleSheetVector newStyleSheets;
   newStyleSheets.push_back(
       std::make_pair(sheet, &sheet->contents()->ruleSet()));
 
   styleEngine().applyRuleSetChanges(document(), ActiveStyleSheetVector(),
@@ skipping 119 matching lines @@
   ActiveStyleSheetVector oldStyleSheets;
   oldStyleSheets.push_back(
       std::make_pair(cssSheet, &cssSheet->contents()->ruleSet()));
   styleEngine().applyRuleSetChanges(shadowRoot, oldStyleSheets,
                                     ActiveStyleSheetVector());
 
   EXPECT_TRUE(styleEngine().treeBoundaryCrossingScopes().isEmpty());
 }
 
 }  // namespace blink