Bug 1425769 - Base class for ShadowRoot and Document to manage style state

Tag #1375

16 files changed, 250 insertions, 296 deletions

diff --git a/chrome/nsChromeRegistry.cpp b/chrome/nsChromeRegistry.cpp
index 4bd8b4dca5..7b72d29c3a 100644
--- a/chrome/nsChromeRegistry.cpp
+++ b/chrome/nsChromeRegistry.cpp
@@ -436,23 +436,22 @@ nsresult nsChromeRegistry::RefreshWindow(nsPIDOMWindowOuter* aWindow)
     NS_ENSURE_SUCCESS(rv, rv);
   }
 
-  int32_t count = document->GetNumberOfStyleSheets();
+  size_t count = document->SheetCount();
 
   // Build an array of style sheets we need to reload.
   nsTArray<RefPtr<StyleSheet>> oldSheets(count);
   nsTArray<RefPtr<StyleSheet>> newSheets(count);
 
   // Iterate over the style sheets.
-  for (int32_t i = 0; i < count; i++) {
+  for (size_t i = 0; i < count; i++) {
     // Get the style sheet
-    StyleSheet* styleSheet = document->GetStyleSheetAt(i);
-    oldSheets.AppendElement(styleSheet);
+    oldSheets.AppendElement(document->SheetAt(i));
   }
 
   // Iterate over our old sheets and kick off a sync load of the new
   // sheet if and only if it's a non-inline sheet with a chrome URL.
   for (StyleSheet* sheet : oldSheets) {
-    MOZ_ASSERT(sheet, "GetStyleSheetAt shouldn't return nullptr for "
+    MOZ_ASSERT(sheet, "SheetAt shouldn't return nullptr for "
                       "in-range sheet indexes");
     nsIURI* uri = sheet->GetSheetURI();
 
diff --git a/dom/base/ShadowRoot.cpp b/dom/base/ShadowRoot.cpp
index f241dfcd55..c4e56f3fbe 100644
--- a/dom/base/ShadowRoot.cpp
+++ b/dom/base/ShadowRoot.cpp
@@ -27,7 +27,7 @@ NS_IMPL_CYCLE_COLLECTION_CLASS(ShadowRoot)
 
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(ShadowRoot,
                                                   DocumentFragment)
-  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mStyleSheetList)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mDOMStyleSheets)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mAssociatedBinding)
   for (auto iter = tmp->mIdentifierMap.ConstIter(); !iter.Done();
        iter.Next()) {
@@ -39,7 +39,7 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(ShadowRoot)
   if (tmp->GetHost()) {
     tmp->GetHost()->RemoveMutationObserver(tmp);
   }
-  NS_IMPL_CYCLE_COLLECTION_UNLINK(mStyleSheetList)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mDOMStyleSheets)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mAssociatedBinding)
   tmp->mIdentifierMap.Clear();
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END_INHERITED(DocumentFragment)
@@ -238,16 +238,30 @@ ShadowRoot::InsertSheet(StyleSheet* aSheet,
 
   linkingElement->SetStyleSheet(aSheet); // This sets the ownerNode on the sheet
 
+  MOZ_DIAGNOSTIC_ASSERT(mProtoBinding->SheetCount() == StyleScope::SheetCount());
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+  // FIXME(emilio, bug 1425759): For now we keep them duplicated, the proto
+  // binding will disappear soon (tm).
+  {
+    size_t i = 0;
+    for (RefPtr<StyleSheet>& sheet : mStyleSheets) {
+      MOZ_DIAGNOSTIC_ASSERT(sheet.get() == mProtoBinding->StyleSheetAt(i++));
+    }
+  }
+#endif
+
   // Find the correct position to insert into the style sheet list (must
   // be in tree order).
-  for (size_t i = 0; i <= mProtoBinding->SheetCount(); i++) {
-    if (i == mProtoBinding->SheetCount()) {
+  for (size_t i = 0; i <= SheetCount(); i++) {
+    if (i == SheetCount()) {
+      AppendStyleSheet(*aSheet);
       mProtoBinding->AppendStyleSheet(aSheet);
       break;
     }
 
-    nsINode* sheetOwningNode = mProtoBinding->StyleSheetAt(i)->GetOwnerNode();
+    nsINode* sheetOwningNode = SheetAt(i)->GetOwnerNode();
     if (nsContentUtils::PositionIsBefore(aLinkingContent, sheetOwningNode)) {
+      InsertSheetAt(i, *aSheet);
       mProtoBinding->InsertStyleSheetAt(i, aSheet);
       break;
     }
@@ -262,6 +276,7 @@ void
 ShadowRoot::RemoveSheet(StyleSheet* aSheet)
 {
   mProtoBinding->RemoveStyleSheet(aSheet);
+  StyleScope::RemoveSheet(*aSheet);
 
   if (aSheet->IsApplicable()) {
     StyleSheetChanged();
@@ -523,16 +538,6 @@ ShadowRoot::SetApplyAuthorStyles(bool aApplyAuthorStyles)
   }
 }
 
