Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

15 files changed, 8212 insertions, 0 deletions

diff --git a/docshell/shistory/ChildSHistory.cpp b/docshell/shistory/ChildSHistory.cpp
new file mode 100644
index 0000000000..87b6a7b3fc
--- /dev/null
+++ b/docshell/shistory/ChildSHistory.cpp
@@ -0,0 +1,294 @@
+/* -*- 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 "mozilla/dom/ChildSHistory.h"
+#include "mozilla/dom/ChildSHistoryBinding.h"
+#include "mozilla/dom/CanonicalBrowsingContext.h"
+#include "mozilla/dom/ContentChild.h"
+#include "mozilla/dom/ContentFrameMessageManager.h"
+#include "nsIXULRuntime.h"
+#include "nsComponentManagerUtils.h"
+#include "nsSHEntry.h"
+#include "nsSHistory.h"
+#include "nsDocShell.h"
+#include "nsXULAppAPI.h"
+
+extern mozilla::LazyLogModule gSHLog;
+
+namespace mozilla {
+namespace dom {
+
+ChildSHistory::ChildSHistory(BrowsingContext* aBrowsingContext)
+    : mBrowsingContext(aBrowsingContext) {}
+
+ChildSHistory::~ChildSHistory() {
+  if (mHistory) {
+    static_cast<nsSHistory*>(mHistory.get())->SetBrowsingContext(nullptr);
+  }
+}
+
+void ChildSHistory::SetBrowsingContext(BrowsingContext* aBrowsingContext) {
+  mBrowsingContext = aBrowsingContext;
+}
+
+void ChildSHistory::SetIsInProcess(bool aIsInProcess) {
+  if (!aIsInProcess) {
+    MOZ_ASSERT_IF(mozilla::SessionHistoryInParent(), !mHistory);
+    if (!mozilla::SessionHistoryInParent()) {
+      RemovePendingHistoryNavigations();
+      if (mHistory) {
+        static_cast<nsSHistory*>(mHistory.get())->SetBrowsingContext(nullptr);
+        mHistory = nullptr;
+      }
+    }
+
+    return;
+  }
+
+  if (mHistory || mozilla::SessionHistoryInParent()) {
+    return;
+  }
+
+  mHistory = new nsSHistory(mBrowsingContext);
+}
+
+int32_t ChildSHistory::Count() {
+  if (mozilla::SessionHistoryInParent()) {
+    uint32_t length = mLength;
+    for (uint32_t i = 0; i < mPendingSHistoryChanges.Length(); ++i) {
+      length += mPendingSHistoryChanges[i].mLengthDelta;
+    }
+
+    return length;
+  }
+  return mHistory->GetCount();
+}
+
+int32_t ChildSHistory::Index() {
+  if (mozilla::SessionHistoryInParent()) {
+    uint32_t index = mIndex;
+    for (uint32_t i = 0; i < mPendingSHistoryChanges.Length(); ++i) {
+      index += mPendingSHistoryChanges[i].mIndexDelta;
+    }
+
+    return index;
+  }
+  int32_t index;
+  mHistory->GetIndex(&index);
+  return index;
+}
+
+nsID ChildSHistory::AddPendingHistoryChange() {
+  int32_t indexDelta = 1;
+  int32_t lengthDelta = (Index() + indexDelta) - (Count() - 1);
+  return AddPendingHistoryChange(indexDelta, lengthDelta);
+}
+
+nsID ChildSHistory::AddPendingHistoryChange(int32_t aIndexDelta,
+                                            int32_t aLengthDelta) {
+  nsID changeID = nsID::GenerateUUID();
+  PendingSHistoryChange change = {changeID, aIndexDelta, aLengthDelta};
+  mPendingSHistoryChanges.AppendElement(change);
+  return changeID;
+}
+
+void ChildSHistory::SetIndexAndLength(uint32_t aIndex, uint32_t aLength,
+                                      const nsID& aChangeID) {
+  mIndex = aIndex;
+  mLength = aLength;
+  mPendingSHistoryChanges.RemoveElementsBy(
+      [aChangeID](const PendingSHistoryChange& aChange) {
+        return aChange.mChangeID == aChangeID;
+      });
+}
+
+void ChildSHistory::Reload(uint32_t aReloadFlags, ErrorResult& aRv) {
+  if (mozilla::SessionHistoryInParent()) {
+    if (XRE_IsParentProcess()) {
+      nsCOMPtr<nsISHistory> shistory =
+          mBrowsingContext->Canonical()->GetSessionHistory();
+      if (shistory) {
+        aRv = shistory->Reload(aReloadFlags);
+      }
+    } else {
+      ContentChild::GetSingleton()->SendHistoryReload(mBrowsingContext,
+                                                      aReloadFlags);
+    }
+
+    return;
+  }
+  nsCOMPtr<nsISHistory> shistory = mHistory;
+  aRv = shistory->Reload(aReloadFlags);
+}
+
+bool ChildSHistory::CanGo(int32_t aOffset) {
+  CheckedInt<int32_t> index = Index();
+  index += aOffset;
+  if (!index.isValid()) {
+    return false;
+  }
+  return index.value() < Count() && index.value() >= 0;
+}
+
+void ChildSHistory::Go(int32_t aOffset, bool aRequireUserInteraction,
+                       bool aUserActivation, ErrorResult& aRv) {
+  CheckedInt<int32_t> index = Index();
+  MOZ_LOG(
+      gSHLog, LogLevel::Debug,
+      ("ChildSHistory::Go(%d), current index = %d", aOffset, index.value()));
+  if (aRequireUserInteraction && aOffset != -1 && aOffset != 1) {
+    NS_ERROR(
+        "aRequireUserInteraction may only be used with an offset of -1 or 1");
+    aRv.Throw(NS_ERROR_INVALID_ARG);
+    return;
+  }
+
+  while (true) {
+    index += aOffset;
+    if (!index.isValid()) {
+      aRv.Throw(NS_ERROR_FAILURE);
+      return;
+    }
+
+    // Check for user interaction if desired, except for the first and last
+    // history entries. We compare with >= to account for the case where
+    // aOffset >= Count().
+    if (!aRequireUserInteraction || index.value() >= Count() - 1 ||
+        index.value() <= 0) {
+      break;
+    }
+    if (mHistory && mHistory->HasUserInteractionAtIndex(index.value())) {
+      break;
+    }
+  }
+
+  GotoIndex(index.value(), aOffset, aRequireUserInteraction, aUserActivation,
+            aRv);
+}
+
+void ChildSHistory::AsyncGo(int32_t aOffset, bool aRequireUserInteraction,
+                            bool aUserActivation, CallerType aCallerType,
+                            ErrorResult& aRv) {
+  CheckedInt<int32_t> index = Index();
+  MOZ_LOG(gSHLog, LogLevel::Debug,
+          ("ChildSHistory::AsyncGo(%d), current index = %d", aOffset,
+           index.value()));
+  nsresult rv = mBrowsingContext->CheckLocationChangeRateLimit(aCallerType);
+  if (NS_FAILED(rv)) {
+    MOZ_LOG(gSHLog, LogLevel::Debug, ("Rejected"));
+    aRv.Throw(rv);
+    return;
+  }
+
+  RefPtr<PendingAsyncHistoryNavigation> asyncNav =
+      new PendingAsyncHistoryNavigation(this, aOffset, aRequireUserInteraction,
+                                        aUserActivation);
+  mPendingNavigations.insertBack(asyncNav);
+  NS_DispatchToCurrentThread(asyncNav.forget());
+}
+
+void ChildSHistory::GotoIndex(int32_t aIndex, int32_t aOffset,
+                              bool aRequireUserInteraction,
+                              bool aUserActivation, ErrorResult& aRv) {
+  MOZ_LOG(gSHLog, LogLevel::Debug,
+          ("ChildSHistory::GotoIndex(%d, %d), epoch %" PRIu64, aIndex, aOffset,
+           mHistoryEpoch));
+  if (mozilla::SessionHistoryInParent()) {
+    if (!mPendingEpoch) {
+      mPendingEpoch = true;
+      RefPtr<ChildSHistory> self(this);
+      NS_DispatchToCurrentThread(
+          NS_NewRunnableFunction("UpdateEpochRunnable", [self] {
+            self->mHistoryEpoch++;
+            self->mPendingEpoch = false;
+          }));
+    }
+
+    nsCOMPtr<nsISHistory> shistory = mHistory;
+    RefPtr<BrowsingContext> bc = mBrowsingContext;
+    bc->HistoryGo(
+        aOffset, mHistoryEpoch, aRequireUserInteraction, aUserActivation,
+        [shistory](Maybe<int32_t>&& aRequestedIndex) {
+          // FIXME Should probably only do this for non-fission.
+          if (aRequestedIndex.isSome() && shistory) {
+            shistory->InternalSetRequestedIndex(aRequestedIndex.value());
+          }
+        });
+  } else {
+    nsCOMPtr<nsISHistory> shistory = mHistory;
+    aRv = shistory->GotoIndex(aIndex, aUserActivation);
+  }
+}
+
+void ChildSHistory::RemovePendingHistoryNavigations() {
+  // Per the spec, this generally shouldn't remove all navigations - it
+  // depends if they're in the same document family or not.  We don't do
+  // that.  Also with SessionHistoryInParent, this can only abort AsyncGo's
+  // that have not yet been sent to the parent - see discussion of point
+  // 2.2 in comments in nsDocShell::UpdateURLAndHistory()
+  MOZ_LOG(gSHLog, LogLevel::Debug,
+          ("ChildSHistory::RemovePendingHistoryNavigations: %zu",
+           mPendingNavigations.length()));
+  mPendingNavigations.clear();
+}
+
+void ChildSHistory::EvictLocalContentViewers() {
+  if (!mozilla::SessionHistoryInParent()) {
+    mHistory->EvictAllContentViewers();
+  }
+}
+
+nsISHistory* ChildSHistory::GetLegacySHistory(ErrorResult& aError) {
+  if (mozilla::SessionHistoryInParent()) {
+    aError.ThrowTypeError(
+        "legacySHistory is not available with session history in the parent.");
+    return nullptr;
+  }
+
+  MOZ_RELEASE_ASSERT(mHistory);
+  return mHistory;
+}
+
+nsISHistory* ChildSHistory::LegacySHistory() {
+  IgnoredErrorResult ignore;
+  nsISHistory* shistory = GetLegacySHistory(ignore);
+  MOZ_RELEASE_ASSERT(shistory);
+  return shistory;
+}
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ChildSHistory)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(ChildSHistory)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(ChildSHistory)
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(ChildSHistory)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(ChildSHistory)
+  if (tmp->mHistory) {
+    static_cast<nsSHistory*>(tmp->mHistory.get())->SetBrowsingContext(nullptr);
+  }
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mBrowsingContext, mHistory)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(ChildSHistory)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mBrowsingContext, mHistory)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+JSObject* ChildSHistory::WrapObject(JSContext* cx,
+                                    JS::Handle<JSObject*> aGivenProto) {
+  return ChildSHistory_Binding::Wrap(cx, this, aGivenProto);
+}
+
+nsISupports* ChildSHistory::GetParentObject() const {
+  return xpc::NativeGlobal(xpc::PrivilegedJunkScope());
+}
+
+}  // namespace dom
+}  // namespace mozilla
diff --git a/docshell/shistory/ChildSHistory.h b/docshell/shistory/ChildSHistory.h
new file mode 100644
index 0000000000..031c91c7da
--- /dev/null
+++ b/docshell/shistory/ChildSHistory.h
@@ -0,0 +1,149 @@
+/* -*- 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/. */
+
+/**
+ * ChildSHistory represents a view of session history from a child process. It
+ * exposes getters for some cached history state, and mutators which are
+ * implemented by communicating with the actual history storage.
+ *
+ * NOTE: Currently session history is in transition, meaning that we're still
+ * using the legacy nsSHistory class internally. The API exposed from this class
+ * should be only the API which we expect to expose when this transition is
+ * complete, and special cases will need to call through the LegacySHistory()
+ * getters.
+ */
+
+#ifndef mozilla_dom_ChildSHistory_h
+#define mozilla_dom_ChildSHistory_h
+
+#include "nsCOMPtr.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "nsWrapperCache.h"
+#include "nsThreadUtils.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/LinkedList.h"
+#include "nsID.h"
+
+class nsISHEntry;
+class nsISHistory;
+
+namespace mozilla::dom {
+
+class BrowsingContext;
+
+class ChildSHistory : public nsISupports, public nsWrapperCache {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(ChildSHistory)
+  nsISupports* GetParentObject() const;
+  JSObject* WrapObject(JSContext* cx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  explicit ChildSHistory(BrowsingContext* aBrowsingContext);
+
+  void SetBrowsingContext(BrowsingContext* aBrowsingContext);
+
+  // Create or destroy the session history implementation in the child process.
+  // This can be removed once session history is stored exclusively in the
+  // parent process.
+  void SetIsInProcess(bool aIsInProcess);
+  bool IsInProcess() { return !!mHistory; }
+
+  int32_t Count();
+  int32_t Index();
+
+  /**
+   * Reload the current entry in the session history.
+   */
+  void Reload(uint32_t aReloadFlags, ErrorResult& aRv);
+
+  /**
+   * The CanGo and Go methods are called with an offset from the current index.
+   * Positive numbers go forward in history, while negative numbers go
+   * backwards.
+   */
+  bool CanGo(int32_t aOffset);
+  void Go(int32_t aOffset, bool aRequireUserInteraction, bool aUserActivation,
+          ErrorResult& aRv);
+  void AsyncGo(int32_t aOffset, bool aRequireUserInteraction,
+               bool aUserActivation, CallerType aCallerType, ErrorResult& aRv);
+
+  // aIndex is the new index, and aOffset is the offset between new and current.
+  void GotoIndex(int32_t aIndex, int32_t aOffset, bool aRequireUserInteraction,
+                 bool aUserActivation, ErrorResult& aRv);
+
+  void RemovePendingHistoryNavigations();
+
+  /**
+   * Evicts all content viewers within the current process.
+   */
+  void EvictLocalContentViewers();
+
+  // GetLegacySHistory and LegacySHistory have been deprecated. Don't
+  // use these, but instead handle the interaction with nsISHistory in
+  // the parent process.
+  nsISHistory* GetLegacySHistory(ErrorResult& aError);
+  nsISHistory* LegacySHistory();
+
+  void SetIndexAndLength(uint32_t aIndex, uint32_t aLength,
+                         const nsID& aChangeId);
+  nsID AddPendingHistoryChange();
+  nsID AddPendingHistoryChange(int32_t aIndexDelta, int32_t aLengthDelta);
+
+ private:
+  virtual ~ChildSHistory();
+
+  class PendingAsyncHistoryNavigation
+      : public Runnable,
+        public mozilla::LinkedListElement<PendingAsyncHistoryNavigation> {
+   public:
+    PendingAsyncHistoryNavigation(ChildSHistory* aHistory, int32_t aOffset,
+                                  bool aRequireUserInteraction,
+                                  bool aUserActivation)
+        : Runnable("PendingAsyncHistoryNavigation"),
+          mHistory(aHistory),
+          mRequireUserInteraction(aRequireUserInteraction),
+          mUserActivation(aUserActivation),
+          mOffset(aOffset) {}
+
+    NS_IMETHOD Run() override {
+      if (isInList()) {
+        remove();
+        mHistory->Go(mOffset, mRequireUserInteraction, mUserActivation,
+                     IgnoreErrors());
+      }
+      return NS_OK;
+    }
+
+   private:
+    RefPtr<ChildSHistory> mHistory;
+    bool mRequireUserInteraction;
+    bool mUserActivation;
+    int32_t mOffset;
+  };
+
+  RefPtr<BrowsingContext> mBrowsingContext;
+  nsCOMPtr<nsISHistory> mHistory;
+  // Can be removed once history-in-parent is the only way
+  mozilla::LinkedList<PendingAsyncHistoryNavigation> mPendingNavigations;
+  int32_t mIndex = -1;
+  int32_t mLength = 0;
+
+  struct PendingSHistoryChange {
+    nsID mChangeID;
+    int32_t mIndexDelta;
+    int32_t mLengthDelta;
+  };
+  AutoTArray<PendingSHistoryChange, 2> mPendingSHistoryChanges;
+
+  // Needs to start 1 above default epoch in parent
+  uint64_t mHistoryEpoch = 1;
+  bool mPendingEpoch = false;
+};
+
+}  // namespace mozilla::dom
+
+#endif /* mozilla_dom_ChildSHistory_h */
diff --git a/docshell/shistory/SessionHistoryEntry.cpp b/docshell/shistory/SessionHistoryEntry.cpp
new file mode 100644
index 0000000000..67829e630e
--- /dev/null
+++ b/docshell/shistory/SessionHistoryEntry.cpp
@@ -0,0 +1,1828 @@
+/* -*- 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 "SessionHistoryEntry.h"
+#include "ipc/IPCMessageUtilsSpecializations.h"
+#include "nsDocShell.h"
+#include "nsDocShellLoadState.h"
+#include "nsFrameLoader.h"
+#include "nsIFormPOSTActionChannel.h"
+#include "nsIHttpChannel.h"
+#include "nsIUploadChannel2.h"
+#include "nsIXULRuntime.h"
+#include "nsSHEntryShared.h"
+#include "nsSHistory.h"
+#include "nsStreamUtils.h"
+#include "nsStructuredCloneContainer.h"
+#include "nsXULAppAPI.h"
+#include "mozilla/PresState.h"
+#include "mozilla/StaticPrefs_fission.h"
+
+#include "mozilla/dom/BrowserParent.h"
+#include "mozilla/dom/CanonicalBrowsingContext.h"
+#include "mozilla/dom/ContentChild.h"
+#include "mozilla/dom/ContentParent.h"
+#include "mozilla/dom/CSPMessageUtils.h"
+#include "mozilla/dom/DocumentBinding.h"
+#include "mozilla/dom/DOMTypes.h"
+#include "mozilla/dom/nsCSPContext.h"
+#include "mozilla/dom/PermissionMessageUtils.h"
+#include "mozilla/dom/ReferrerInfoUtils.h"
+#include "mozilla/ipc/IPDLParamTraits.h"
+#include "mozilla/ipc/ProtocolUtils.h"
+#include "mozilla/ipc/URIUtils.h"
+
+extern mozilla::LazyLogModule gSHLog;
+
+namespace mozilla {
+namespace dom {
+
+SessionHistoryInfo::SessionHistoryInfo(nsDocShellLoadState* aLoadState,
+                                       nsIChannel* aChannel)
+    : mURI(aLoadState->URI()),
+      mOriginalURI(aLoadState->OriginalURI()),
+      mResultPrincipalURI(aLoadState->ResultPrincipalURI()),
+      mUnstrippedURI(aLoadState->GetUnstrippedURI()),
+      mLoadType(aLoadState->LoadType()),
+      mSrcdocData(aLoadState->SrcdocData().IsVoid()
+                      ? Nothing()
+                      : Some(aLoadState->SrcdocData())),
+      mBaseURI(aLoadState->BaseURI()),
+      mLoadReplace(aLoadState->LoadReplace()),
+      mHasUserInteraction(false),
+      mHasUserActivation(aLoadState->HasValidUserGestureActivation()),
+      mSharedState(SharedState::Create(
+          aLoadState->TriggeringPrincipal(), aLoadState->PrincipalToInherit(),
+          aLoadState->PartitionedPrincipalToInherit(), aLoadState->Csp(),
+          /* FIXME Is this correct? */
+          aLoadState->TypeHint())) {
+  // Pull the upload stream off of the channel instead of the load state, as
+  // ownership has already been transferred from the load state to the channel.