-StyleSheetList*
-ShadowRoot::StyleSheets()
-{
-  if (!mStyleSheetList) {
-    mStyleSheetList = new ShadowRootStyleSheetList(this);
-  }
-
-  return mStyleSheetList;
-}
-
 void
 ShadowRoot::AttributeChanged(nsIDocument* aDocument,
                              Element* aElement,
@@ -651,40 +656,3 @@ ShadowRoot::Clone(mozilla::dom::NodeInfo *aNodeInfo, nsINode **aResult) const
   *aResult = nullptr;
   return NS_ERROR_DOM_DATA_CLONE_ERR;
 }
-
-NS_IMPL_CYCLE_COLLECTION_INHERITED(ShadowRootStyleSheetList, StyleSheetList,
-                                   mShadowRoot)
-
-NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION_INHERITED(ShadowRootStyleSheetList)
-NS_INTERFACE_MAP_END_INHERITING(StyleSheetList)
-
-NS_IMPL_ADDREF_INHERITED(ShadowRootStyleSheetList, StyleSheetList)
-NS_IMPL_RELEASE_INHERITED(ShadowRootStyleSheetList, StyleSheetList)
-
-ShadowRootStyleSheetList::ShadowRootStyleSheetList(ShadowRoot* aShadowRoot)
-  : mShadowRoot(aShadowRoot)
-{
-  MOZ_COUNT_CTOR(ShadowRootStyleSheetList);
-}
-
-ShadowRootStyleSheetList::~ShadowRootStyleSheetList()
-{
-  MOZ_COUNT_DTOR(ShadowRootStyleSheetList);
-}
-
-StyleSheet*
-ShadowRootStyleSheetList::IndexedGetter(uint32_t aIndex, bool& aFound)
-{
-  aFound = aIndex < mShadowRoot->mProtoBinding->SheetCount();
-  if (!aFound) {
-    return nullptr;
-  }
-  return mShadowRoot->mProtoBinding->StyleSheetAt(aIndex);
-}
-
-uint32_t
-ShadowRootStyleSheetList::Length()
-{
-  return mShadowRoot->mProtoBinding->SheetCount();
-}
-
diff --git a/dom/base/ShadowRoot.h b/dom/base/ShadowRoot.h
index c525ba8e8f..a24c8138e0 100644
--- a/dom/base/ShadowRoot.h
+++ b/dom/base/ShadowRoot.h
@@ -8,8 +8,7 @@
 #define mozilla_dom_shadowroot_h__
 
 #include "mozilla/dom/DocumentFragment.h"
-#include "mozilla/dom/StyleSheetList.h"
-#include "mozilla/StyleSheet.h"
+#include "mozilla/dom/StyleScope.h"
 #include "nsCOMPtr.h"
 #include "nsCycleCollectionParticipant.h"
 #include "nsIContentInlines.h"
@@ -27,12 +26,11 @@ class EventChainPreVisitor;
 namespace dom {
 
 class Element;
-class ShadowRootStyleSheetList;
 
 class ShadowRoot final : public DocumentFragment,
+                         public StyleScope,
                          public nsStubMutationObserver
 {
-  friend class ShadowRootStyleSheetList;
 public:
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(ShadowRoot,
                                            DocumentFragment)
@@ -49,15 +47,20 @@ public:
 
   // Shadow DOM v1
   Element* Host();
-  ShadowRootMode Mode()
+  ShadowRootMode Mode() const
   {
     return mMode;
   }
-  bool IsClosed()
+  bool IsClosed() const
   {
     return mMode == ShadowRootMode::Closed;
   }
 
+  // StyleScope.
+  nsINode& AsNode() final
+  {
+    return *this;
+  }
 
   // [deprecated] Shadow DOM v0
   void AddToIdTable(Element* aElement, nsIAtom* aId);
@@ -66,7 +69,10 @@ public:
   void RemoveSheet(StyleSheet* aSheet);
   bool ApplyAuthorStyles();
   void SetApplyAuthorStyles(bool aApplyAuthorStyles);
-  StyleSheetList* StyleSheets();
+  StyleSheetList* StyleSheets()
+  {
+    return &StyleScope::EnsureDOMStyleSheets();
+  }
 
   /**
    * Distributes all the explicit children of the pool host to the content
@@ -155,8 +161,6 @@ protected:
   // owns |mProtoBinding|.
   RefPtr<nsXBLBinding> mAssociatedBinding;
 
-  RefPtr<ShadowRootStyleSheetList> mStyleSheetList;
-
   // A boolean that indicates that an insertion point was added or removed
   // from this ShadowRoot and that the nodes need to be redistributed into
   // the insertion points. After this flag is set, nodes will be distributed
@@ -172,28 +176,6 @@ protected:
   nsresult Clone(mozilla::dom::NodeInfo *aNodeInfo, nsINode **aResult) const override;
 };
 
-class ShadowRootStyleSheetList : public StyleSheetList
-{
-public:
-  explicit ShadowRootStyleSheetList(ShadowRoot* aShadowRoot);
-
-  NS_DECL_ISUPPORTS_INHERITED
-  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(ShadowRootStyleSheetList, StyleSheetList)
-
-  virtual nsINode* GetParentObject() const override
-  {
-    return mShadowRoot;
-  }
-
-  uint32_t Length() override;
-  StyleSheet* IndexedGetter(uint32_t aIndex, bool& aFound) override;
-
-protected:
-  virtual ~ShadowRootStyleSheetList();
-
-  RefPtr<ShadowRoot> mShadowRoot;
-};
-
 } // namespace dom
 } // namespace mozilla
 
diff --git a/dom/base/StyleScope.cpp b/dom/base/StyleScope.cpp
new file mode 100644
index 0000000000..6c708a8ba1
--- /dev/null
+++ b/dom/base/StyleScope.cpp
@@ -0,0 +1,27 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "StyleScope.h"
+#include "mozilla/dom/StyleSheetList.h"
+
+namespace mozilla {
+namespace dom {
+
+StyleScope::~StyleScope()
+{
+}
+
+StyleSheetList&
+StyleScope::EnsureDOMStyleSheets()
+{
+  if (!mDOMStyleSheets) {
+    mDOMStyleSheets = new StyleSheetList(*this);
+  }
+  return *mDOMStyleSheets;
+}
+
+}
+}
diff --git a/dom/base/StyleScope.h b/dom/base/StyleScope.h
new file mode 100644
index 0000000000..c8957b069b
--- /dev/null
+++ b/dom/base/StyleScope.h
@@ -0,0 +1,81 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_StyleScope_h__
+#define mozilla_dom_StyleScope_h__
+
+#include "nsTArray.h"
+
+class nsINode;
+
+namespace mozilla {
+class StyleSheet;
+
+namespace dom {
+
+class StyleSheetList;
+
+/**
+ * A class meant to be shared by ShadowRoot and Document, that holds a list of
+ * stylesheets.
+ *
+ * TODO(emilio, bug 1418159): In the future this should hold most of the
+ * relevant style state, this should allow us to fix bug 548397.
+ */
+class StyleScope
+{
+public:
+  virtual nsINode& AsNode() = 0;
+
+  const nsINode& AsNode() const
+  {
+    return const_cast<StyleScope&>(*this).AsNode();
+  }
+
+  StyleSheet* SheetAt(size_t aIndex) const
+  {
+    return mStyleSheets.SafeElementAt(aIndex);
+  }
+
+  size_t SheetCount() const
+  {
+    return mStyleSheets.Length();
+  }
+
+  int32_t IndexOfSheet(const StyleSheet& aSheet) const
+  {
+    return mStyleSheets.IndexOf(&aSheet);
+  }
+
+  void InsertSheetAt(size_t aIndex, StyleSheet& aSheet)
+  {
+    mStyleSheets.InsertElementAt(aIndex, &aSheet);
+  }
+
+  void RemoveSheet(StyleSheet& aSheet)
+  {
+    mStyleSheets.RemoveElement(&aSheet);
+  }
+
+  void AppendStyleSheet(StyleSheet& aSheet)
+  {
+    mStyleSheets.AppendElement(&aSheet);
+  }
+
+  StyleSheetList& EnsureDOMStyleSheets();
+
+  ~StyleScope();
+
+protected:
+  nsTArray<RefPtr<mozilla::StyleSheet>> mStyleSheets;
+  RefPtr<mozilla::dom::StyleSheetList> mDOMStyleSheets;
+};
+
+}
+
+}
+
+#endif
diff --git a/dom/base/StyleSheetList.cpp b/dom/base/StyleSheetList.cpp
index 7274b5ea5d..42db3179dd 100644
--- a/dom/base/StyleSheetList.cpp
+++ b/dom/base/StyleSheetList.cpp
@@ -8,6 +8,7 @@
 
 #include "mozilla/CSSStyleSheet.h"
 #include "mozilla/dom/StyleSheetListBinding.h"
+#include "nsStubDocumentObserver.h"
 
 namespace mozilla {
 namespace dom {
@@ -17,7 +18,8 @@ NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_0(StyleSheetList)
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(StyleSheetList)
   NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
   NS_INTERFACE_MAP_ENTRY(nsIDOMStyleSheetList)
-  NS_INTERFACE_MAP_ENTRY(nsISupports)
+  NS_INTERFACE_MAP_ENTRY(nsIMutationObserver)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIDOMStyleSheetList)
 NS_INTERFACE_MAP_END
 
 NS_IMPL_CYCLE_COLLECTING_ADDREF(StyleSheetList)
@@ -43,5 +45,24 @@ StyleSheetList::SlowItem(uint32_t aIndex, nsIDOMStyleSheet** aItem)
   return NS_OK;
 }
 