+  if (nsCOMPtr<nsIUploadChannel2> postChannel = do_QueryInterface(aChannel)) {
+    int64_t contentLength;
+    MOZ_ALWAYS_SUCCEEDS(postChannel->CloneUploadStream(
+        &contentLength, getter_AddRefs(mPostData)));
+    MOZ_ASSERT_IF(mPostData, NS_InputStreamIsCloneable(mPostData));
+  }
+
+  if (nsCOMPtr<nsIHttpChannel> httpChannel = do_QueryInterface(aChannel)) {
+    mReferrerInfo = httpChannel->GetReferrerInfo();
+  }
+
+  MaybeUpdateTitleFromURI();
+}
+
+SessionHistoryInfo::SessionHistoryInfo(
+    const SessionHistoryInfo& aSharedStateFrom, nsIURI* aURI)
+    : mURI(aURI), mSharedState(aSharedStateFrom.mSharedState) {
+  MaybeUpdateTitleFromURI();
+}
+
+SessionHistoryInfo::SessionHistoryInfo(
+    nsIURI* aURI, nsIPrincipal* aTriggeringPrincipal,
+    nsIPrincipal* aPrincipalToInherit,
+    nsIPrincipal* aPartitionedPrincipalToInherit,
+    nsIContentSecurityPolicy* aCsp, const nsACString& aContentType)
+    : mURI(aURI),
+      mSharedState(SharedState::Create(
+          aTriggeringPrincipal, aPrincipalToInherit,
+          aPartitionedPrincipalToInherit, aCsp, aContentType)) {
+  MaybeUpdateTitleFromURI();
+}
+
+SessionHistoryInfo::SessionHistoryInfo(
+    nsIChannel* aChannel, uint32_t aLoadType,
+    nsIPrincipal* aPartitionedPrincipalToInherit,
+    nsIContentSecurityPolicy* aCsp) {
+  aChannel->GetURI(getter_AddRefs(mURI));
+  mLoadType = aLoadType;
+
+  nsCOMPtr<nsILoadInfo> loadInfo;
+  aChannel->GetLoadInfo(getter_AddRefs(loadInfo));
+
+  loadInfo->GetResultPrincipalURI(getter_AddRefs(mResultPrincipalURI));
+  loadInfo->GetUnstrippedURI(getter_AddRefs(mUnstrippedURI));
+  loadInfo->GetTriggeringPrincipal(
+      getter_AddRefs(mSharedState.Get()->mTriggeringPrincipal));
+  loadInfo->GetPrincipalToInherit(
+      getter_AddRefs(mSharedState.Get()->mPrincipalToInherit));
+
+  mSharedState.Get()->mPartitionedPrincipalToInherit =
+      aPartitionedPrincipalToInherit;
+  mSharedState.Get()->mCsp = aCsp;
+  aChannel->GetContentType(mSharedState.Get()->mContentType);
+  aChannel->GetOriginalURI(getter_AddRefs(mOriginalURI));
+
+  uint32_t loadFlags;
+  aChannel->GetLoadFlags(&loadFlags);
+  mLoadReplace = !!(loadFlags & nsIChannel::LOAD_REPLACE);
+
+  MaybeUpdateTitleFromURI();
+
+  if (nsCOMPtr<nsIHttpChannel> httpChannel = do_QueryInterface(aChannel)) {
+    mReferrerInfo = httpChannel->GetReferrerInfo();
+  }
+}
+
+void SessionHistoryInfo::Reset(nsIURI* aURI, const nsID& aDocShellID,
+                               bool aDynamicCreation,
+                               nsIPrincipal* aTriggeringPrincipal,
+                               nsIPrincipal* aPrincipalToInherit,
+                               nsIPrincipal* aPartitionedPrincipalToInherit,
+                               nsIContentSecurityPolicy* aCsp,
+                               const nsACString& aContentType) {
+  mURI = aURI;
+  mOriginalURI = nullptr;
+  mResultPrincipalURI = nullptr;
+  mUnstrippedURI = nullptr;
+  mReferrerInfo = nullptr;
+  // Default title is the URL.
+  nsAutoCString spec;
+  if (NS_SUCCEEDED(mURI->GetSpec(spec))) {
+    CopyUTF8toUTF16(spec, mTitle);
+  }
+  mPostData = nullptr;
+  mLoadType = 0;
+  mScrollPositionX = 0;
+  mScrollPositionY = 0;
+  mStateData = nullptr;
+  mSrcdocData = Nothing();
+  mBaseURI = nullptr;
+  mLoadReplace = false;
+  mURIWasModified = false;
+  mScrollRestorationIsManual = false;
+  mPersist = false;
+  mHasUserInteraction = false;
+  mHasUserActivation = false;
+
+  mSharedState.Get()->mTriggeringPrincipal = aTriggeringPrincipal;
+  mSharedState.Get()->mPrincipalToInherit = aPrincipalToInherit;
+  mSharedState.Get()->mPartitionedPrincipalToInherit =
+      aPartitionedPrincipalToInherit;
+  mSharedState.Get()->mCsp = aCsp;
+  mSharedState.Get()->mContentType = aContentType;
+  mSharedState.Get()->mLayoutHistoryState = nullptr;
+}
+
+void SessionHistoryInfo::MaybeUpdateTitleFromURI() {
+  if (mTitle.IsEmpty() && mURI) {
+    // Default title is the URL.
+    nsAutoCString spec;
+    if (NS_SUCCEEDED(mURI->GetSpec(spec))) {
+      AppendUTF8toUTF16(spec, mTitle);
+    }
+  }
+}
+
+already_AddRefed<nsIInputStream> SessionHistoryInfo::GetPostData() const {
+  // Return a clone of our post data stream. Our caller will either be
+  // transferring this stream to a different SessionHistoryInfo, or passing it
+  // off to necko/another process which will consume it, and we want to preserve
+  // our local instance.
+  nsCOMPtr<nsIInputStream> postData;
+  if (mPostData) {
+    MOZ_ALWAYS_SUCCEEDS(
+        NS_CloneInputStream(mPostData, getter_AddRefs(postData)));
+  }
+  return postData.forget();
+}
+
+void SessionHistoryInfo::SetPostData(nsIInputStream* aPostData) {
+  MOZ_ASSERT_IF(aPostData, NS_InputStreamIsCloneable(aPostData));
+  mPostData = aPostData;
+}
+
+uint64_t SessionHistoryInfo::SharedId() const {
+  return mSharedState.Get()->mId;
+}
+
+nsILayoutHistoryState* SessionHistoryInfo::GetLayoutHistoryState() {
+  return mSharedState.Get()->mLayoutHistoryState;
+}
+
+void SessionHistoryInfo::SetLayoutHistoryState(nsILayoutHistoryState* aState) {
+  mSharedState.Get()->mLayoutHistoryState = aState;
+  if (mSharedState.Get()->mLayoutHistoryState) {
+    mSharedState.Get()->mLayoutHistoryState->SetScrollPositionOnly(
+        !mSharedState.Get()->mSaveLayoutState);
+  }
+}
+
+nsIPrincipal* SessionHistoryInfo::GetTriggeringPrincipal() const {
+  return mSharedState.Get()->mTriggeringPrincipal;
+}
+
+nsIPrincipal* SessionHistoryInfo::GetPrincipalToInherit() const {
+  return mSharedState.Get()->mPrincipalToInherit;
+}
+
+nsIPrincipal* SessionHistoryInfo::GetPartitionedPrincipalToInherit() const {
+  return mSharedState.Get()->mPartitionedPrincipalToInherit;
+}
+
+nsIContentSecurityPolicy* SessionHistoryInfo::GetCsp() const {
+  return mSharedState.Get()->mCsp;
+}
+
+uint32_t SessionHistoryInfo::GetCacheKey() const {
+  return mSharedState.Get()->mCacheKey;
+}
+
+void SessionHistoryInfo::SetCacheKey(uint32_t aCacheKey) {
+  mSharedState.Get()->mCacheKey = aCacheKey;
+}
+
+bool SessionHistoryInfo::IsSubFrame() const {
+  return mSharedState.Get()->mIsFrameNavigation;
+}
+
+void SessionHistoryInfo::SetSaveLayoutStateFlag(bool aSaveLayoutStateFlag) {
+  MOZ_ASSERT(XRE_IsParentProcess());
+  static_cast<SHEntrySharedParentState*>(mSharedState.Get())->mSaveLayoutState =
+      aSaveLayoutStateFlag;
+}
+
+void SessionHistoryInfo::FillLoadInfo(nsDocShellLoadState& aLoadState) const {
+  aLoadState.SetOriginalURI(mOriginalURI);
+  aLoadState.SetMaybeResultPrincipalURI(Some(mResultPrincipalURI));
+  aLoadState.SetUnstrippedURI(mUnstrippedURI);
+  aLoadState.SetLoadReplace(mLoadReplace);
+  nsCOMPtr<nsIInputStream> postData = GetPostData();
+  aLoadState.SetPostDataStream(postData);
+  aLoadState.SetReferrerInfo(mReferrerInfo);
+
+  aLoadState.SetTypeHint(mSharedState.Get()->mContentType);
+  aLoadState.SetTriggeringPrincipal(mSharedState.Get()->mTriggeringPrincipal);
+  aLoadState.SetPrincipalToInherit(mSharedState.Get()->mPrincipalToInherit);
+  aLoadState.SetPartitionedPrincipalToInherit(
+      mSharedState.Get()->mPartitionedPrincipalToInherit);
+  aLoadState.SetCsp(mSharedState.Get()->mCsp);
+
+  // Do not inherit principal from document (security-critical!);
+  uint32_t flags = nsDocShell::InternalLoad::INTERNAL_LOAD_FLAGS_NONE;
+
+  // Passing nullptr as aSourceDocShell gives the same behaviour as before
+  // aSourceDocShell was introduced. According to spec we should be passing
+  // the source browsing context that was used when the history entry was
+  // first created. bug 947716 has been created to address this issue.
+  nsAutoString srcdoc;
+  nsCOMPtr<nsIURI> baseURI;
+  if (mSrcdocData) {
+    srcdoc = mSrcdocData.value();
+    baseURI = mBaseURI;
+    flags |= nsDocShell::InternalLoad::INTERNAL_LOAD_FLAGS_IS_SRCDOC;
+  } else {
+    srcdoc = VoidString();
+  }
+  aLoadState.SetSrcdocData(srcdoc);
+  aLoadState.SetBaseURI(baseURI);
+  aLoadState.SetInternalLoadFlags(flags);
+
+  aLoadState.SetFirstParty(true);
+
+  // When we create a load state from the history info we already know if
+  // https-first was able to upgrade the request from http to https. There is no
+  // point in re-retrying to upgrade.