+void
+StyleSheetList::NodeWillBeDestroyed(const nsINode* aNode)
+{
+  mStyleScope = nullptr;
+}
+
+StyleSheetList::StyleSheetList(StyleScope& aScope)
+  : mStyleScope(&aScope)
+{
+  mStyleScope->AsNode().AddMutationObserver(this);
+}
+
+StyleSheetList::~StyleSheetList()
+{
+  if (mStyleScope) {
+    mStyleScope->AsNode().RemoveMutationObserver(this);
+  }
+}
+
 } // namespace dom
 } // namespace mozilla
diff --git a/dom/base/StyleSheetList.h b/dom/base/StyleSheetList.h
index dfedc22145..ea5c33a983 100644
--- a/dom/base/StyleSheetList.h
+++ b/dom/base/StyleSheetList.h
@@ -7,8 +7,10 @@
 #ifndef mozilla_dom_StyleSheetList_h
 #define mozilla_dom_StyleSheetList_h
 
+#include "mozilla/dom/StyleScope.h"
 #include "nsIDOMStyleSheetList.h"
 #include "nsWrapperCache.h"
+#include "nsStubDocumentObserver.h"
 
 class nsINode;
 
@@ -17,28 +19,54 @@ class StyleSheet;
 
 namespace dom {
 
-class StyleSheetList : public nsIDOMStyleSheetList
-                     , public nsWrapperCache
+class StyleSheetList final : public nsIDOMStyleSheetList
+                           , public nsWrapperCache
+                           , public nsStubDocumentObserver
 {
 public:
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
-  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(StyleSheetList)
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_AMBIGUOUS(StyleSheetList, nsIDOMStyleSheetList)
+
   NS_DECL_NSIDOMSTYLESHEETLIST
 
+  NS_DECL_NSIMUTATIONOBSERVER_NODEWILLBEDESTROYED
+
+  explicit StyleSheetList(StyleScope& aScope);
+
   virtual JSObject* WrapObject(JSContext* aCx, JS::Handle<JSObject*> aGivenProto) override final;
 
-  virtual nsINode* GetParentObject() const = 0;
+  nsINode* GetParentObject() const
+  {
+    return mStyleScope ? &mStyleScope->AsNode() : nullptr;
+  }
 
-  virtual uint32_t Length() = 0;
-  virtual StyleSheet* IndexedGetter(uint32_t aIndex, bool& aFound) = 0;
-  StyleSheet* Item(uint32_t aIndex)
+  uint32_t Length() const
+  {
+    return mStyleScope ? mStyleScope->SheetCount() : 0;
+  }
+
+  StyleSheet* IndexedGetter(uint32_t aIndex, bool& aFound) const
+  {
+    if (!mStyleScope) {
+      aFound = false;
+      return nullptr;
+    }
+
+    StyleSheet* sheet = mStyleScope->SheetAt(aIndex);
+    aFound = !!sheet;
+    return sheet;
+  }
+
+  StyleSheet* Item(uint32_t aIndex) const
   {
     bool dummy = false;
     return IndexedGetter(aIndex, dummy);
   }
 
 protected:
-  virtual ~StyleSheetList() {}
+  virtual ~StyleSheetList();
+
+  StyleScope* mStyleScope; // Weak, cleared on "NodeWillBeDestroyed".
 };
 
 } // namespace dom
diff --git a/dom/base/moz.build b/dom/base/moz.build
index 89f1785ca7..5acb49d4ea 100755
--- a/dom/base/moz.build
+++ b/dom/base/moz.build
@@ -213,6 +213,7 @@ EXPORTS.mozilla.dom += [
     'SimpleTreeIterator.h',
     'StructuredCloneHolder.h',
     'StructuredCloneTags.h',
+    'StyleScope.h',
     'StyleSheetList.h',
     'SubtleCrypto.h',
     'TabGroup.h',
@@ -359,6 +360,7 @@ SOURCES += [
     'ScriptSettings.cpp',
     'ShadowRoot.cpp',
     'StructuredCloneHolder.cpp',
+    'StyleScope.cpp',
     'StyleSheetList.cpp',
     'SubtleCrypto.cpp',
     'TabGroup.cpp',
diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index d2763edddf..745d170f18 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -570,78 +570,6 @@ struct nsRadioGroupStruct
   bool mGroupSuffersFromValueMissing;
 };
 
-
-nsDOMStyleSheetList::nsDOMStyleSheetList(nsIDocument *aDocument)
-{
-  mLength = -1;
-  // Not reference counted to avoid circular references.
-  // The document will tell us when its going away.
-  mDocument = aDocument;
-  mDocument->AddObserver(this);
-}
-
-nsDOMStyleSheetList::~nsDOMStyleSheetList()
-{
-  if (mDocument) {
-    mDocument->RemoveObserver(this);
-  }
-}
-
-NS_IMPL_ISUPPORTS_INHERITED(nsDOMStyleSheetList, StyleSheetList,
-                            nsIDocumentObserver,
-                            nsIMutationObserver)
-
-uint32_t
-nsDOMStyleSheetList::Length()
-{
-  if (!mDocument) {
-    return 0;
-  }
-
-  // XXX Find the number and then cache it. We'll use the
-  // observer notification to figure out if new ones have
-  // been added or removed.
-  if (-1 == mLength) {
-    mLength = mDocument->GetNumberOfStyleSheets();
-  }
-  return mLength;
-}
-
-StyleSheet*
-nsDOMStyleSheetList::IndexedGetter(uint32_t aIndex, bool& aFound)
-{
-  if (!mDocument || aIndex >= (uint32_t)mDocument->GetNumberOfStyleSheets()) {
-    aFound = false;
-    return nullptr;
-  }
-  aFound = true;
-  return mDocument->GetStyleSheetAt(aIndex);
-}
-
-void
-nsDOMStyleSheetList::NodeWillBeDestroyed(const nsINode *aNode)
-{
-  mDocument = nullptr;
-}
-
-void
-nsDOMStyleSheetList::StyleSheetAdded(StyleSheet* aStyleSheet,
-                                     bool aDocumentSheet)
-{
-  if (aDocumentSheet && -1 != mLength) {
-    mLength++;
-  }
-}
-
-void
-nsDOMStyleSheetList::StyleSheetRemoved(StyleSheet* aStyleSheet,
-                                       bool aDocumentSheet)
-{
-  if (aDocumentSheet && -1 != mLength) {
-    mLength--;
-  }
-}
-
 // nsOnloadBlocker implementation
 NS_IMPL_ISUPPORTS(nsOnloadBlocker, nsIRequest)
 
@@ -1199,10 +1127,10 @@ nsDOMStyleSheetSetList::EnsureFresh()
             // no document, for sure
   }
 
-  int32_t count = mDocument->GetNumberOfStyleSheets();
+  size_t count = mDocument->SheetCount();
   nsAutoString title;
-  for (int32_t index = 0; index < count; index++) {
-    StyleSheet* sheet = mDocument->GetStyleSheetAt(index);
+  for (size_t index = 0; index < count; index++) {
+    StyleSheet* sheet = mDocument->SheetAt(index);
     NS_ASSERTION(sheet, "Null sheet in sheet list!");
     // XXXheycam ServoStyleSheets don't expose their title yet.
     if (sheet->IsServo()) {
@@ -3930,24 +3858,6 @@ nsDocument::AddOnDemandBuiltInUASheet(StyleSheet* aSheet)
   NotifyStyleSheetAdded(aSheet, false);
 }
 
-int32_t
-nsDocument::GetNumberOfStyleSheets() const
-{
-  return mStyleSheets.Length();
-}
-
-StyleSheet*
-nsDocument::GetStyleSheetAt(int32_t aIndex) const
-{
-  return mStyleSheets.SafeElementAt(aIndex, nullptr);
-}
-
-int32_t
-nsDocument::GetIndexOfStyleSheet(const StyleSheet* aSheet) const
-{
-  return mStyleSheets.IndexOf(aSheet);
-}
-
 void
 nsDocument::AddStyleSheetToStyleSets(StyleSheet* aSheet)
 {
@@ -4088,9 +3998,9 @@ nsDocument::UpdateStyleSheets(nsTArray<RefPtr<StyleSheet>>& aOldSheets,
 }
 
 void
-nsDocument::InsertStyleSheetAt(StyleSheet* aSheet, int32_t aIndex)
+nsDocument::InsertStyleSheetAt(StyleSheet* aSheet, size_t aIndex)
 {
-  NS_PRECONDITION(aSheet, "null ptr");
+  MOZ_ASSERT(aSheet);
 
   mStyleSheets.InsertElementAt(aIndex, aSheet);
 
@@ -5815,15 +5725,6 @@ nsDocument::GetStyleSheets(nsIDOMStyleSheetList** aStyleSheets)
   return NS_OK;
 }
 
-StyleSheetList*
-nsDocument::StyleSheets()
-{
-  if (!mDOMStyleSheets) {
-    mDOMStyleSheets = new nsDOMStyleSheetList(this);
-  }
-  return mDOMStyleSheets;
-}
-
 NS_IMETHODIMP
 nsDocument::GetMozSelectedStyleSheetSet(nsAString& aSheetSet)
 {
@@ -5837,10 +5738,10 @@ nsIDocument::GetSelectedStyleSheetSet(nsAString& aSheetSet)
   aSheetSet.Truncate();
 