+  aLoadState.SetIsExemptFromHTTPSOnlyMode(true);
+}
+/* static */
+SessionHistoryInfo::SharedState SessionHistoryInfo::SharedState::Create(
+    nsIPrincipal* aTriggeringPrincipal, nsIPrincipal* aPrincipalToInherit,
+    nsIPrincipal* aPartitionedPrincipalToInherit,
+    nsIContentSecurityPolicy* aCsp, const nsACString& aContentType) {
+  if (XRE_IsParentProcess()) {
+    return SharedState(new SHEntrySharedParentState(
+        aTriggeringPrincipal, aPrincipalToInherit,
+        aPartitionedPrincipalToInherit, aCsp, aContentType));
+  }
+
+  return SharedState(MakeUnique<SHEntrySharedState>(
+      aTriggeringPrincipal, aPrincipalToInherit, aPartitionedPrincipalToInherit,
+      aCsp, aContentType));
+}
+
+SessionHistoryInfo::SharedState::SharedState() { Init(); }
+
+SessionHistoryInfo::SharedState::SharedState(
+    const SessionHistoryInfo::SharedState& aOther) {
+  Init(aOther);
+}
+
+SessionHistoryInfo::SharedState::SharedState(
+    const Maybe<const SessionHistoryInfo::SharedState&>& aOther) {
+  if (aOther.isSome()) {
+    Init(aOther.ref());
+  } else {
+    Init();
+  }
+}
+
+SessionHistoryInfo::SharedState::~SharedState() {
+  if (XRE_IsParentProcess()) {
+    mParent
+        .RefPtr<SHEntrySharedParentState>::~RefPtr<SHEntrySharedParentState>();
+  } else {
+    mChild.UniquePtr<SHEntrySharedState>::~UniquePtr<SHEntrySharedState>();
+  }
+}
+
+SessionHistoryInfo::SharedState& SessionHistoryInfo::SharedState::operator=(
+    const SessionHistoryInfo::SharedState& aOther) {
+  if (this != &aOther) {
+    if (XRE_IsParentProcess()) {
+      mParent = aOther.mParent;
+    } else {
+      mChild = MakeUnique<SHEntrySharedState>(*aOther.mChild);
+    }
+  }
+  return *this;
+}
+
+SHEntrySharedState* SessionHistoryInfo::SharedState::Get() const {
+  if (XRE_IsParentProcess()) {
+    return mParent;
+  }
+
+  return mChild.get();
+}
+
+void SessionHistoryInfo::SharedState::ChangeId(uint64_t aId) {
+  if (XRE_IsParentProcess()) {
+    mParent->ChangeId(aId);
+  } else {
+    mChild->mId = aId;
+  }
+}
+
+void SessionHistoryInfo::SharedState::Init() {
+  if (XRE_IsParentProcess()) {
+    new (&mParent)
+        RefPtr<SHEntrySharedParentState>(new SHEntrySharedParentState());
+  } else {
+    new (&mChild)
+        UniquePtr<SHEntrySharedState>(MakeUnique<SHEntrySharedState>());
+  }
+}
+
+void SessionHistoryInfo::SharedState::Init(
+    const SessionHistoryInfo::SharedState& aOther) {
+  if (XRE_IsParentProcess()) {
+    new (&mParent) RefPtr<SHEntrySharedParentState>(aOther.mParent);
+  } else {
+    new (&mChild) UniquePtr<SHEntrySharedState>(
+        MakeUnique<SHEntrySharedState>(*aOther.mChild));
+  }
+}
+
+static uint64_t gLoadingSessionHistoryInfoLoadId = 0;
+
+nsTHashMap<nsUint64HashKey, SessionHistoryEntry::LoadingEntry>*
+    SessionHistoryEntry::sLoadIdToEntry = nullptr;
+
+LoadingSessionHistoryInfo::LoadingSessionHistoryInfo(
+    SessionHistoryEntry* aEntry)
+    : mInfo(aEntry->Info()), mLoadId(++gLoadingSessionHistoryInfoLoadId) {
+  SessionHistoryEntry::SetByLoadId(mLoadId, aEntry);
+}
+
+LoadingSessionHistoryInfo::LoadingSessionHistoryInfo(
+    SessionHistoryEntry* aEntry, const LoadingSessionHistoryInfo* aInfo)
+    : mInfo(aEntry->Info()),
+      mLoadId(aInfo->mLoadId),
+      mLoadIsFromSessionHistory(aInfo->mLoadIsFromSessionHistory),
+      mOffset(aInfo->mOffset),
+      mLoadingCurrentEntry(aInfo->mLoadingCurrentEntry) {
+  MOZ_ASSERT(SessionHistoryEntry::GetByLoadId(mLoadId)->mEntry == aEntry);
+}
+
+LoadingSessionHistoryInfo::LoadingSessionHistoryInfo(
+    const SessionHistoryInfo& aInfo)
+    : mInfo(aInfo), mLoadId(UINT64_MAX) {}
+
+already_AddRefed<nsDocShellLoadState>
+LoadingSessionHistoryInfo::CreateLoadInfo() const {
+  RefPtr<nsDocShellLoadState> loadState(
+      new nsDocShellLoadState(mInfo.GetURI()));
+
+  mInfo.FillLoadInfo(*loadState);
+
+  loadState->SetLoadingSessionHistoryInfo(*this);
+
+  return loadState.forget();
+}
+
+static uint32_t gEntryID;
+
+SessionHistoryEntry::LoadingEntry* SessionHistoryEntry::GetByLoadId(
+    uint64_t aLoadId) {
+  MOZ_ASSERT(XRE_IsParentProcess());
+  if (!sLoadIdToEntry) {
+    return nullptr;
+  }
+
+  return sLoadIdToEntry->Lookup(aLoadId).DataPtrOrNull();
+}
+
+void SessionHistoryEntry::SetByLoadId(uint64_t aLoadId,
+                                      SessionHistoryEntry* aEntry) {
+  if (!sLoadIdToEntry) {
+    sLoadIdToEntry = new nsTHashMap<nsUint64HashKey, LoadingEntry>();
+  }
+
+  MOZ_LOG(
+      gSHLog, LogLevel::Verbose,
+      ("SessionHistoryEntry::SetByLoadId(%" PRIu64 " - %p)", aLoadId, aEntry));
+  sLoadIdToEntry->InsertOrUpdate(
+      aLoadId, LoadingEntry{
+                   .mEntry = aEntry,
+                   .mInfoSnapshotForValidation =
+                       MakeUnique<SessionHistoryInfo>(aEntry->Info()),
+               });
+}
+
+void SessionHistoryEntry::RemoveLoadId(uint64_t aLoadId) {
+  MOZ_ASSERT(XRE_IsParentProcess());
+  if (!sLoadIdToEntry) {
+    return;
+  }
+
+  MOZ_LOG(gSHLog, LogLevel::Verbose,
+          ("SHEntry::RemoveLoadId(%" PRIu64 ")", aLoadId));
+  sLoadIdToEntry->Remove(aLoadId);
+}
+
+SessionHistoryEntry::SessionHistoryEntry()
+    : mInfo(new SessionHistoryInfo()), mID(++gEntryID) {
+  MOZ_ASSERT(mozilla::SessionHistoryInParent());
+}
+
+SessionHistoryEntry::SessionHistoryEntry(nsDocShellLoadState* aLoadState,
+                                         nsIChannel* aChannel)
+    : mInfo(new SessionHistoryInfo(aLoadState, aChannel)), mID(++gEntryID) {
+  MOZ_ASSERT(mozilla::SessionHistoryInParent());
+}
+
+SessionHistoryEntry::SessionHistoryEntry(SessionHistoryInfo* aInfo)
+    : mInfo(MakeUnique<SessionHistoryInfo>(*aInfo)), mID(++gEntryID) {
+  MOZ_ASSERT(mozilla::SessionHistoryInParent());
+}
+
+SessionHistoryEntry::SessionHistoryEntry(const SessionHistoryEntry& aEntry)
+    : mInfo(MakeUnique<SessionHistoryInfo>(*aEntry.mInfo)),
+      mParent(aEntry.mParent),
+      mID(aEntry.mID),
+      mBCHistoryLength(aEntry.mBCHistoryLength) {
+  MOZ_ASSERT(mozilla::SessionHistoryInParent());
+}
+
+SessionHistoryEntry::~SessionHistoryEntry() {
+  // Null out the mParent pointers on all our kids.
+  for (nsISHEntry* entry : mChildren) {
+    if (entry) {
+      entry->SetParent(nullptr);
+    }
+  }
+
+  if (sLoadIdToEntry) {
+    sLoadIdToEntry->RemoveIf(
+        [this](auto& aIter) { return aIter.Data().mEntry == this; });
+    if (sLoadIdToEntry->IsEmpty()) {
+      delete sLoadIdToEntry;
+      sLoadIdToEntry = nullptr;
+    }
+  }
+}
+
+NS_IMPL_ISUPPORTS(SessionHistoryEntry, nsISHEntry, SessionHistoryEntry,
+                  nsISupportsWeakReference)
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetURI(nsIURI** aURI) {
+  nsCOMPtr<nsIURI> uri = mInfo->mURI;
+  uri.forget(aURI);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetURI(nsIURI* aURI) {
+  mInfo->mURI = aURI;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetOriginalURI(nsIURI** aOriginalURI) {
+  nsCOMPtr<nsIURI> originalURI = mInfo->mOriginalURI;
+  originalURI.forget(aOriginalURI);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetOriginalURI(nsIURI* aOriginalURI) {
+  mInfo->mOriginalURI = aOriginalURI;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetResultPrincipalURI(nsIURI** aResultPrincipalURI) {
+  nsCOMPtr<nsIURI> resultPrincipalURI = mInfo->mResultPrincipalURI;
+  resultPrincipalURI.forget(aResultPrincipalURI);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetResultPrincipalURI(nsIURI* aResultPrincipalURI) {
+  mInfo->mResultPrincipalURI = aResultPrincipalURI;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetUnstrippedURI(nsIURI** aUnstrippedURI) {
+  nsCOMPtr<nsIURI> unstrippedURI = mInfo->mUnstrippedURI;
+  unstrippedURI.forget(aUnstrippedURI);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetUnstrippedURI(nsIURI* aUnstrippedURI) {
+  mInfo->mUnstrippedURI = aUnstrippedURI;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetLoadReplace(bool* aLoadReplace) {
+  *aLoadReplace = mInfo->mLoadReplace;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetLoadReplace(bool aLoadReplace) {
+  mInfo->mLoadReplace = aLoadReplace;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetTitle(nsAString& aTitle) {
+  aTitle = mInfo->mTitle;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetTitle(const nsAString& aTitle) {
+  mInfo->SetTitle(aTitle);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetName(nsAString& aName) {
+  aName = mInfo->mName;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetName(const nsAString& aName) {
+  mInfo->mName = aName;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetIsSubFrame(bool* aIsSubFrame) {
+  *aIsSubFrame = SharedInfo()->mIsFrameNavigation;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetIsSubFrame(bool aIsSubFrame) {
+  SharedInfo()->mIsFrameNavigation = aIsSubFrame;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetHasUserInteraction(bool* aFlag) {
+  // The back button and menulist deal with root/top-level
+  // session history entries, thus we annotate only the root entry.
+  if (!mParent) {
+    *aFlag = mInfo->mHasUserInteraction;
+  } else {
+    nsCOMPtr<nsISHEntry> root = nsSHistory::GetRootSHEntry(this);
+    root->GetHasUserInteraction(aFlag);
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetHasUserInteraction(bool aFlag) {
+  // The back button and menulist deal with root/top-level
+  // session history entries, thus we annotate only the root entry.
+  if (!mParent) {
+    mInfo->mHasUserInteraction = aFlag;
+  } else {
+    nsCOMPtr<nsISHEntry> root = nsSHistory::GetRootSHEntry(this);
+    root->SetHasUserInteraction(aFlag);
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetHasUserActivation(bool* aFlag) {
+  *aFlag = mInfo->mHasUserActivation;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetHasUserActivation(bool aFlag) {
+  mInfo->mHasUserActivation = aFlag;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetReferrerInfo(nsIReferrerInfo** aReferrerInfo) {
+  nsCOMPtr<nsIReferrerInfo> referrerInfo = mInfo->mReferrerInfo;
+  referrerInfo.forget(aReferrerInfo);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetReferrerInfo(nsIReferrerInfo* aReferrerInfo) {
+  mInfo->mReferrerInfo = aReferrerInfo;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetContentViewer(nsIContentViewer** aContentViewer) {
+  *aContentViewer = nullptr;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetContentViewer(nsIContentViewer* aContentViewer) {
+  MOZ_CRASH("This lives in the child process");
+  return NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetIsInBFCache(bool* aResult) {
+  *aResult = !!SharedInfo()->mFrameLoader;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetSticky(bool* aSticky) {
+  *aSticky = SharedInfo()->mSticky;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetSticky(bool aSticky) {
+  SharedInfo()->mSticky = aSticky;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetWindowState(nsISupports** aWindowState) {
+  MOZ_CRASH("This lives in the child process");
+  return NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetWindowState(nsISupports* aWindowState) {
+  MOZ_CRASH("This lives in the child process");
+  return NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetRefreshURIList(nsIMutableArray** aRefreshURIList) {
+  MOZ_CRASH("This lives in the child process");
+  return NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetRefreshURIList(nsIMutableArray* aRefreshURIList) {
+  MOZ_CRASH("This lives in the child process");
+  return NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetPostData(nsIInputStream** aPostData) {
+  *aPostData = mInfo->GetPostData().take();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetPostData(nsIInputStream* aPostData) {
+  mInfo->SetPostData(aPostData);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetHasPostData(bool* aResult) {
+  *aResult = mInfo->HasPostData();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetLayoutHistoryState(
+    nsILayoutHistoryState** aLayoutHistoryState) {
+  nsCOMPtr<nsILayoutHistoryState> layoutHistoryState =
+      SharedInfo()->mLayoutHistoryState;
+  layoutHistoryState.forget(aLayoutHistoryState);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetLayoutHistoryState(
+    nsILayoutHistoryState* aLayoutHistoryState) {
+  SharedInfo()->mLayoutHistoryState = aLayoutHistoryState;
+  if (SharedInfo()->mLayoutHistoryState) {
+    SharedInfo()->mLayoutHistoryState->SetScrollPositionOnly(
+        !SharedInfo()->mSaveLayoutState);
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetParent(nsISHEntry** aParent) {
+  nsCOMPtr<nsISHEntry> parent = do_QueryReferent(mParent);
+  parent.forget(aParent);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetParent(nsISHEntry* aParent) {
+  mParent = do_GetWeakReference(aParent);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetLoadType(uint32_t* aLoadType) {
+  *aLoadType = mInfo->mLoadType;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetLoadType(uint32_t aLoadType) {
+  mInfo->mLoadType = aLoadType;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetID(uint32_t* aID) {
+  *aID = mID;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetID(uint32_t aID) {
+  mID = aID;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetCacheKey(uint32_t* aCacheKey) {
+  *aCacheKey = SharedInfo()->mCacheKey;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetCacheKey(uint32_t aCacheKey) {
+  SharedInfo()->mCacheKey = aCacheKey;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetSaveLayoutStateFlag(bool* aSaveLayoutStateFlag) {
+  *aSaveLayoutStateFlag = SharedInfo()->mSaveLayoutState;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetSaveLayoutStateFlag(bool aSaveLayoutStateFlag) {
+  SharedInfo()->mSaveLayoutState = aSaveLayoutStateFlag;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetContentType(nsACString& aContentType) {
+  aContentType = SharedInfo()->mContentType;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetContentType(const nsACString& aContentType) {
+  SharedInfo()->mContentType = aContentType;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetURIWasModified(bool* aURIWasModified) {
+  *aURIWasModified = mInfo->mURIWasModified;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetURIWasModified(bool aURIWasModified) {
+  mInfo->mURIWasModified = aURIWasModified;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetTriggeringPrincipal(
+    nsIPrincipal** aTriggeringPrincipal) {
+  nsCOMPtr<nsIPrincipal> triggeringPrincipal =
+      SharedInfo()->mTriggeringPrincipal;
+  triggeringPrincipal.forget(aTriggeringPrincipal);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetTriggeringPrincipal(
+    nsIPrincipal* aTriggeringPrincipal) {
+  SharedInfo()->mTriggeringPrincipal = aTriggeringPrincipal;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetPrincipalToInherit(nsIPrincipal** aPrincipalToInherit) {
+  nsCOMPtr<nsIPrincipal> principalToInherit = SharedInfo()->mPrincipalToInherit;
+  principalToInherit.forget(aPrincipalToInherit);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetPrincipalToInherit(nsIPrincipal* aPrincipalToInherit) {
+  SharedInfo()->mPrincipalToInherit = aPrincipalToInherit;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetPartitionedPrincipalToInherit(
+    nsIPrincipal** aPartitionedPrincipalToInherit) {
+  nsCOMPtr<nsIPrincipal> partitionedPrincipalToInherit =
+      SharedInfo()->mPartitionedPrincipalToInherit;
+  partitionedPrincipalToInherit.forget(aPartitionedPrincipalToInherit);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetPartitionedPrincipalToInherit(
+    nsIPrincipal* aPartitionedPrincipalToInherit) {
+  SharedInfo()->mPartitionedPrincipalToInherit = aPartitionedPrincipalToInherit;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetCsp(nsIContentSecurityPolicy** aCsp) {
+  nsCOMPtr<nsIContentSecurityPolicy> csp = SharedInfo()->mCsp;
+  csp.forget(aCsp);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetCsp(nsIContentSecurityPolicy* aCsp) {
+  SharedInfo()->mCsp = aCsp;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetStateData(nsIStructuredCloneContainer** aStateData) {
+  RefPtr<nsStructuredCloneContainer> stateData = mInfo->mStateData;
+  stateData.forget(aStateData);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetStateData(nsIStructuredCloneContainer* aStateData) {
+  mInfo->mStateData = static_cast<nsStructuredCloneContainer*>(aStateData);
+  return NS_OK;
+}
+
+const nsID& SessionHistoryEntry::DocshellID() const {
+  return SharedInfo()->mDocShellID;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetDocshellID(nsID& aDocshellID) {
+  aDocshellID = DocshellID();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetDocshellID(const nsID& aDocshellID) {
+  SharedInfo()->mDocShellID = aDocshellID;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetIsSrcdocEntry(bool* aIsSrcdocEntry) {
+  *aIsSrcdocEntry = mInfo->mSrcdocData.isSome();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetSrcdocData(nsAString& aSrcdocData) {
+  aSrcdocData = mInfo->mSrcdocData.valueOr(EmptyString());
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetSrcdocData(const nsAString& aSrcdocData) {
+  mInfo->mSrcdocData = Some(nsString(aSrcdocData));
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetBaseURI(nsIURI** aBaseURI) {
+  nsCOMPtr<nsIURI> baseURI = mInfo->mBaseURI;
+  baseURI.forget(aBaseURI);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetBaseURI(nsIURI* aBaseURI) {
+  mInfo->mBaseURI = aBaseURI;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetScrollRestorationIsManual(
+    bool* aScrollRestorationIsManual) {
+  *aScrollRestorationIsManual = mInfo->mScrollRestorationIsManual;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetScrollRestorationIsManual(
+    bool aScrollRestorationIsManual) {
+  mInfo->mScrollRestorationIsManual = aScrollRestorationIsManual;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetLoadedInThisProcess(bool* aLoadedInThisProcess) {
+  // FIXME
+  //*aLoadedInThisProcess = mInfo->mLoadedInThisProcess;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetShistory(nsISHistory** aShistory) {
+  nsCOMPtr<nsISHistory> sHistory = do_QueryReferent(SharedInfo()->mSHistory);
+  sHistory.forget(aShistory);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetShistory(nsISHistory* aShistory) {
+  nsWeakPtr shistory = do_GetWeakReference(aShistory);
+  // mSHistory can not be changed once it's set
+  MOZ_ASSERT(!SharedInfo()->mSHistory || (SharedInfo()->mSHistory == shistory));
+  SharedInfo()->mSHistory = shistory;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetLastTouched(uint32_t* aLastTouched) {
+  *aLastTouched = SharedInfo()->mLastTouched;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetLastTouched(uint32_t aLastTouched) {
+  SharedInfo()->mLastTouched = aLastTouched;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetChildCount(int32_t* aChildCount) {
+  *aChildCount = mChildren.Length();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetPersist(bool* aPersist) {
+  *aPersist = mInfo->mPersist;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetPersist(bool aPersist) {
+  mInfo->mPersist = aPersist;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetScrollPosition(int32_t* aX, int32_t* aY) {
+  *aX = mInfo->mScrollPositionX;
+  *aY = mInfo->mScrollPositionY;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetScrollPosition(int32_t aX, int32_t aY) {
+  mInfo->mScrollPositionX = aX;
+  mInfo->mScrollPositionY = aY;
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(void)
+SessionHistoryEntry::GetViewerBounds(nsIntRect& bounds) {
+  bounds = SharedInfo()->mViewerBounds;
+}
+
+NS_IMETHODIMP_(void)
+SessionHistoryEntry::SetViewerBounds(const nsIntRect& bounds) {
+  SharedInfo()->mViewerBounds = bounds;
+}
+
+NS_IMETHODIMP_(void)
+SessionHistoryEntry::AddChildShell(nsIDocShellTreeItem* shell) {
+  MOZ_CRASH("This lives in the child process");
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::ChildShellAt(int32_t index,
+                                  nsIDocShellTreeItem** _retval) {
+  MOZ_CRASH("This lives in the child process");
+  return NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP_(void)
+SessionHistoryEntry::ClearChildShells() {
+  MOZ_CRASH("This lives in the child process");
+}
+
+NS_IMETHODIMP_(void)
+SessionHistoryEntry::SyncPresentationState() {
+  MOZ_CRASH("This lives in the child process");
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::InitLayoutHistoryState(
+    nsILayoutHistoryState** aLayoutHistoryState) {
+  if (!SharedInfo()->mLayoutHistoryState) {
+    nsCOMPtr<nsILayoutHistoryState> historyState;
+    historyState = NS_NewLayoutHistoryState();
+    SetLayoutHistoryState(historyState);
+  }
+
+  return GetLayoutHistoryState(aLayoutHistoryState);
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::Create(
+    nsIURI* aURI, const nsAString& aTitle, nsIInputStream* aInputStream,
+    uint32_t aCacheKey, const nsACString& aContentType,
+    nsIPrincipal* aTriggeringPrincipal, nsIPrincipal* aPrincipalToInherit,
+    nsIPrincipal* aPartitionedPrincipalToInherit,
+    nsIContentSecurityPolicy* aCsp, const nsID& aDocshellID,
+    bool aDynamicCreation, nsIURI* aOriginalURI, nsIURI* aResultPrincipalURI,
+    nsIURI* aUnstrippedURI, bool aLoadReplace, nsIReferrerInfo* aReferrerInfo,
+    const nsAString& aSrcdoc, bool aSrcdocEntry, nsIURI* aBaseURI,
+    bool aSaveLayoutState, bool aExpired, bool aUserActivation) {
+  MOZ_CRASH("Might need to implement this");
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::Clone(nsISHEntry** aEntry) {
+  RefPtr<SessionHistoryEntry> entry = new SessionHistoryEntry(*this);
+
+  // These are not copied for some reason, we're not sure why.