   // Look through our sheets, find the selected set title
-  int32_t count = GetNumberOfStyleSheets();
+  size_t count = SheetCount();
   nsAutoString title;
-  for (int32_t index = 0; index < count; index++) {
-    StyleSheet* sheet = GetStyleSheetAt(index);
+  for (size_t index = 0; index < count; index++) {
+    StyleSheet* sheet = SheetAt(index);
     NS_ASSERTION(sheet, "Null sheet in sheet list!");
 
     // XXXheycam Make this work with ServoStyleSheets.
@@ -5957,10 +5858,10 @@ nsDocument::EnableStyleSheetsForSetInternal(const nsAString& aSheetSet,
                                             bool aUpdateCSSLoader)
 {
   BeginUpdate(UPDATE_STYLE);
-  int32_t count = GetNumberOfStyleSheets();
+  size_t count = SheetCount();
   nsAutoString title;
-  for (int32_t index = 0; index < count; index++) {
-    StyleSheet* sheet = GetStyleSheetAt(index);
+  for (size_t index = 0; index < count; index++) {
+    StyleSheet* sheet = SheetAt(index);
     NS_ASSERTION(sheet, "Null sheet in sheet list!");
 
     // XXXheycam Make this work with ServoStyleSheets.
@@ -9668,9 +9569,9 @@ nsIDocument::CreateStaticClone(nsIDocShell* aCloneContainer)
 
       clonedDoc->mOriginalDocument->mStaticCloneCount++;
 
-      int32_t sheetsCount = GetNumberOfStyleSheets();
-      for (int32_t i = 0; i < sheetsCount; ++i) {
-        RefPtr<StyleSheet> sheet = GetStyleSheetAt(i);
+      size_t sheetsCount = SheetCount();
+      for (size_t i = 0; i < sheetsCount; ++i) {
+        RefPtr<StyleSheet> sheet = SheetAt(i);
         if (sheet) {
           if (sheet->IsApplicable()) {
             // XXXheycam Need to make ServoStyleSheet cloning work.
diff --git a/dom/base/nsDocument.h b/dom/base/nsDocument.h
index 931bdd89d8..4ab2858408 100644
--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -293,36 +293,6 @@ public:
   nsDocHeaderData*  mNext;
 };
 
-class nsDOMStyleSheetList : public mozilla::dom::StyleSheetList,
-                            public nsStubDocumentObserver
-{
-public:
-  explicit nsDOMStyleSheetList(nsIDocument* aDocument);
-
-  NS_DECL_ISUPPORTS_INHERITED
-
-  // nsIDocumentObserver
-  NS_DECL_NSIDOCUMENTOBSERVER_STYLESHEETADDED
-  NS_DECL_NSIDOCUMENTOBSERVER_STYLESHEETREMOVED
-
-  // nsIMutationObserver
-  NS_DECL_NSIMUTATIONOBSERVER_NODEWILLBEDESTROYED
-
-  virtual nsINode* GetParentObject() const override
-  {
-    return mDocument;
-  }
-
-  uint32_t Length() override;
-  mozilla::StyleSheet* IndexedGetter(uint32_t aIndex, bool& aFound) override;
-
-protected:
-  virtual ~nsDOMStyleSheetList();
-
-  int32_t       mLength;
-  nsIDocument*  mDocument;
-};
-
 class nsOnloadBlocker final : public nsIRequest
 {
 public:
@@ -624,14 +594,6 @@ public:
 
   virtual void EnsureOnDemandBuiltInUASheet(mozilla::StyleSheet* aSheet) override;
 
-  /**
-   * Get the (document) style sheets owned by this document.
-   * These are ordered, highest priority last
-   */
-  virtual int32_t GetNumberOfStyleSheets() const override;
-  virtual mozilla::StyleSheet* GetStyleSheetAt(int32_t aIndex) const override;
-  virtual int32_t GetIndexOfStyleSheet(
-      const mozilla::StyleSheet* aSheet) const override;
   virtual void AddStyleSheet(mozilla::StyleSheet* aSheet) override;
   virtual void RemoveStyleSheet(mozilla::StyleSheet* aSheet) override;
 
@@ -642,7 +604,7 @@ public:
   virtual void RemoveStyleSheetFromStyleSets(mozilla::StyleSheet* aSheet);
 
   virtual void InsertStyleSheetAt(mozilla::StyleSheet* aSheet,
-                                  int32_t aIndex) override;
+                                  size_t aIndex) override;
   virtual void SetStyleSheetApplicableState(mozilla::StyleSheet* aSheet,
                                             bool aApplicable) override;
 
@@ -1128,7 +1090,7 @@ public:
   // WebIDL bits
   virtual mozilla::dom::DOMImplementation*
     GetImplementation(mozilla::ErrorResult& rv) override;
-  virtual mozilla::dom::StyleSheetList* StyleSheets() override;
+
   virtual void SetSelectedStyleSheetSet(const nsAString& aSheetSet) override;
   virtual void GetLastStyleSheetSet(nsString& aSheetSet) override;
   virtual mozilla::dom::DOMStringList* StyleSheetSets() override;
@@ -1350,7 +1312,6 @@ protected:
   // EndLoad() has already happened.
   nsWeakPtr mWeakSink;
 
-  nsTArray<RefPtr<mozilla::StyleSheet>> mStyleSheets;
   nsTArray<RefPtr<mozilla::StyleSheet>> mOnDemandBuiltInUASheets;
   nsTArray<RefPtr<mozilla::StyleSheet>> mAdditionalSheets[AdditionalSheetTypeCount];
 
@@ -1392,7 +1353,6 @@ public:
   static bool IsWebComponentsEnabled(nsPIDOMWindowInner* aWindow);
 
   RefPtr<mozilla::EventListenerManager> mListenerManager;
-  RefPtr<mozilla::dom::StyleSheetList> mDOMStyleSheets;
   RefPtr<nsDOMStyleSheetSetList> mStyleSheetSetList;
   RefPtr<nsScriptLoader> mScriptLoader;
   nsDocHeaderData* mHeaderData;
diff --git a/dom/base/nsIDocument.h b/dom/base/nsIDocument.h
index 5ccb18f780..b4fda21c1a 100644
--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -34,6 +34,7 @@
 #include "prclist.h"
 #include "mozilla/UniquePtr.h"
 #include "mozilla/CORSMode.h"
+#include "mozilla/dom/StyleScope.h"
 #include "mozilla/LinkedList.h"
 #include "mozilla/StyleBackendType.h"
 #include "mozilla/StyleSheet.h"
@@ -196,7 +197,8 @@ class nsContentList;
 
 // Document interface.  This is implemented by all document objects in
 // Gecko.
-class nsIDocument : public nsINode
+class nsIDocument : public nsINode,
+                    public mozilla::dom::StyleScope
 {
   typedef mozilla::dom::GlobalObject GlobalObject;
 
@@ -1069,40 +1071,24 @@ public:
    */
   virtual void EnsureOnDemandBuiltInUASheet(mozilla::StyleSheet* aSheet) = 0;
 
-  /**
-   * Get the number of (document) stylesheets
-   *
-   * @return the number of stylesheets
-   * @throws no exceptions
-   */
-  virtual int32_t GetNumberOfStyleSheets() const = 0;
+  nsINode& AsNode() final
+  {
+    return *this;
+  }
 
-  /**
-   * Get a particular stylesheet
-   * @param aIndex the index the stylesheet lives at.  This is zero-based
-   * @return the stylesheet at aIndex.  Null if aIndex is out of range.
-   * @throws no exceptions
-   */
-  virtual mozilla::StyleSheet* GetStyleSheetAt(int32_t aIndex) const = 0;
+  mozilla::dom::StyleSheetList* StyleSheets()
+  {
+    return &StyleScope::EnsureDOMStyleSheets();
+  }
 
   /**
    * Insert a sheet at a particular spot in the stylesheet list (zero-based)
    * @param aSheet the sheet to insert
-   * @param aIndex the index to insert at.  This index will be
-   *   adjusted for the "special" sheets.
+   * @param aIndex the index to insert at.
    * @throws no exceptions
    */
   virtual void InsertStyleSheetAt(mozilla::StyleSheet* aSheet,
-                                  int32_t aIndex) = 0;
-
-  /**
-   * Get the index of a particular stylesheet.  This will _always_
-   * consider the "special" sheets as part of the sheet list.
-   * @param aSheet the sheet to get the index of
-   * @return aIndex the index of the sheet in the full list
-   */
-  virtual int32_t GetIndexOfStyleSheet(
-      const mozilla::StyleSheet* aSheet) const = 0;
+                                  size_t aIndex) = 0;
 
   /**
    * Replace the stylesheets in aOldSheets with the stylesheets in
@@ -1159,7 +1145,7 @@ public:
    */
   template<typename T>
   size_t FindDocStyleSheetInsertionPoint(const nsTArray<T>& aDocSheets,
-                                         mozilla::StyleSheet* aSheet);
+                                         const mozilla::StyleSheet& aSheet);
 
   /**
    * Get this document's CSSLoader.  This is guaranteed to not return null.
@@ -2699,7 +2685,6 @@ public:
     return mVisibilityState;
   }
 #endif
-  virtual mozilla::dom::StyleSheetList* StyleSheets() = 0;
   void GetSelectedStyleSheetSet(nsAString& aSheetSet);
   virtual void SetSelectedStyleSheetSet(const nsAString& aSheetSet) = 0;
   virtual void GetLastStyleSheetSet(nsString& aSheetSet) = 0;
diff --git a/dom/base/nsIDocumentInlines.h b/dom/base/nsIDocumentInlines.h
index 708a1ae91f..5b95a1bd3a 100644
--- a/dom/base/nsIDocumentInlines.h
+++ b/dom/base/nsIDocumentInlines.h
@@ -20,19 +20,19 @@ template<typename T>
 size_t
 nsIDocument::FindDocStyleSheetInsertionPoint(
     const nsTArray<T>& aDocSheets,
-    mozilla::StyleSheet* aSheet)
+    const mozilla::StyleSheet& aSheet)
 {
   nsStyleSheetService* sheetService = nsStyleSheetService::GetInstance();
 