+  entry->mInfo->mLoadType = 0;
+  entry->mInfo->mScrollPositionX = 0;
+  entry->mInfo->mScrollPositionY = 0;
+  entry->mInfo->mScrollRestorationIsManual = false;
+
+  entry->mInfo->mHasUserInteraction = false;
+
+  entry.forget(aEntry);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(nsDocShellEditorData*)
+SessionHistoryEntry::ForgetEditorData() {
+  MOZ_CRASH("This lives in the child process");
+  return nullptr;
+}
+
+NS_IMETHODIMP_(void)
+SessionHistoryEntry::SetEditorData(nsDocShellEditorData* aData) {
+  NS_WARNING("This lives in the child process");
+}
+
+NS_IMETHODIMP_(bool)
+SessionHistoryEntry::HasDetachedEditor() {
+  NS_WARNING("This lives in the child process");
+  return false;
+}
+
+NS_IMETHODIMP_(bool)
+SessionHistoryEntry::IsDynamicallyAdded() {
+  return SharedInfo()->mDynamicallyCreated;
+}
+
+void SessionHistoryEntry::SetWireframe(const Maybe<Wireframe>& aWireframe) {
+  mWireframe = aWireframe;
+}
+
+void SessionHistoryEntry::SetIsDynamicallyAdded(bool aDynamic) {
+  MOZ_ASSERT_IF(SharedInfo()->mDynamicallyCreated, aDynamic);
+  SharedInfo()->mDynamicallyCreated = aDynamic;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::HasDynamicallyAddedChild(bool* aHasDynamicallyAddedChild) {
+  for (const auto& child : mChildren) {
+    if (child && child->IsDynamicallyAdded()) {
+      *aHasDynamicallyAddedChild = true;
+      return NS_OK;
+    }
+  }
+  *aHasDynamicallyAddedChild = false;
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(bool)
+SessionHistoryEntry::HasBFCacheEntry(SHEntrySharedParentState* aEntry) {
+  return SharedInfo() == aEntry;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::AdoptBFCacheEntry(nsISHEntry* aEntry) {
+  nsCOMPtr<SessionHistoryEntry> she = do_QueryInterface(aEntry);
+  NS_ENSURE_STATE(she && she->mInfo->mSharedState.Get());
+
+  mInfo->mSharedState =
+      static_cast<SessionHistoryEntry*>(aEntry)->mInfo->mSharedState;
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::AbandonBFCacheEntry() {
+  MOZ_CRASH("This lives in the child process");
+  return NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SharesDocumentWith(nsISHEntry* aEntry,
+                                        bool* aSharesDocumentWith) {
+  SessionHistoryEntry* entry = static_cast<SessionHistoryEntry*>(aEntry);
+
+  MOZ_ASSERT_IF(entry->SharedInfo() != SharedInfo(),
+                entry->SharedInfo()->GetId() != SharedInfo()->GetId());
+
+  *aSharesDocumentWith = entry->SharedInfo() == SharedInfo();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetLoadTypeAsHistory() {
+  mInfo->mLoadType = LOAD_HISTORY;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::AddChild(nsISHEntry* aChild, int32_t aOffset,
+                              bool aUseRemoteSubframes) {
+  nsCOMPtr<SessionHistoryEntry> child = do_QueryInterface(aChild);
+  MOZ_ASSERT_IF(aChild, child);
+  AddChild(child, aOffset, aUseRemoteSubframes);
+
+  return NS_OK;
+}
+
+void SessionHistoryEntry::AddChild(SessionHistoryEntry* aChild, int32_t aOffset,
+                                   bool aUseRemoteSubframes) {
+  if (aChild) {
+    aChild->SetParent(this);
+  }
+
+  if (aOffset < 0) {
+    mChildren.AppendElement(aChild);
+    return;
+  }
+
+  //
+  // Bug 52670: Ensure children are added in order.
+  //
+  //  Later frames in the child list may load faster and get appended
+  //  before earlier frames, causing session history to be scrambled.
+  //  By growing the list here, they are added to the right position.
+
+  int32_t length = mChildren.Length();
+
+  //  Assert that aOffset will not be so high as to grow us a lot.
+  NS_ASSERTION(aOffset < length + 1023, "Large frames array!\n");
+
+  // If the new child is dynamically added, try to add it to aOffset, but if
+  // there are non-dynamically added children, the child must be after those.
+  if (aChild && aChild->IsDynamicallyAdded()) {
+    int32_t lastNonDyn = aOffset - 1;
+    for (int32_t i = aOffset; i < length; ++i) {
+      SessionHistoryEntry* entry = mChildren[i];
+      if (entry) {
+        if (entry->IsDynamicallyAdded()) {
+          break;
+        }
+
+        lastNonDyn = i;
+      }
+    }
+
+    // If aOffset is larger than Length(), we must first truncate the array.
+    if (aOffset > length) {
+      mChildren.SetLength(aOffset);
+    }
+
+    mChildren.InsertElementAt(lastNonDyn + 1, aChild);
+
+    return;
+  }
+
+  // If the new child isn't dynamically added, it should be set to aOffset.
+  // If there are dynamically added children before that, those must be moved
+  // to be after aOffset.
+  if (length > 0) {
+    int32_t start = std::min(length - 1, aOffset);
+    int32_t dynEntryIndex = -1;
+    DebugOnly<SessionHistoryEntry*> dynEntry = nullptr;
+    for (int32_t i = start; i >= 0; --i) {
+      SessionHistoryEntry* entry = mChildren[i];
+      if (entry) {
+        if (!entry->IsDynamicallyAdded()) {
+          break;
+        }
+
+        dynEntryIndex = i;
+        dynEntry = entry;
+      }
+    }
+
+    if (dynEntryIndex >= 0) {
+      mChildren.InsertElementsAt(dynEntryIndex, aOffset - dynEntryIndex + 1);
+      NS_ASSERTION(mChildren[aOffset + 1] == dynEntry, "Whaat?");
+    }
+  }
+
+  // Make sure there isn't anything at aOffset.
+  if ((uint32_t)aOffset < mChildren.Length()) {
+    SessionHistoryEntry* oldChild = mChildren[aOffset];
+    if (oldChild && oldChild != aChild) {
+      // Under Fission, this can happen when a network-created iframe starts
+      // out in-process, moves out-of-process, and then switches back. At that
+      // point, we'll create a new network-created DocShell at the same index
+      // where we already have an entry for the original network-created
+      // DocShell.
+      //
+      // This should ideally stop being an issue once the Fission-aware
+      // session history rewrite is complete.
+      NS_ASSERTION(
+          aUseRemoteSubframes || NS_IsAboutBlank(oldChild->Info().GetURI()),
+          "Adding a child where we already have a child? This may misbehave");
+      oldChild->SetParent(nullptr);
+    }
+  } else {
+    mChildren.SetLength(aOffset + 1);
+  }
+
+  mChildren.ReplaceElementAt(aOffset, aChild);
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::RemoveChild(nsISHEntry* aChild) {
+  NS_ENSURE_TRUE(aChild, NS_ERROR_FAILURE);
+
+  nsCOMPtr<SessionHistoryEntry> child = do_QueryInterface(aChild);
+  MOZ_ASSERT(child);
+  RemoveChild(child);
+
+  return NS_OK;
+}
+
+void SessionHistoryEntry::RemoveChild(SessionHistoryEntry* aChild) {
+  bool childRemoved = false;
+  if (aChild->IsDynamicallyAdded()) {
+    childRemoved = mChildren.RemoveElement(aChild);
+  } else {
+    int32_t index = mChildren.IndexOf(aChild);
+    if (index >= 0) {
+      // Other alive non-dynamic child docshells still keep mChildOffset,
+      // so we don't want to change the indices here.
+      mChildren.ReplaceElementAt(index, nullptr);
+      childRemoved = true;
+    }
+  }
+
+  if (childRemoved) {
+    aChild->SetParent(nullptr);
+
+    // reduce the child count, i.e. remove empty children at the end
+    for (int32_t i = mChildren.Length() - 1; i >= 0 && !mChildren[i]; --i) {
+      mChildren.RemoveElementAt(i);
+    }
+  }
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetChildAt(int32_t aIndex, nsISHEntry** aChild) {
+  nsCOMPtr<nsISHEntry> child = mChildren.SafeElementAt(aIndex);
+  child.forget(aChild);
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(void)
+SessionHistoryEntry::GetChildSHEntryIfHasNoDynamicallyAddedChild(
+    int32_t aChildOffset, nsISHEntry** aChild) {
+  *aChild = nullptr;
+
+  bool dynamicallyAddedChild = false;
+  HasDynamicallyAddedChild(&dynamicallyAddedChild);
+  if (dynamicallyAddedChild) {
+    return;
+  }
+
+  // If the user did a shift-reload on this frameset page,
+  // we don't want to load the subframes from history.
+  if (IsForceReloadType(mInfo->mLoadType) || mInfo->mLoadType == LOAD_REFRESH) {
+    return;
+  }
+
+  /* Before looking for the subframe's url, check
+   * the expiration status of the parent. If the parent
+   * has expired from cache, then subframes will not be
+   * loaded from history in certain situations.
+   * If the user pressed reload and the parent frame has expired
+   *  from cache, we do not want to load the child frame from history.
+   */
+  if (SharedInfo()->mExpired && (mInfo->mLoadType == LOAD_RELOAD_NORMAL)) {
+    // The parent has expired. Return null.
+    *aChild = nullptr;
+    return;
+  }
+  // Get the child subframe from session history.
+  GetChildAt(aChildOffset, aChild);
+  if (*aChild) {
+    // Set the parent's Load Type on the child
+    (*aChild)->SetLoadType(mInfo->mLoadType);
+  }
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::ReplaceChild(nsISHEntry* aNewChild) {
+  NS_ENSURE_STATE(aNewChild);
+
+  nsCOMPtr<SessionHistoryEntry> newChild = do_QueryInterface(aNewChild);
+  MOZ_ASSERT(newChild);
+  return ReplaceChild(newChild) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+bool SessionHistoryEntry::ReplaceChild(SessionHistoryEntry* aNewChild) {
+  const nsID& docshellID = aNewChild->DocshellID();
+
+  for (uint32_t i = 0; i < mChildren.Length(); ++i) {
+    if (mChildren[i] && docshellID == mChildren[i]->DocshellID()) {
+      mChildren[i]->SetParent(nullptr);
+      mChildren.ReplaceElementAt(i, aNewChild);
+      aNewChild->SetParent(this);
+
+      return true;
+    }
+  }
+
+  return false;
+}
+
+NS_IMETHODIMP_(void)
+SessionHistoryEntry::ClearEntry() {
+  int32_t childCount = GetChildCount();
+  // Remove all children of this entry
+  for (int32_t i = childCount; i > 0; --i) {
+    nsCOMPtr<nsISHEntry> child;
+    GetChildAt(i - 1, getter_AddRefs(child));
+    RemoveChild(child);
+  }
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::CreateLoadInfo(nsDocShellLoadState** aLoadState) {
+  NS_WARNING("We shouldn't be calling this!");
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetBfcacheID(uint64_t* aBfcacheID) {
+  *aBfcacheID = SharedInfo()->mId;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::GetWireframe(JSContext* aCx,
+                                  JS::MutableHandle<JS::Value> aOut) {
+  if (mWireframe.isNothing()) {
+    aOut.set(JS::NullValue());
+  } else if (NS_WARN_IF(!mWireframe->ToObjectInternal(aCx, aOut))) {
+    return NS_ERROR_FAILURE;
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SessionHistoryEntry::SetWireframe(JSContext* aCx, JS::Handle<JS::Value> aArg) {
+  if (aArg.isNullOrUndefined()) {
+    mWireframe = Nothing();
+    return NS_OK;
+  }
+
+  Wireframe wireframe;
+  if (aArg.isObject() && wireframe.Init(aCx, aArg)) {
+    mWireframe = Some(std::move(wireframe));
+    return NS_OK;
+  }
+
+  return NS_ERROR_INVALID_ARG;
+}
+
+NS_IMETHODIMP_(void)
+SessionHistoryEntry::SyncTreesForSubframeNavigation(
+    nsISHEntry* aEntry, mozilla::dom::BrowsingContext* aTopBC,
+    mozilla::dom::BrowsingContext* aIgnoreBC) {
+  // XXX Keep this in sync with nsSHEntry::SyncTreesForSubframeNavigation.
+  //
+  // We need to sync up the browsing context and session history trees for
+  // subframe navigation.  If the load was in a subframe, we forward up to
+  // the top browsing context, which will then recursively sync up all browsing
+  // contexts to their corresponding entries in the new session history tree. If
+  // we don't do this, then we can cache a content viewer on the wrong cloned
+  // entry, and subsequently restore it at the wrong time.
+  nsCOMPtr<nsISHEntry> newRootEntry = nsSHistory::GetRootSHEntry(aEntry);
+  if (newRootEntry) {
+    // newRootEntry is now the new root entry.
+    // Find the old root entry as well.
+
+    // Need a strong ref. on |oldRootEntry| so it isn't destroyed when
+    // SetChildHistoryEntry() does SwapHistoryEntries() (bug 304639).
+    nsCOMPtr<nsISHEntry> oldRootEntry = nsSHistory::GetRootSHEntry(this);
+
+    if (oldRootEntry) {
+      nsSHistory::SwapEntriesData data = {aIgnoreBC, newRootEntry, nullptr};
+      nsSHistory::SetChildHistoryEntry(oldRootEntry, aTopBC, 0, &data);
+    }
+  }
+}
+
+void SessionHistoryEntry::ReplaceWith(const SessionHistoryEntry& aSource) {
+  mInfo = MakeUnique<SessionHistoryInfo>(*aSource.mInfo);
+  mChildren.Clear();
+}
+
+SHEntrySharedParentState* SessionHistoryEntry::SharedInfo() const {
+  return static_cast<SHEntrySharedParentState*>(mInfo->mSharedState.Get());
+}
+
+void SessionHistoryEntry::SetFrameLoader(nsFrameLoader* aFrameLoader) {
+  MOZ_DIAGNOSTIC_ASSERT(!aFrameLoader || !SharedInfo()->mFrameLoader);
+  // If the pref is disabled, we still allow evicting the existing entries.
+  MOZ_RELEASE_ASSERT(!aFrameLoader || mozilla::BFCacheInParent());
+  SharedInfo()->SetFrameLoader(aFrameLoader);
+  if (aFrameLoader) {
+    if (BrowsingContext* bc = aFrameLoader->GetMaybePendingBrowsingContext()) {
+      bc->PreOrderWalk([&](BrowsingContext* aContext) {
+        if (BrowserParent* bp = aContext->Canonical()->GetBrowserParent()) {
+          bp->Deactivated();
+        }
+      });
+    }
+
+    // When a new frameloader is stored, try to evict some older
+    // frameloaders. Non-SHIP session history has a similar call in
+    // nsDocumentViewer::Show.
+    nsCOMPtr<nsISHistory> shistory;
+    GetShistory(getter_AddRefs(shistory));
+    if (shistory) {
+      int32_t index = 0;
+      shistory->GetIndex(&index);
+      shistory->EvictOutOfRangeContentViewers(index);
+    }
+  }
+}
+
+nsFrameLoader* SessionHistoryEntry::GetFrameLoader() {
+  return SharedInfo()->mFrameLoader;
+}
+
+void SessionHistoryEntry::SetInfo(SessionHistoryInfo* aInfo) {
+  // FIXME Assert that we're not changing shared state!
+  mInfo = MakeUnique<SessionHistoryInfo>(*aInfo);
+}
+
+}  // namespace dom
+
+namespace ipc {
+
+void IPDLParamTraits<dom::SessionHistoryInfo>::Write(
+    IPC::MessageWriter* aWriter, IProtocol* aActor,
+    const dom::SessionHistoryInfo& aParam) {
+  nsCOMPtr<nsIInputStream> postData = aParam.GetPostData();
+
+  Maybe<std::tuple<uint32_t, dom::ClonedMessageData>> stateData;
+  if (aParam.mStateData) {
+    stateData.emplace();
+    // FIXME: We should fail more aggressively if this fails, as currently we'll
+    // just early return and the deserialization will break.
+    NS_ENSURE_SUCCESS_VOID(
+        aParam.mStateData->GetFormatVersion(&std::get<0>(*stateData)));
+    NS_ENSURE_TRUE_VOID(
+        aParam.mStateData->BuildClonedMessageData(std::get<1>(*stateData)));
+  }
+
+  WriteIPDLParam(aWriter, aActor, aParam.mURI);
+  WriteIPDLParam(aWriter, aActor, aParam.mOriginalURI);
+  WriteIPDLParam(aWriter, aActor, aParam.mResultPrincipalURI);
+  WriteIPDLParam(aWriter, aActor, aParam.mUnstrippedURI);
+  WriteIPDLParam(aWriter, aActor, aParam.mReferrerInfo);
+  WriteIPDLParam(aWriter, aActor, aParam.mTitle);
+  WriteIPDLParam(aWriter, aActor, aParam.mName);
+  WriteIPDLParam(aWriter, aActor, postData);
+  WriteIPDLParam(aWriter, aActor, aParam.mLoadType);
+  WriteIPDLParam(aWriter, aActor, aParam.mScrollPositionX);
+  WriteIPDLParam(aWriter, aActor, aParam.mScrollPositionY);
+  WriteIPDLParam(aWriter, aActor, stateData);
+  WriteIPDLParam(aWriter, aActor, aParam.mSrcdocData);
+  WriteIPDLParam(aWriter, aActor, aParam.mBaseURI);
+  WriteIPDLParam(aWriter, aActor, aParam.mLoadReplace);
+  WriteIPDLParam(aWriter, aActor, aParam.mURIWasModified);
+  WriteIPDLParam(aWriter, aActor, aParam.mScrollRestorationIsManual);
+  WriteIPDLParam(aWriter, aActor, aParam.mPersist);
+  WriteIPDLParam(aWriter, aActor, aParam.mHasUserInteraction);
+  WriteIPDLParam(aWriter, aActor, aParam.mHasUserActivation);
+  WriteIPDLParam(aWriter, aActor, aParam.mSharedState.Get()->mId);
+  WriteIPDLParam(aWriter, aActor,
+                 aParam.mSharedState.Get()->mTriggeringPrincipal);
+  WriteIPDLParam(aWriter, aActor,
+                 aParam.mSharedState.Get()->mPrincipalToInherit);
+  WriteIPDLParam(aWriter, aActor,
+                 aParam.mSharedState.Get()->mPartitionedPrincipalToInherit);
+  WriteIPDLParam(aWriter, aActor, aParam.mSharedState.Get()->mCsp);
+  WriteIPDLParam(aWriter, aActor, aParam.mSharedState.Get()->mContentType);
+  WriteIPDLParam(aWriter, aActor,
+                 aParam.mSharedState.Get()->mLayoutHistoryState);
+  WriteIPDLParam(aWriter, aActor, aParam.mSharedState.Get()->mCacheKey);
+  WriteIPDLParam(aWriter, aActor,
+                 aParam.mSharedState.Get()->mIsFrameNavigation);
+  WriteIPDLParam(aWriter, aActor, aParam.mSharedState.Get()->mSaveLayoutState);
+}
+
+bool IPDLParamTraits<dom::SessionHistoryInfo>::Read(
+    IPC::MessageReader* aReader, IProtocol* aActor,
+    dom::SessionHistoryInfo* aResult) {
+  Maybe<std::tuple<uint32_t, dom::ClonedMessageData>> stateData;
+  uint64_t sharedId;
+  if (!ReadIPDLParam(aReader, aActor, &aResult->mURI) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mOriginalURI) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mResultPrincipalURI) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mUnstrippedURI) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mReferrerInfo) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mTitle) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mName) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mPostData) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mLoadType) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mScrollPositionX) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mScrollPositionY) ||
+      !ReadIPDLParam(aReader, aActor, &stateData) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mSrcdocData) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mBaseURI) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mLoadReplace) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mURIWasModified) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mScrollRestorationIsManual) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mPersist) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mHasUserInteraction) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mHasUserActivation) ||
+      !ReadIPDLParam(aReader, aActor, &sharedId)) {
+    aActor->FatalError("Error reading fields for SessionHistoryInfo");
+    return false;
+  }
+
+  nsCOMPtr<nsIPrincipal> triggeringPrincipal;
+  nsCOMPtr<nsIPrincipal> principalToInherit;
+  nsCOMPtr<nsIPrincipal> partitionedPrincipalToInherit;
+  nsCOMPtr<nsIContentSecurityPolicy> csp;
+  nsCString contentType;
+  if (!ReadIPDLParam(aReader, aActor, &triggeringPrincipal) ||
+      !ReadIPDLParam(aReader, aActor, &principalToInherit) ||
+      !ReadIPDLParam(aReader, aActor, &partitionedPrincipalToInherit) ||
+      !ReadIPDLParam(aReader, aActor, &csp) ||
+      !ReadIPDLParam(aReader, aActor, &contentType)) {
+    aActor->FatalError("Error reading fields for SessionHistoryInfo");
+    return false;
+  }
+
+  // We should always see a cloneable input stream passed to SessionHistoryInfo.
+  // This is because it will be cloneable when first read in the parent process
+  // from the nsHttpChannel (which forces streams to be cloneable), and future
+  // streams in content will be wrapped in
+  // nsMIMEInputStream(RemoteLazyInputStream) which is also cloneable.
+  if (aResult->mPostData && !NS_InputStreamIsCloneable(aResult->mPostData)) {
+    aActor->FatalError(
+        "Unexpected non-cloneable postData for SessionHistoryInfo");
+    return false;
+  }
+
+  dom::SHEntrySharedParentState* sharedState = nullptr;
+  if (XRE_IsParentProcess()) {
+    sharedState = dom::SHEntrySharedParentState::Lookup(sharedId);
+  }
+
+  if (sharedState) {
+    aResult->mSharedState.Set(sharedState);
+
+    MOZ_ASSERT(triggeringPrincipal
+                   ? triggeringPrincipal->Equals(
+                         aResult->mSharedState.Get()->mTriggeringPrincipal)
+                   : !aResult->mSharedState.Get()->mTriggeringPrincipal,
+               "We don't expect this to change!");
+    MOZ_ASSERT(principalToInherit
+                   ? principalToInherit->Equals(
+                         aResult->mSharedState.Get()->mPrincipalToInherit)
+                   : !aResult->mSharedState.Get()->mPrincipalToInherit,
+               "We don't expect this to change!");
+    MOZ_ASSERT(
+        partitionedPrincipalToInherit
+            ? partitionedPrincipalToInherit->Equals(
+                  aResult->mSharedState.Get()->mPartitionedPrincipalToInherit)
+            : !aResult->mSharedState.Get()->mPartitionedPrincipalToInherit,
+        "We don't expect this to change!");
+    MOZ_ASSERT(
+        csp ? nsCSPContext::Equals(csp, aResult->mSharedState.Get()->mCsp)
+            : !aResult->mSharedState.Get()->mCsp,
+        "We don't expect this to change!");
+    MOZ_ASSERT(contentType.Equals(aResult->mSharedState.Get()->mContentType),
+               "We don't expect this to change!");
+  } else {
+    aResult->mSharedState.ChangeId(sharedId);
+    aResult->mSharedState.Get()->mTriggeringPrincipal =
+        triggeringPrincipal.forget();
+    aResult->mSharedState.Get()->mPrincipalToInherit =
+        principalToInherit.forget();
+    aResult->mSharedState.Get()->mPartitionedPrincipalToInherit =
+        partitionedPrincipalToInherit.forget();
+    aResult->mSharedState.Get()->mCsp = csp.forget();
+    aResult->mSharedState.Get()->mContentType = contentType;
+  }
+
+  if (!ReadIPDLParam(aReader, aActor,
+                     &aResult->mSharedState.Get()->mLayoutHistoryState) ||
+      !ReadIPDLParam(aReader, aActor,
+                     &aResult->mSharedState.Get()->mCacheKey) ||
+      !ReadIPDLParam(aReader, aActor,
+                     &aResult->mSharedState.Get()->mIsFrameNavigation) ||
+      !ReadIPDLParam(aReader, aActor,
+                     &aResult->mSharedState.Get()->mSaveLayoutState)) {
+    aActor->FatalError("Error reading fields for SessionHistoryInfo");
+    return false;
+  }
+
+  if (stateData.isSome()) {
+    uint32_t version = std::get<0>(*stateData);
+    aResult->mStateData = new nsStructuredCloneContainer(version);
+    aResult->mStateData->StealFromClonedMessageData(std::get<1>(*stateData));
+  }
+  MOZ_ASSERT_IF(stateData.isNothing(), !aResult->mStateData);
+  return true;
+}
+
+void IPDLParamTraits<dom::LoadingSessionHistoryInfo>::Write(
+    IPC::MessageWriter* aWriter, IProtocol* aActor,
+    const dom::LoadingSessionHistoryInfo& aParam) {
+  WriteIPDLParam(aWriter, aActor, aParam.mInfo);
+  WriteIPDLParam(aWriter, aActor, aParam.mLoadId);
+  WriteIPDLParam(aWriter, aActor, aParam.mLoadIsFromSessionHistory);
+  WriteIPDLParam(aWriter, aActor, aParam.mOffset);
+  WriteIPDLParam(aWriter, aActor, aParam.mLoadingCurrentEntry);
+  WriteIPDLParam(aWriter, aActor, aParam.mForceMaybeResetName);
+}
+
+bool IPDLParamTraits<dom::LoadingSessionHistoryInfo>::Read(
+    IPC::MessageReader* aReader, IProtocol* aActor,
+    dom::LoadingSessionHistoryInfo* aResult) {
+  if (!ReadIPDLParam(aReader, aActor, &aResult->mInfo) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mLoadId) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mLoadIsFromSessionHistory) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mOffset) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mLoadingCurrentEntry) ||
+      !ReadIPDLParam(aReader, aActor, &aResult->mForceMaybeResetName)) {
+    aActor->FatalError("Error reading fields for LoadingSessionHistoryInfo");
+    return false;
+  }
+
+  return true;
+}
+
+void IPDLParamTraits<nsILayoutHistoryState*>::Write(
+    IPC::MessageWriter* aWriter, IProtocol* aActor,
+    nsILayoutHistoryState* aParam) {
+  if (aParam) {
+    WriteIPDLParam(aWriter, aActor, true);
+    bool scrollPositionOnly = false;
+    nsTArray<nsCString> keys;
+    nsTArray<mozilla::PresState> states;
+    aParam->GetContents(&scrollPositionOnly, keys, states);
+    WriteIPDLParam(aWriter, aActor, scrollPositionOnly);
+    WriteIPDLParam(aWriter, aActor, keys);
+    WriteIPDLParam(aWriter, aActor, states);
+  } else {
+    WriteIPDLParam(aWriter, aActor, false);
+  }
+}
+
+bool IPDLParamTraits<nsILayoutHistoryState*>::Read(
+    IPC::MessageReader* aReader, IProtocol* aActor,
+    RefPtr<nsILayoutHistoryState>* aResult) {
+  bool hasLayoutHistoryState = false;
+  if (!ReadIPDLParam(aReader, aActor, &hasLayoutHistoryState)) {
+    aActor->FatalError("Error reading fields for nsILayoutHistoryState");
+    return false;
+  }
+
+  if (hasLayoutHistoryState) {
+    bool scrollPositionOnly = false;
+    nsTArray<nsCString> keys;
+    nsTArray<mozilla::PresState> states;
+    if (!ReadIPDLParam(aReader, aActor, &scrollPositionOnly) ||
+        !ReadIPDLParam(aReader, aActor, &keys) ||
+        !ReadIPDLParam(aReader, aActor, &states)) {
+      aActor->FatalError("Error reading fields for nsILayoutHistoryState");
+    }
+
+    if (keys.Length() != states.Length()) {
+      aActor->FatalError("Error reading fields for nsILayoutHistoryState");
+      return false;
+    }
+
+    *aResult = NS_NewLayoutHistoryState();
+    (*aResult)->SetScrollPositionOnly(scrollPositionOnly);
+    for (uint32_t i = 0; i < keys.Length(); ++i) {
+      PresState& state = states[i];
+      UniquePtr<PresState> newState = MakeUnique<PresState>(state);
+      (*aResult)->AddState(keys[i], std::move(newState));
+    }
+  }
+  return true;
+}
+
+void IPDLParamTraits<mozilla::dom::Wireframe>::Write(
+    IPC::MessageWriter* aWriter, IProtocol* aActor,
+    const mozilla::dom::Wireframe& aParam) {
+  WriteParam(aWriter, aParam.mCanvasBackground);
+  WriteParam(aWriter, aParam.mRects);
+}
+
+bool IPDLParamTraits<mozilla::dom::Wireframe>::Read(
+    IPC::MessageReader* aReader, IProtocol* aActor,
+    mozilla::dom::Wireframe* aResult) {
+  return ReadParam(aReader, &aResult->mCanvasBackground) &&
+         ReadParam(aReader, &aResult->mRects);
+}
+
+}  // namespace ipc
+}  // namespace mozilla
+
+namespace IPC {
+// Allow sending mozilla::dom::WireframeRectType enums over IPC.
+template <>
+struct ParamTraits<mozilla::dom::WireframeRectType>
+    : public ContiguousEnumSerializer<
+          mozilla::dom::WireframeRectType,
+          mozilla::dom::WireframeRectType::Image,
+          mozilla::dom::WireframeRectType::EndGuard_> {};
+
+template <>
+struct ParamTraits<mozilla::dom::WireframeTaggedRect> {
+  static void Write(MessageWriter* aWriter,
+                    const mozilla::dom::WireframeTaggedRect& aParam);
+  static bool Read(MessageReader* aReader,
+                   mozilla::dom::WireframeTaggedRect* aResult);
+};
+
+void ParamTraits<mozilla::dom::WireframeTaggedRect>::Write(
+    MessageWriter* aWriter, const mozilla::dom::WireframeTaggedRect& aParam) {
+  WriteParam(aWriter, aParam.mColor);
+  WriteParam(aWriter, aParam.mType);
+  WriteParam(aWriter, aParam.mX);
+  WriteParam(aWriter, aParam.mY);
+  WriteParam(aWriter, aParam.mWidth);
+  WriteParam(aWriter, aParam.mHeight);
+}
+
+bool ParamTraits<mozilla::dom::WireframeTaggedRect>::Read(
+    IPC::MessageReader* aReader, mozilla::dom::WireframeTaggedRect* aResult) {
+  return ReadParam(aReader, &aResult->mColor) &&
+         ReadParam(aReader, &aResult->mType) &&
+         ReadParam(aReader, &aResult->mX) && ReadParam(aReader, &aResult->mY) &&
+         ReadParam(aReader, &aResult->mWidth) &&
+         ReadParam(aReader, &aResult->mHeight);
+}
+}  // namespace IPC
diff --git a/docshell/shistory/SessionHistoryEntry.h b/docshell/shistory/SessionHistoryEntry.h
new file mode 100644
index 0000000000..8eeb5ad2a9
--- /dev/null
+++ b/docshell/shistory/SessionHistoryEntry.h
@@ -0,0 +1,510 @@
+/* -*- 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_SessionHistoryEntry_h
+#define mozilla_dom_SessionHistoryEntry_h
+
+#include "mozilla/dom/DocumentBinding.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/UniquePtr.h"
+#include "nsILayoutHistoryState.h"
+#include "nsISHEntry.h"
+#include "nsSHEntryShared.h"
+#include "nsStructuredCloneContainer.h"
+#include "nsTHashMap.h"
+#include "nsWeakReference.h"
+
+class nsDocShellLoadState;
+class nsIChannel;
+class nsIInputStream;
+class nsIReferrerInfo;
+class nsISHistory;
+class nsIURI;
+
+namespace mozilla::ipc {
+template <typename P>
+struct IPDLParamTraits;
+}
+
+namespace mozilla {
+namespace dom {
+
+struct LoadingSessionHistoryInfo;
+class SessionHistoryEntry;
+class SHEntrySharedParentState;
+
+// SessionHistoryInfo stores session history data for a load. It can be sent
+// over IPC and is used in both the parent and the child processes.