   // lowest index first
-  int32_t newDocIndex = GetIndexOfStyleSheet(aSheet);
-
-  int32_t count = aDocSheets.Length();
-  int32_t index;
-  for (index = 0; index < count; index++) {
-    mozilla::StyleSheet* sheet = static_cast<mozilla::StyleSheet*>(
-      aDocSheets[index]);
-    int32_t sheetDocIndex = GetIndexOfStyleSheet(sheet);
+  int32_t newDocIndex = IndexOfSheet(aSheet);
+
+  size_t count = aDocSheets.Length();
+  size_t index = 0;
+  for (; index < count; index++) {
+    auto* sheet = static_cast<mozilla::StyleSheet*>(aDocSheets[index]);
+    MOZ_ASSERT(sheet);
+    int32_t sheetDocIndex = IndexOfSheet(*sheet);
     if (sheetDocIndex > newDocIndex)
       break;
 
diff --git a/layout/inspector/inDOMUtils.cpp b/layout/inspector/inDOMUtils.cpp
index 8e547cc240..58afc5f760 100644
--- a/layout/inspector/inDOMUtils.cpp
+++ b/layout/inspector/inDOMUtils.cpp
@@ -114,8 +114,8 @@ inDOMUtils::GetAllStyleSheets(nsIDOMDocument *aDocument, uint32_t *aLength,
   }
 
   // Get the document sheets.
-  for (int32_t i = 0; i < document->GetNumberOfStyleSheets(); i++) {
-    sheets.AppendElement(document->GetStyleSheetAt(i));
+  for (size_t i = 0; i < document->SheetCount(); i++) {
+    sheets.AppendElement(document->SheetAt(i));
   }
 
   nsISupports** ret = static_cast<nsISupports**>(moz_xmalloc(sheets.Length() *
diff --git a/layout/style/Loader.cpp b/layout/style/Loader.cpp
index d4edfe81d0..9894ce8f45 100644
--- a/layout/style/Loader.cpp
+++ b/layout/style/Loader.cpp
@@ -1334,7 +1334,7 @@ Loader::InsertSheetInDoc(StyleSheet* aSheet,
 
   // XXX Need to cancel pending sheet loads for this element, if any
 
-  int32_t sheetCount = aDocument->GetNumberOfStyleSheets();
+  int32_t sheetCount = aDocument->SheetCount();
 
   /*
    * Start the walk at the _end_ of the list, since in the typical
@@ -1346,7 +1346,7 @@ Loader::InsertSheetInDoc(StyleSheet* aSheet,
    */
   int32_t insertionPoint;
   for (insertionPoint = sheetCount - 1; insertionPoint >= 0; --insertionPoint) {
-    StyleSheet* curSheet = aDocument->GetStyleSheetAt(insertionPoint);
+    StyleSheet* curSheet = aDocument->SheetAt(insertionPoint);
     NS_ASSERTION(curSheet, "There must be a sheet here!");
     nsCOMPtr<nsINode> sheetOwner = curSheet->GetOwnerNode();
     if (sheetOwner && !aLinkingContent) {
diff --git a/layout/style/ServoStyleSet.cpp b/layout/style/ServoStyleSet.cpp
index 519d17aa85..331e2322a1 100644
--- a/layout/style/ServoStyleSet.cpp
+++ b/layout/style/ServoStyleSet.cpp
@@ -339,7 +339,7 @@ ServoStyleSet::AddDocStyleSheet(ServoStyleSheet* aSheet,
   mSheets[SheetType::Doc].RemoveElement(aSheet);
 
   size_t index =
-    aDocument->FindDocStyleSheetInsertionPoint(mSheets[SheetType::Doc], aSheet);
+    aDocument->FindDocStyleSheetInsertionPoint(mSheets[SheetType::Doc], *aSheet);
   mSheets[SheetType::Doc].InsertElementAt(index, aSheet);
 
   // Maintain a mirrored list of sheets on the servo side.
diff --git a/layout/style/nsStyleSet.cpp b/layout/style/nsStyleSet.cpp
index 414eee4d43..558b9fa7f1 100644
--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -734,7 +734,7 @@ nsStyleSet::AddDocStyleSheet(CSSStyleSheet* aSheet, nsIDocument* aDocument)
 
   bool present = sheets.RemoveElement(aSheet);
 
-  size_t index = aDocument->FindDocStyleSheetInsertionPoint(sheets, aSheet);
+  size_t index = aDocument->FindDocStyleSheetInsertionPoint(sheets, *aSheet);
   sheets.InsertElementAt(index, aSheet);
 
   if (!present) {