+class SessionHistoryInfo {
+ public:
+  SessionHistoryInfo() = default;
+  SessionHistoryInfo(const SessionHistoryInfo& aInfo) = default;
+  SessionHistoryInfo(nsDocShellLoadState* aLoadState, nsIChannel* aChannel);
+  SessionHistoryInfo(const SessionHistoryInfo& aSharedStateFrom, nsIURI* aURI);
+  SessionHistoryInfo(nsIURI* aURI, nsIPrincipal* aTriggeringPrincipal,
+                     nsIPrincipal* aPrincipalToInherit,
+                     nsIPrincipal* aPartitionedPrincipalToInherit,
+                     nsIContentSecurityPolicy* aCsp,
+                     const nsACString& aContentType);
+  SessionHistoryInfo(nsIChannel* aChannel, uint32_t aLoadType,
+                     nsIPrincipal* aPartitionedPrincipalToInherit,
+                     nsIContentSecurityPolicy* aCsp);
+
+  void Reset(nsIURI* aURI, const nsID& aDocShellID, bool aDynamicCreation,
+             nsIPrincipal* aTriggeringPrincipal,
+             nsIPrincipal* aPrincipalToInherit,
+             nsIPrincipal* aPartitionedPrincipalToInherit,
+             nsIContentSecurityPolicy* aCsp, const nsACString& aContentType);
+
+  bool operator==(const SessionHistoryInfo& aInfo) const {
+    return false;  // FIXME
+  }
+
+  nsIURI* GetURI() const { return mURI; }
+  void SetURI(nsIURI* aURI) { mURI = aURI; }
+
+  nsIURI* GetOriginalURI() const { return mOriginalURI; }
+  void SetOriginalURI(nsIURI* aOriginalURI) { mOriginalURI = aOriginalURI; }
+
+  nsIURI* GetUnstrippedURI() const { return mUnstrippedURI; }
+  void SetUnstrippedURI(nsIURI* aUnstrippedURI) {
+    mUnstrippedURI = aUnstrippedURI;
+  }
+
+  nsIURI* GetResultPrincipalURI() const { return mResultPrincipalURI; }
+  void SetResultPrincipalURI(nsIURI* aResultPrincipalURI) {
+    mResultPrincipalURI = aResultPrincipalURI;
+  }
+
+  nsIReferrerInfo* GetReferrerInfo() { return mReferrerInfo; }
+  void SetReferrerInfo(nsIReferrerInfo* aReferrerInfo) {
+    mReferrerInfo = aReferrerInfo;
+  }
+
+  bool HasPostData() const { return mPostData; }
+  already_AddRefed<nsIInputStream> GetPostData() const;
+  void SetPostData(nsIInputStream* aPostData);
+
+  void GetScrollPosition(int32_t* aScrollPositionX, int32_t* aScrollPositionY) {
+    *aScrollPositionX = mScrollPositionX;
+    *aScrollPositionY = mScrollPositionY;
+  }
+
+  void SetScrollPosition(int32_t aScrollPositionX, int32_t aScrollPositionY) {
+    mScrollPositionX = aScrollPositionX;
+    mScrollPositionY = aScrollPositionY;
+  }
+
+  bool GetScrollRestorationIsManual() const {
+    return mScrollRestorationIsManual;
+  }
+  const nsAString& GetTitle() { return mTitle; }
+  void SetTitle(const nsAString& aTitle) {
+    mTitle = aTitle;
+    MaybeUpdateTitleFromURI();
+  }
+
+  const nsAString& GetName() { return mName; }
+  void SetName(const nsAString& aName) { mName = aName; }
+
+  void SetScrollRestorationIsManual(bool aIsManual) {
+    mScrollRestorationIsManual = aIsManual;
+  }
+
+  nsStructuredCloneContainer* GetStateData() const { return mStateData; }
+  void SetStateData(nsStructuredCloneContainer* aStateData) {
+    mStateData = aStateData;
+  }
+
+  void SetLoadReplace(bool aLoadReplace) { mLoadReplace = aLoadReplace; }
+
+  void SetURIWasModified(bool aURIWasModified) {
+    mURIWasModified = aURIWasModified;
+  }
+  bool GetURIWasModified() const { return mURIWasModified; }
+
+  void SetHasUserInteraction(bool aHasUserInteraction) {
+    mHasUserInteraction = aHasUserInteraction;
+  }
+  bool GetHasUserInteraction() const { return mHasUserInteraction; }
+
+  uint64_t SharedId() const;
+
+  nsILayoutHistoryState* GetLayoutHistoryState();
+  void SetLayoutHistoryState(nsILayoutHistoryState* aState);
+
+  nsIPrincipal* GetTriggeringPrincipal() const;
+
+  nsIPrincipal* GetPrincipalToInherit() const;
+
+  nsIPrincipal* GetPartitionedPrincipalToInherit() const;
+
+  nsIContentSecurityPolicy* GetCsp() const;
+
+  uint32_t GetCacheKey() const;
+  void SetCacheKey(uint32_t aCacheKey);
+
+  bool IsSubFrame() const;
+
+  bool SharesDocumentWith(const SessionHistoryInfo& aOther) const {
+    return SharedId() == aOther.SharedId();
+  }
+
+  void FillLoadInfo(nsDocShellLoadState& aLoadState) const;
+
+  uint32_t LoadType() { return mLoadType; }
+
+  void SetSaveLayoutStateFlag(bool aSaveLayoutStateFlag);
+
+  bool GetPersist() const { return mPersist; }
+
+ private:
+  friend class SessionHistoryEntry;
+  friend struct mozilla::ipc::IPDLParamTraits<SessionHistoryInfo>;
+
+  void MaybeUpdateTitleFromURI();
+
+  nsCOMPtr<nsIURI> mURI;
+  nsCOMPtr<nsIURI> mOriginalURI;
+  nsCOMPtr<nsIURI> mResultPrincipalURI;
+  nsCOMPtr<nsIURI> mUnstrippedURI;
+  nsCOMPtr<nsIReferrerInfo> mReferrerInfo;
+  nsString mTitle;
+  nsString mName;
+  nsCOMPtr<nsIInputStream> mPostData;
+  uint32_t mLoadType = 0;
+  int32_t mScrollPositionX = 0;
+  int32_t mScrollPositionY = 0;
+  RefPtr<nsStructuredCloneContainer> mStateData;
+  Maybe<nsString> mSrcdocData;
+  nsCOMPtr<nsIURI> mBaseURI;
+
+  bool mLoadReplace = false;
+  bool mURIWasModified = false;
+  bool mScrollRestorationIsManual = false;
+  bool mPersist = true;
+  bool mHasUserInteraction = false;
+  bool mHasUserActivation = false;
+
+  union SharedState {
+    SharedState();
+    explicit SharedState(const SharedState& aOther);
+    explicit SharedState(const Maybe<const SharedState&>& aOther);
+    ~SharedState();
+
+    SharedState& operator=(const SharedState& aOther);
+
+    SHEntrySharedState* Get() const;
+
+    void Set(SHEntrySharedParentState* aState) { mParent = aState; }
+
+    void ChangeId(uint64_t aId);
+
+    static SharedState Create(nsIPrincipal* aTriggeringPrincipal,
+                              nsIPrincipal* aPrincipalToInherit,
+                              nsIPrincipal* aPartitionedPrincipalToInherit,
+                              nsIContentSecurityPolicy* aCsp,
+                              const nsACString& aContentType);
+
+   private:
+    explicit SharedState(SHEntrySharedParentState* aParent)
+        : mParent(aParent) {}
+    explicit SharedState(UniquePtr<SHEntrySharedState>&& aChild)
+        : mChild(std::move(aChild)) {}
+
+    void Init();
+    void Init(const SharedState& aOther);
+
+    // In the parent process this holds a strong reference to the refcounted
+    // SHEntrySharedParentState. In the child processes this holds an owning
+    // pointer to a SHEntrySharedState.
+    RefPtr<SHEntrySharedParentState> mParent;
+    UniquePtr<SHEntrySharedState> mChild;
+  };
+
+  SharedState mSharedState;
+};
+
+struct LoadingSessionHistoryInfo {
+  LoadingSessionHistoryInfo() = default;
+  explicit LoadingSessionHistoryInfo(SessionHistoryEntry* aEntry);
+  // Initializes mInfo using aEntry and otherwise copies the values from aInfo.
+  LoadingSessionHistoryInfo(SessionHistoryEntry* aEntry,
+                            const LoadingSessionHistoryInfo* aInfo);
+  // For about:blank only.
+  explicit LoadingSessionHistoryInfo(const SessionHistoryInfo& aInfo);
+
+  already_AddRefed<nsDocShellLoadState> CreateLoadInfo() const;
+
+  SessionHistoryInfo mInfo;
+
+  uint64_t mLoadId = 0;
+
+  // The following three member variables are used to inform about a load from
+  // the session history. The session-history-in-child approach has just
+  // an nsISHEntry in the nsDocShellLoadState and access to the nsISHistory,
+  // but session-history-in-parent needs to pass needed information explicitly
+  // to the relevant child process.
+  bool mLoadIsFromSessionHistory = false;
+  // mOffset and mLoadingCurrentEntry are relevant only if
+  // mLoadIsFromSessionHistory is true.
+  int32_t mOffset = 0;
+  // If we're loading from the current entry we want to treat it as not a
+  // same-document navigation (see nsDocShell::IsSameDocumentNavigation).
+  bool mLoadingCurrentEntry = false;
+  // If mForceMaybeResetName.isSome() is true then the parent process has
+  // determined whether the BC's name should be cleared and stored in session
+  // history (see https://html.spec.whatwg.org/#history-traversal step 4.2).
+  // This is used when we're replacing the BC for BFCache in the parent. In
+  // other cases mForceMaybeResetName.isSome() will be false and the child
+  // process should be able to make that determination itself.
+  Maybe<bool> mForceMaybeResetName;
+};
+
+// HistoryEntryCounterForBrowsingContext is used to count the number of entries
+// which are added to the session history for a particular browsing context.
+// If a SessionHistoryEntry is cloned because of navigation in some other
+// browsing context, that doesn't cause the counter value to be increased.
+// The browsing context specific counter is needed to make it easier to
+// synchronously update history.length value in a child process when
+// an iframe is removed from DOM.
+class HistoryEntryCounterForBrowsingContext {
+ public:
+  HistoryEntryCounterForBrowsingContext()
+      : mCounter(new RefCountedCounter()), mHasModified(false) {
+    ++(*this);
+  }
+
+  HistoryEntryCounterForBrowsingContext(
+      const HistoryEntryCounterForBrowsingContext& aOther)
+      : mCounter(aOther.mCounter), mHasModified(false) {}
+
+  HistoryEntryCounterForBrowsingContext(
+      HistoryEntryCounterForBrowsingContext&& aOther) = delete;
+
+  ~HistoryEntryCounterForBrowsingContext() {
+    if (mHasModified) {
+      --(*mCounter);
+    }
+  }
+
+  void CopyValueFrom(const HistoryEntryCounterForBrowsingContext& aOther) {
+    if (mHasModified) {
+      --(*mCounter);
+    }
+    mCounter = aOther.mCounter;
+    mHasModified = false;
+  }
+
+  HistoryEntryCounterForBrowsingContext& operator=(
+      const HistoryEntryCounterForBrowsingContext& aOther) = delete;
+
+  HistoryEntryCounterForBrowsingContext& operator++() {
+    mHasModified = true;
+    ++(*mCounter);
+    return *this;
+  }
+
+  operator uint32_t() const { return *mCounter; }
+
+  bool Modified() { return mHasModified; }
+
+  void SetModified(bool aModified) { mHasModified = aModified; }
+
+  void Reset() {
+    if (mHasModified) {
+      --(*mCounter);
+    }
+    mCounter = new RefCountedCounter();
+    mHasModified = false;
+  }
+
+ private:
+  class RefCountedCounter {
+   public:
+    NS_INLINE_DECL_REFCOUNTING(
+        mozilla::dom::HistoryEntryCounterForBrowsingContext::RefCountedCounter)
+
+    RefCountedCounter& operator++() {
+      ++mCounter;
+      return *this;
+    }
+
+    RefCountedCounter& operator--() {
+      --mCounter;
+      return *this;
+    }
+
+    operator uint32_t() const { return mCounter; }
+
+   private:
+    ~RefCountedCounter() = default;
+
+    uint32_t mCounter = 0;
+  };
+
+  RefPtr<RefCountedCounter> mCounter;
+  bool mHasModified;
+};
+
+// SessionHistoryEntry is used to store session history data in the parent
+// process. It holds a SessionHistoryInfo, some state shared amongst multiple
+// SessionHistoryEntries, a parent and children.
+#define NS_SESSIONHISTORYENTRY_IID                   \
+  {                                                  \
+    0x5b66a244, 0x8cec, 0x4caa, {                    \
+      0xaa, 0x0a, 0x78, 0x92, 0xfd, 0x17, 0xa6, 0x67 \
+    }                                                \
+  }
+
+class SessionHistoryEntry : public nsISHEntry, public nsSupportsWeakReference {
+ public:
+  SessionHistoryEntry(nsDocShellLoadState* aLoadState, nsIChannel* aChannel);
+  SessionHistoryEntry();
+  explicit SessionHistoryEntry(SessionHistoryInfo* aInfo);
+  explicit SessionHistoryEntry(const SessionHistoryEntry& aEntry);
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISHENTRY
+  NS_DECLARE_STATIC_IID_ACCESSOR(NS_SESSIONHISTORYENTRY_IID)
+
+  bool IsInSessionHistory() {
+    SessionHistoryEntry* entry = this;
+    while (nsCOMPtr<SessionHistoryEntry> parent =
+               do_QueryReferent(entry->mParent)) {
+      entry = parent;
+    }
+    return entry->SharedInfo()->mSHistory &&
+           entry->SharedInfo()->mSHistory->IsAlive();
+  }
+
+  void ReplaceWith(const SessionHistoryEntry& aSource);
+
+  const SessionHistoryInfo& Info() const { return *mInfo; }
+
+  SHEntrySharedParentState* SharedInfo() const;
+
+  void SetFrameLoader(nsFrameLoader* aFrameLoader);
+  nsFrameLoader* GetFrameLoader();
+
+  void AddChild(SessionHistoryEntry* aChild, int32_t aOffset,
+                bool aUseRemoteSubframes);
+  void RemoveChild(SessionHistoryEntry* aChild);
+  // Finds the child with the same docshell ID as aNewChild, replaces it with
+  // aNewChild and returns true. If there is no child with the same docshell ID
+  // then it returns false.
+  bool ReplaceChild(SessionHistoryEntry* aNewChild);
+
+  void SetInfo(SessionHistoryInfo* aInfo);
+
+  bool ForInitialLoad() { return mForInitialLoad; }
+  void SetForInitialLoad(bool aForInitialLoad) {
+    mForInitialLoad = aForInitialLoad;
+  }
+
+  const nsID& DocshellID() const;
+
+  HistoryEntryCounterForBrowsingContext& BCHistoryLength() {
+    return mBCHistoryLength;
+  }
+
+  void SetBCHistoryLength(HistoryEntryCounterForBrowsingContext& aCounter) {
+    mBCHistoryLength.CopyValueFrom(aCounter);
+  }
+
+  void ClearBCHistoryLength() { mBCHistoryLength.Reset(); }
+
+  void SetIsDynamicallyAdded(bool aDynamic);
+
+  void SetWireframe(const Maybe<Wireframe>& aWireframe);
+
+  struct LoadingEntry {
+    // A pointer to the entry being loaded. Will be cleared by the
+    // SessionHistoryEntry destructor, at latest.
+    SessionHistoryEntry* mEntry;
+    // Snapshot of the entry's SessionHistoryInfo when the load started, to be
+    // used for validation purposes only.
+    UniquePtr<SessionHistoryInfo> mInfoSnapshotForValidation;
+  };
+
+  // Get an entry based on LoadingSessionHistoryInfo's mLoadId. Parent process
+  // only.
+  static LoadingEntry* GetByLoadId(uint64_t aLoadId);
+  static void SetByLoadId(uint64_t aLoadId, SessionHistoryEntry* aEntry);
+  static void RemoveLoadId(uint64_t aLoadId);
+
+  const nsTArray<RefPtr<SessionHistoryEntry>>& Children() { return mChildren; }
+
+ private:
+  friend struct LoadingSessionHistoryInfo;
+  virtual ~SessionHistoryEntry();
+
+  UniquePtr<SessionHistoryInfo> mInfo;
+  nsWeakPtr mParent;
+  uint32_t mID;
+  nsTArray<RefPtr<SessionHistoryEntry>> mChildren;
+  Maybe<Wireframe> mWireframe;
+
+  bool mForInitialLoad = false;
+
+  HistoryEntryCounterForBrowsingContext mBCHistoryLength;
+
+  static nsTHashMap<nsUint64HashKey, LoadingEntry>* sLoadIdToEntry;
+};
+
+NS_DEFINE_STATIC_IID_ACCESSOR(SessionHistoryEntry, NS_SESSIONHISTORYENTRY_IID)
+
+}  // namespace dom
+
+namespace ipc {
+
+class IProtocol;
+
+// Allow sending SessionHistoryInfo objects over IPC.
+template <>
+struct IPDLParamTraits<dom::SessionHistoryInfo> {
+  static void Write(IPC::MessageWriter* aWriter, IProtocol* aActor,
+                    const dom::SessionHistoryInfo& aParam);
+  static bool Read(IPC::MessageReader* aReader, IProtocol* aActor,
+                   dom::SessionHistoryInfo* aResult);
+};
+
+// Allow sending LoadingSessionHistoryInfo objects over IPC.
+template <>
+struct IPDLParamTraits<dom::LoadingSessionHistoryInfo> {
+  static void Write(IPC::MessageWriter* aWriter, IProtocol* aActor,
+                    const dom::LoadingSessionHistoryInfo& aParam);
+  static bool Read(IPC::MessageReader* aReader, IProtocol* aActor,
+                   dom::LoadingSessionHistoryInfo* aResult);
+};
+
+// Allow sending nsILayoutHistoryState objects over IPC.
+template <>
+struct IPDLParamTraits<nsILayoutHistoryState*> {
+  static void Write(IPC::MessageWriter* aWriter, IProtocol* aActor,
+                    nsILayoutHistoryState* aParam);
+  static bool Read(IPC::MessageReader* aReader, IProtocol* aActor,
+                   RefPtr<nsILayoutHistoryState>* aResult);
+};
+
+// Allow sending dom::Wireframe objects over IPC.
+template <>
+struct IPDLParamTraits<mozilla::dom::Wireframe> {
+  static void Write(IPC::MessageWriter* aWriter, IProtocol* aActor,
+                    const mozilla::dom::Wireframe& aParam);
+  static bool Read(IPC::MessageReader* aReader, IProtocol* aActor,
+                   mozilla::dom::Wireframe* aResult);
+};
+
+}  // namespace ipc
+
+}  // namespace mozilla
+
+inline nsISupports* ToSupports(mozilla::dom::SessionHistoryEntry* aEntry) {
+  return static_cast<nsISHEntry*>(aEntry);
+}
+
+#endif /* mozilla_dom_SessionHistoryEntry_h */
diff --git a/docshell/shistory/moz.build b/docshell/shistory/moz.build
new file mode 100644
index 0000000000..f3b86c207b
--- /dev/null
+++ b/docshell/shistory/moz.build
@@ -0,0 +1,42 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# 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/.
+
+XPIDL_SOURCES += [
+    "nsIBFCacheEntry.idl",
+    "nsISHEntry.idl",
+    "nsISHistory.idl",
+    "nsISHistoryListener.idl",
+]
+
+XPIDL_MODULE = "shistory"
+
+EXPORTS += [
+    "nsSHEntry.h",
+    "nsSHEntryShared.h",
+    "nsSHistory.h",
+]
+
+EXPORTS.mozilla.dom += [
+    "ChildSHistory.h",
+    "SessionHistoryEntry.h",
+]
+
+UNIFIED_SOURCES += [
+    "ChildSHistory.cpp",
+    "nsSHEntry.cpp",
+    "nsSHEntryShared.cpp",
+    "nsSHistory.cpp",
+    "SessionHistoryEntry.cpp",
+]
+
+LOCAL_INCLUDES += [
+    "/docshell/base",
+    "/dom/base",
+]
+
+FINAL_LIBRARY = "xul"
+
+include("/ipc/chromium/chromium-config.mozbuild")
diff --git a/docshell/shistory/nsIBFCacheEntry.idl b/docshell/shistory/nsIBFCacheEntry.idl
new file mode 100644
index 0000000000..2e24c67e35
--- /dev/null
+++ b/docshell/shistory/nsIBFCacheEntry.idl
@@ -0,0 +1,16 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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 "nsISupports.idl"
+
+/**
+ * This interface lets you evict a document from the back/forward cache.
+ */
+[scriptable, builtinclass, uuid(a576060e-c7df-4d81-aa8c-5b52bd6ad25d)]
+interface nsIBFCacheEntry : nsISupports
+{
+  void RemoveFromBFCacheSync();
+  void RemoveFromBFCacheAsync();
+};
diff --git a/docshell/shistory/nsISHEntry.idl b/docshell/shistory/nsISHEntry.idl
new file mode 100644
index 0000000000..28161e8885
--- /dev/null
+++ b/docshell/shistory/nsISHEntry.idl
@@ -0,0 +1,476 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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/. */
+
+/**
+ * The interface to nsISHentry. Each document or subframe in
+ * Session History will have a nsISHEntry associated with it which will
+ * hold all information required to recreate the document from history
+ */
+
+#include "nsISupports.idl"
+
+interface nsIContentSecurityPolicy;
+interface nsIMutableArray;
+interface nsILayoutHistoryState;
+interface nsIContentViewer;
+interface nsIURI;
+interface nsIInputStream;
+interface nsIDocShellTreeItem;
+interface nsIStructuredCloneContainer;
+interface nsIBFCacheEntry;
+interface nsIPrincipal;
+interface nsISHistory;
+interface nsIReferrerInfo;
+
+%{C++
+#include "nsRect.h"
+class nsDocShellEditorData;
+
+namespace mozilla {
+namespace dom {
+
+class SHEntrySharedParentState;
+
+}
+}
+class nsSHEntryShared;
+class nsDocShellLoadState;
+struct EntriesAndBrowsingContextData;
+%}
+[ref] native nsIntRect(nsIntRect);
+[ptr] native nsDocShellEditorDataPtr(nsDocShellEditorData);
+[ptr] native nsDocShellLoadStatePtr(nsDocShellLoadState);
+[ptr] native SHEntrySharedParentStatePtr(mozilla::dom::SHEntrySharedParentState);
+webidl BrowsingContext;
+
+[builtinclass, scriptable, uuid(0dad26b8-a259-42c7-93f1-2fa7fc076e45)]
+interface nsISHEntry : nsISupports
+{
+    /**
+     * The URI of the current entry.
+     */
+    [infallible] attribute nsIURI URI;
+
+    /**
+     * The original URI of the current entry. If an entry is the result of a
+     * redirect this attribute holds the original URI.
+     */
+    [infallible] attribute nsIURI originalURI;
+
+    /**
+     * URL as stored from nsILoadInfo.resultPrincipalURI.  See nsILoadInfo
+     * for more details.
+     */
+    [infallible] attribute nsIURI resultPrincipalURI;
+
+    /**
+     * If non-null, the URI as it was before query stripping was performed.
+     */
+    [infallible] attribute nsIURI unstrippedURI;
+
+    /**
+     *  This flag remembers whether channel has LOAD_REPLACE set.
+     */
+    [infallible] attribute boolean loadReplace;
+
+    /**
+     * The title of the current entry.
+     */
+    // XXX: make it [infallible] when AString supports that (bug 1491187).
+    attribute AString title;
+
+    /**
+     * The name of the browsing context.
+     */
+    attribute AString name;
+
+    /**
+     * Was the entry created as a result of a subframe navigation?
+     * - Will be 'false' when a frameset page is visited for the first time.
+     * - Will be 'true' for all history entries created as a result of a
+     *   subframe navigation.
+     */
+    [infallible] attribute boolean isSubFrame;
+
+    /**
+     * Whether the user interacted with the page while this entry was active.
+     * This includes interactions with subframe documents associated with
+     * child entries that are rooted at this entry.
+     * This field will only be set on top-level entries.
+     */
+    [infallible] attribute boolean hasUserInteraction;
+
+    /**
+     * Whether the load that created this entry was triggered by user activation.
+     * (e.g.: The user clicked a link)
+     * Remembering this flag enables replaying the sec-fetch-* headers.
+     */
+    [infallible] attribute boolean hasUserActivation;
+
+    /** Referrer Info*/
+    [infallible] attribute nsIReferrerInfo referrerInfo;
+
+    /** Content viewer, for fast restoration of presentation */
+    [infallible] attribute nsIContentViewer contentViewer;
+
+    [infallible] readonly attribute boolean isInBFCache;
+
+    /** Whether the content viewer is marked "sticky" */
+    [infallible] attribute boolean sticky;
+
+    /** Saved state of the global window object */
+    [infallible] attribute nsISupports windowState;
+
+    /** Saved refresh URI list for the content viewer */
+    [infallible] attribute nsIMutableArray refreshURIList;
+
+    /** Post Data for the document */
+    [infallible] attribute nsIInputStream postData;
+    [infallible] readonly attribute boolean hasPostData;
+
+    /** LayoutHistoryState for scroll position and form values */
+    [infallible] attribute nsILayoutHistoryState layoutHistoryState;
+
+    /** parent of this entry */
+    [infallible] attribute nsISHEntry parent;
+
+    /**
+     * The loadType for this entry. This is typically loadHistory except
+     * when reload is pressed, it has the appropriate reload flag
+     */
+    [infallible] attribute unsigned long loadType;
+
+    /**
+     * An ID to help identify this entry from others during
+     * subframe navigation
+     */
+    [infallible] attribute unsigned long ID;
+
+    /** The cache key for the entry */
+    [infallible] attribute unsigned long cacheKey;
+
+    /** Should the layoutHistoryState be saved? */
+    [infallible] attribute boolean saveLayoutStateFlag;
+
+    /**
+     * attribute to indicate the content-type of the document that this
+     * is a session history entry for
+     */
+    // XXX: make it [infallible] when ACString supports that (bug 1491187).
+    attribute ACString contentType;
+
+    /**
+     * If we created this SHEntry via history.pushState or modified it via
+     * history.replaceState, and if we changed the SHEntry's URI via the
+     * push/replaceState call, and if the SHEntry's new URI differs from its
+     * old URI by more than just the hash, then we set this field to true.
+     *
+     * Additionally, if this SHEntry was created by calling pushState from a
+     * SHEntry whose URI was modified, this SHEntry's URIWasModified field is
+     * true.
+     */
+    [infallible] attribute boolean URIWasModified;
+
+    /**
+     * Get the principal, if any, that was associated with the channel
+     * that the document that was loaded to create this history entry
+     * came from.
+     */
+    [infallible] attribute nsIPrincipal triggeringPrincipal;
+
+    /**
+     * Get the principal, if any, that is used when the inherit flag
+     * is set.
+     */
+    [infallible] attribute nsIPrincipal principalToInherit;
+
+    /**
+     * Get the storage principal, if any, that is used when the inherit flag is
+     * set.
+     */
+    [infallible] attribute nsIPrincipal partitionedPrincipalToInherit;
+
+    /**
+     * Get the csp, if any, that was used for this document load. That
+     * is not the CSP that was applied to subresource loads within the
+     * document, but the CSP that was applied to this document load.
+     */
+    [infallible] attribute nsIContentSecurityPolicy csp;
+
+    /**
+     * Get/set data associated with this history state via a pushState() call,
+     * serialized using structured clone.
+     **/
+    [infallible] attribute nsIStructuredCloneContainer stateData;
+
+    /**
+     * The history ID of the docshell.
+     */
+    // Would be [infallible], but we don't support that property for nsIDPtr.
+    attribute nsIDRef docshellID;
+
+    /**
+     * True if this SHEntry corresponds to a document created by a srcdoc
+     * iframe. Set when a value is assigned to srcdocData.
+     */
+    [infallible] readonly attribute boolean isSrcdocEntry;
+
+    /**
+     * Contents of the srcdoc attribute in a srcdoc iframe to be loaded instead
+     * of the URI.  Similar to a Data URI, this information is needed to
+     * recreate the document at a later stage.
+     * Setting this sets isSrcdocEntry to true
+     */
+    // XXX: make it [infallible] when AString supports that (bug 1491187).
+    attribute AString srcdocData;
+
+    /**
+     * When isSrcdocEntry is true, this contains the baseURI of the srcdoc
+     * document for use in situations where it cannot otherwise be determined,
+     * for example with view-source.
+     */
+    [infallible] attribute nsIURI baseURI;
+
+    /**
+     * Sets/gets the current scroll restoration state,
+     * if true == "manual", false == "auto".
+     */
+    [infallible] attribute boolean scrollRestorationIsManual;
+
+    /**
+     * Flag to indicate that the history entry was originally loaded in the
+     * current process. This flag does not survive a browser process switch.
+     */
+    [infallible] readonly attribute boolean loadedInThisProcess;
+
+    /**
+     * The session history it belongs to. This is set only on the root entries.
+     */
+    [noscript, infallible] attribute nsISHistory shistory;
+
+    /**
+     * A number that is assigned by the sHistory when the entry is activated
+     */
+    [noscript, infallible] attribute unsigned long lastTouched;
+
+    /**
+     * The current number of nsISHEntries which are immediate children of this
+     * SHEntry.
+     */
+    [infallible] readonly attribute long childCount;
+
+    /**
+     * When an entry is serving is within nsISHistory's array of entries, this
+     * property specifies if it should persist. If not it will be replaced by
+     * new additions to the list.
+     */
+    [infallible] attribute boolean persist;
+
+    /**
+     * Set/Get the visual viewport scroll position if session history is
+     * changed through anchor navigation or pushState.
+     */
+    void setScrollPosition(in long x, in long y);
+    void getScrollPosition(out long x, out long y);
+
+    /**
+     * Saved position and dimensions of the content viewer; we must adjust the
+     * root view's widget accordingly if this has changed when the presentation
+     * is restored.
+     */
+    [noscript, notxpcom] void getViewerBounds(in nsIntRect bounds);
+    [noscript, notxpcom] void setViewerBounds([const] in nsIntRect bounds);
+
+    /**
+     * Saved child docshells corresponding to contentViewer.  The child shells
+     * are restored as children of the parent docshell, in this order, when the
+     * parent docshell restores a saved presentation.
+     */
+
+    /** Append a child shell to the end of our list. */
+    [noscript, notxpcom] void addChildShell(in nsIDocShellTreeItem shell);
+
+    /**
+     * Get the child shell at |index|; returns null if |index| is out of bounds.
+     */
+    [noscript] nsIDocShellTreeItem childShellAt(in long index);
+
+    /**
+     * Clear the child shell list.
+     */
+    [noscript, notxpcom] void clearChildShells();
+
+    /**
+     * Ensure that the cached presentation members are self-consistent.
+     * If either |contentViewer| or |windowState| are null, then all of the
+     * following members are cleared/reset:
+     *  contentViewer, sticky, windowState, viewerBounds, childShells,
+     *  refreshURIList.
+     */
+    [noscript, notxpcom] void syncPresentationState();
+
+    /**
+     * Initialises `layoutHistoryState` if it doesn't already exist
+     * and returns a reference to it.
+     */
+    nsILayoutHistoryState initLayoutHistoryState();
+
+    /** Additional ways to create an entry */
+    [noscript] void create(in nsIURI URI, in AString title,
+                           in nsIInputStream inputStream,
+                           in unsigned long cacheKey,
+                           in ACString contentType,
+                           in nsIPrincipal triggeringPrincipal,
+                           in nsIPrincipal principalToInherit,
+                           in nsIPrincipal partitionedPrincipalToInherit,
+                           in nsIContentSecurityPolicy aCsp,
+                           in nsIDRef docshellID,
+                           in boolean dynamicCreation,
+                           in nsIURI originalURI,
+                           in nsIURI resultPrincipalURI,
+                           in nsIURI unstrippedURI,
+                           in bool loadReplace,
+                           in nsIReferrerInfo referrerInfo,
+                           in AString srcdoc,
+                           in bool srcdocEntry,
+                           in nsIURI baseURI,
+                           in bool saveLayoutState,
+                           in bool expired,
+                           in bool userActivation);
+
+    nsISHEntry clone();
+
+    /**
+     * Gets the owning pointer to the editor data assosicated with
+     * this shistory entry. This forgets its pointer, so free it when
+     * you're done.
+     */
+    [noscript, notxpcom] nsDocShellEditorDataPtr forgetEditorData();
+
+    /**
+     * Sets the owning pointer to the editor data assosicated with
+     * this shistory entry. Unless forgetEditorData() is called, this
+     * shentry will destroy the editor data when it's destroyed.
+     */
+    [noscript, notxpcom] void setEditorData(in nsDocShellEditorDataPtr aData);
+
+    /** Returns true if this shistory entry is storing a detached editor. */
+    [noscript, notxpcom] boolean hasDetachedEditor();
+
+    /**
+     * Returns true if the related docshell was added because of
+     * dynamic addition of an iframe/frame.
+     */
+    [noscript, notxpcom] boolean isDynamicallyAdded();
+
+    /**
+     * Returns true if any of the child entries returns true
+     * when isDynamicallyAdded is called on it.
+     */
+    boolean hasDynamicallyAddedChild();
+
+    /**
+     * Does this SHEntry point to the given BFCache entry? If so, evicting
+     * the BFCache entry will evict the SHEntry, since the two entries
+     * correspond to the same document.
+     */
+    [noscript, notxpcom]
+    boolean hasBFCacheEntry(in SHEntrySharedParentStatePtr aEntry);
+
+    /**
+     * Adopt aEntry's BFCacheEntry, so now both this and aEntry point to
+     * aEntry's BFCacheEntry.
+     */
+    void adoptBFCacheEntry(in nsISHEntry aEntry);
+
+    /**
+     * Create a new BFCache entry and drop our reference to our old one. This
+     * call unlinks this SHEntry from any other SHEntries for its document.
+     */
+    void abandonBFCacheEntry();
+
+    /**
+     * Does this SHEntry correspond to the same document as aEntry? This is
+     * true iff the two SHEntries have the same BFCacheEntry. So in particular,
+     * sharesDocumentWith(aEntry) is guaranteed to return true if it's
+     * preceded by a call to adoptBFCacheEntry(aEntry).
+     */
+    boolean sharesDocumentWith(in nsISHEntry aEntry);
+
+    /**
+     * Sets an SHEntry to reflect that it is a history type load. This is the
+     * equivalent to doing
+     *
+     * shEntry.loadType = 4;
+     *
+     * in js, but is easier to maintain and less opaque.
+     */
+    void setLoadTypeAsHistory();
+
+    /**
+     * Add a new child SHEntry. If offset is -1 adds to the end of the list.
+     */
+    void AddChild(in nsISHEntry aChild, in long aOffset,
+                  [optional,default(false)] in bool aUseRemoteSubframes);
+
+    /**
+     * Remove a child SHEntry.
+     */
+    [noscript] void RemoveChild(in nsISHEntry aChild);
+
+    /**
+     * Get child at an index.
+     */
+    nsISHEntry GetChildAt(in long aIndex);
+
+    /**
+     * If this entry has no dynamically added child, get the child SHEntry
+     * at the given offset. The loadtype of the returned entry is set
+     * to its parent's loadtype.
+     */
+    [notxpcom] void GetChildSHEntryIfHasNoDynamicallyAddedChild(in long aChildOffset,
+                                                                out nsISHEntry aChild);
+
+    /**
+     * Replaces a child which is for the same docshell as aNewChild
+     * with aNewChild.
+     * @throw if nothing was replaced.
+     */
+    [noscript] void ReplaceChild(in nsISHEntry aNewChild);
+
+    /**
+    * Remove all children of this entry and call abandonBFCacheEntry.
+    */
+    [notxpcom] void ClearEntry();
+
+    /**
+     * Create nsDocShellLoadState and fill it with information.
+     * Don't set nsSHEntry here to avoid serializing it.
+     */
+    [noscript] nsDocShellLoadStatePtr CreateLoadInfo();
+
+    [infallible] readonly attribute unsigned long long bfcacheID;
+
+    /**
+     * Sync up the docshell and session history trees for subframe navigation.
+     *
+     * @param aEntry                    new entry
+     * @param aTopBC                    top BC corresponding to the root ancestor
+                                        of the docshell that called this method
+     * @param aIgnoreBC                 current BC
+     */
+    [notxpcom] void SyncTreesForSubframeNavigation(in nsISHEntry aEntry,
+                                                   in BrowsingContext aTopBC,
+                                                   in BrowsingContext aIgnoreBC);
+
+    /**
+     * If browser.history.collectWireframes is true, this will get populated
+     * with a Wireframe upon document navigation / pushState. This will only
+     * be set for nsISHEntry's accessed in the parent process with
+     * sessionHistoryInParent enabled. See Document.webidl for more details on
+     * what a Wireframe is.
+     */
+    [implicit_jscontext] attribute jsval wireframe;
+};
diff --git a/docshell/shistory/nsISHistory.idl b/docshell/shistory/nsISHistory.idl
new file mode 100644
index 0000000000..8ea23693d9
--- /dev/null
+++ b/docshell/shistory/nsISHistory.idl
@@ -0,0 +1,291 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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 "nsISupports.idl"
+
+interface nsIBFCacheEntry;
+interface nsISHEntry;
+interface nsISHistoryListener;
+interface nsIURI;
+webidl BrowsingContext;
+
+%{C++
+#include "nsTArrayForwardDeclare.h"
+#include "mozilla/Maybe.h"
+struct EntriesAndBrowsingContextData;
+namespace mozilla {
+namespace dom {
+class SHEntrySharedParentState;
+} // namespace dom
+} // namespace mozilla
+%}
+
+[ref] native nsDocshellIDArray(nsTArray<nsID>);
+native MaybeInt32(mozilla::Maybe<int32_t>);
+[ptr] native SHEntrySharedParentStatePtr(mozilla::dom::SHEntrySharedParentState);
+/**
+ * An interface to the primary properties of the Session History
+ * component. In an embedded browser environment, the nsIWebBrowser
+ * object creates an instance of session history for each open window.
+ * A handle to the session history object can be obtained from
+ * nsIWebNavigation. In a non-embedded situation, the  owner of the
+ * session history component must create a instance of it and set
+ * it in the nsIWebNavigation object.
+ * This interface is accessible from javascript.
+ */
+
+[builtinclass, scriptable, uuid(7b807041-e60a-4384-935f-af3061d8b815)]
+interface nsISHistory: nsISupports
+{
+  /**
+   * A readonly property of the interface that returns
+   * the number of toplevel documents currently available
+   * in session history.
+   */
+  [infallible] readonly attribute long count;
+
+  /**
+   * The index of the current document in session history. Not infallible
+   * because setting can fail if the assigned value is out of range.
+   */
+  attribute long index;
+
+  /**
+   * A readonly property of the interface that returns
+   * the index of the last document that started to load and
+   * didn't finished yet. When document finishes the loading
+   * value -1 is returned.
+   */
+  [infallible] readonly attribute long requestedIndex;
+
+  /**
+   * Artifically set the |requestedIndex| for this nsISHEntry to the given
+   * index. This is used when resuming a cross-process load from a different
+   * process.
+   */
+  [noscript, notxpcom]
+  void internalSetRequestedIndex(in long aRequestedIndex);
+
+  /**
+   * Get the history entry at a given index. Returns non-null on success.
+   *
+   * @param index             The index value whose entry is requested.
+   *                          The oldest entry is located at index == 0.
+   * @return                  The found entry; never null.
+   */
+  nsISHEntry getEntryAtIndex(in long aIndex);
+
+  /**
+   * Called to purge older documents from history.
+   * Documents can be removed from session history for various
+   * reasons. For example to  control memory usage of the browser, to
+   * prevent users from loading documents from history, to erase evidence of
+   * prior page loads etc...
+   *
+   * @param numEntries        The number of toplevel documents to be
+   *                          purged from history. During purge operation,
+   *                          the latest documents are maintained and older
+   *                          'numEntries' documents are removed from history.
+   * @throws                  <code>NS_SUCCESS_LOSS_OF_INSIGNIFICANT_DATA</code>
+   *                          Purge was vetod.
+   * @throws                  <code>NS_ERROR_FAILURE</code> numEntries is
+   *                          invalid or out of bounds with the size of history.
+   */
+  void purgeHistory(in long aNumEntries);
+
+  /**
+   * Called to register a listener for the session history component.
+   * Listeners are notified when pages are loaded or purged from history.
+   *
+   * @param aListener         Listener object to be notified for all
+   *                          page loads that initiate in session history.
+   *
+   * @note                    A listener object must implement
+   *                          nsISHistoryListener and nsSupportsWeakReference
+   *
+   * @see nsISHistoryListener
+   * @see nsSupportsWeakReference
+   */
+  void addSHistoryListener(in nsISHistoryListener aListener);
+
+  /**
+   * Called to remove a listener for the session history component.
+   * Listeners are notified when pages are loaded from history.
+   *
+   * @param aListener         Listener object to be removed from
+   *                          session history.
+   *
+   * @note                    A listener object must implement
+   *                          nsISHistoryListener and nsSupportsWeakReference
+   * @see nsISHistoryListener
+   * @see nsSupportsWeakReference
+   */
+  void removeSHistoryListener(in nsISHistoryListener aListener);
+
+  void reloadCurrentEntry();
+
+  /**
+   * Load the entry at the particular index.
+   */
+  [noscript]
+  void gotoIndex(in long aIndex, in boolean aUserActivation);
+
+  /**
+   * If an element exists at the particular index and
+   * whether it has user interaction.
+   */
+  [noscript,notxpcom]
+  boolean hasUserInteractionAtIndex(in long aIndex);
+
+  /**
+   * Called to obtain the index to a given history entry.
+   *
+   * @param aEntry            The entry to obtain the index of.
+   *
+   * @return                  <code>NS_OK</code> index for the history entry
+   *                          is obtained successfully.
+   *                          <code>NS_ERROR_FAILURE</code> Error in obtaining
+   *                          index for the given history entry.
+   */
+  [noscript, notxpcom]
+  long getIndexOfEntry(in nsISHEntry aEntry);
+
+  /**
+   * Add a new Entry to the History List.
+   *
+   * @param aEntry            The entry to add.
+   * @param aPersist          If true this specifies that the entry should
+   *                          persist in the list. If false, this means that
+   *                          when new entries are added this element will not
+   *                          appear in the session history list.
+   */
+  void addEntry(in nsISHEntry aEntry, in boolean aPersist);
+
+  /**
+   * Update the index maintained by sessionHistory
+   */
+  void updateIndex();
+
+  /**
+   * Replace the nsISHEntry at a particular index
+   *
+   * @param aIndex            The index at which the entry should be replaced.
+   * @param aReplaceEntry     The replacement entry for the index.
+   */
+  void replaceEntry(in long aIndex, in nsISHEntry aReplaceEntry);
+
+  /**
+   * Notifies all registered session history listeners about an impending
+   * reload.
+   *
+   * @return                  Whether the operation can proceed.
+   */
+  boolean notifyOnHistoryReload();
+
+  /**
+   * Evict content viewers which don't lie in the "safe" range around aIndex.
+   * In practice, this should leave us with no more than gHistoryMaxViewers
+   * viewers associated with this SHistory object.
+   *
+   * Also make sure that the total number of content viewers in all windows is
+   * not greater than our global max; if it is, evict viewers as appropriate.
+   *
+   * @param aIndex           The index around which the "safe" range is
+   *                         centered.  In general, if you just navigated the
+   *                         history, aIndex should be the index history was
+   *                         navigated to.
+   */
+  void evictOutOfRangeContentViewers(in long aIndex);
+
+  /**
+   * Evict the content viewer associated with a bfcache entry that has timed
+   * out.
+   */
+  [noscript, notxpcom]
+  void evictExpiredContentViewerForEntry(in SHEntrySharedParentStatePtr aEntry);
+
+  /**
+   * Evict all the content viewers in this session history
+   */
+  void evictAllContentViewers();
+
+  /**
+   * Add a BFCache entry to expiration tracker so it gets evicted on
+   * expiration.
+   */
+  [noscript, notxpcom]
+  void addToExpirationTracker(in SHEntrySharedParentStatePtr aEntry);
+
+  /**
+   * Remove a BFCache entry from expiration tracker.
+   */
+  [noscript, notxpcom]
+  void removeFromExpirationTracker(in SHEntrySharedParentStatePtr aEntry);
+
+  /**
+   * Remove dynamic entries found at given index.
+   *
+   * @param aIndex           Index to remove dynamic entries from. It will be
+   *                         passed to RemoveEntries as aStartIndex.
+   * @param aEntry (optional)  The entry to start looking in for dynamic
+   *                         entries. Only the dynamic descendants of the
+   *                         entry will be removed. If not given, all dynamic
+   *                         entries at the index will be removed.
+   */
+  [noscript, notxpcom]
+  void RemoveDynEntries(in long aIndex, in nsISHEntry aEntry);
+
+  /**
+   * Similar to RemoveDynEntries, but instead of specifying an index, use the
+   * given BFCacheEntry to find the index and remove dynamic entries from the
+   * index.
+   *
+   * The method takes no effect if the bfcache entry is not or no longer hold
+   * by the SHistory instance.
+   *
+   * @param aEntry           The bfcache entry to look up for index to remove
+   *                         dynamic entries from.
+   */
+  [noscript, notxpcom]
+  void RemoveDynEntriesForBFCacheEntry(in nsIBFCacheEntry aEntry);
+
+  /**
+   * Removes entries from the history if their docshellID is in
+   * aIDs array.
+   */
+  [noscript, notxpcom]
+  void RemoveEntries(in nsDocshellIDArray aIDs, in long aStartIndex);
+
+  /**
+   * Collect docshellIDs from aEntry's children and remove those
+   * entries from history.
+   *
+   * @param aEntry           Children docshellID's will be collected from
+   *                         this entry and passed to RemoveEntries as aIDs.
+  */
+  [noscript, notxpcom]
+  void RemoveFrameEntries(in nsISHEntry aEntry);
+
+  [noscript]
+  void Reload(in unsigned long aReloadFlags);
+
+  [notxpcom] void EnsureCorrectEntryAtCurrIndex(in nsISHEntry aEntry);
+
+  [notxpcom] void EvictContentViewersOrReplaceEntry(in nsISHEntry aNewSHEntry, in bool aReplace);
+
+  nsISHEntry createEntry();
+
+  [noscript] void AddToRootSessionHistory(in bool aCloneChildren, in nsISHEntry aOSHE,
+                                          in BrowsingContext aRootBC, in nsISHEntry aEntry,
+                                          in unsigned long aLoadType,
+                                          in bool aShouldPersist,
+                                          out MaybeInt32 aPreviousEntryIndex,
+                                          out MaybeInt32 aLoadedEntryIndex);
+
+  [noscript] void AddChildSHEntryHelper(in nsISHEntry aCloneRef, in nsISHEntry aNewEntry,
+                                        in BrowsingContext aRootBC, in bool aCloneChildren);
+
+  [noscript, notxpcom] boolean isEmptyOrHasEntriesForSingleTopLevelPage();
+};
diff --git a/docshell/shistory/nsISHistoryListener.idl b/docshell/shistory/nsISHistoryListener.idl
new file mode 100644
index 0000000000..569cb25dca
--- /dev/null
+++ b/docshell/shistory/nsISHistoryListener.idl
@@ -0,0 +1,88 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ *
+ * 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 "nsISupports.idl"
+
+interface nsIURI;
+
+/**
+ * nsISHistoryListener defines the interface one can implement to receive
+ * notifications about activities in session history and (for reloads) to be
+ * able to cancel them.
+ *
+ * A session history listener will be notified when pages are added, removed
+ * and loaded from session history. In the case of reloads, it can prevent them
+ * from happening by returning false from the corresponding callback method.
+ *
+ * A session history listener can be registered on a particular nsISHistory
+ * instance via the nsISHistory::addSHistoryListener() method.
+ *
+ * Listener methods should not alter the session history. Things are likely to
+ * go haywire if they do.
+ */
+[scriptable, uuid(125c0833-746a-400e-9b89-d2d18545c08a)]
+interface nsISHistoryListener : nsISupports
+{
+  /**
+   * Called when a new document is added to session history. New documents are
+   * added to session history by docshell when new pages are loaded in a frame
+   * or content area, for example via nsIWebNavigation::loadURI()
+   *
+   * @param aNewURI     The URI of the document to be added to session history.
+   * @param aOldIndex   The index of the current history item before the
+   *                    operation.
+   */
+  void OnHistoryNewEntry(in nsIURI aNewURI, in long aOldIndex);
+
+  /**
+   * Called before the current document is reloaded, for example due to a
+   * nsIWebNavigation::reload() call.
+   */
+  boolean OnHistoryReload();
+
+  /**
+   * Called before navigating to a session history entry by index, for example,
+   * when nsIWebNavigation::gotoIndex() is called.
+   */
+  void OnHistoryGotoIndex();
+
+  /**
+   * Called before entries are removed from the start of session history.
+   * Entries can be removed from session history for various reasons, for
+   * example to control the memory usage of the browser, to prevent users from
+   * loading documents from history, to erase evidence of prior page loads, etc.
+   *
+   * To purge documents from session history call nsISHistory::PurgeHistory().
+   *
+   * @param aNumEntries  The number of entries being removed.
+   */
+  void OnHistoryPurge(in long aNumEntries);
+
+  /**
+   * Called before entries are removed from the end of session history. This
+   * occurs when navigating to a new page while on a previous session entry.
+   *
+   * @param aNumEntries  The number of entries being removed.
+   */
+  void OnHistoryTruncate(in long aNumEntries);
+
+  /**
+   * Called before an entry is replaced in the session history. Entries are
+   * replaced when navigating away from non-persistent history entries (such as
+   * about pages) and when history.replaceState is called.
+   */
+  void OnHistoryReplaceEntry();
+
+
+  /**
+   * Called whenever a content viewer is evicted. A content viewer is evicted
+   * whenever a bfcache entry has timed out or the number of total content
+   * viewers has exceeded the global max. This is used for testing only.
+   *
+   * @param aNumEvicted - number of content viewers evicted
+   */
+  void OnContentViewerEvicted(in unsigned long aNumEvicted);
+};
diff --git a/docshell/shistory/nsSHEntry.cpp b/docshell/shistory/nsSHEntry.cpp
new file mode 100644
index 0000000000..0b250c33ff
--- /dev/null
+++ b/docshell/shistory/nsSHEntry.cpp
@@ -0,0 +1,1131 @@
+/* -*- 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 "nsSHEntry.h"
+
+#include <algorithm>
+
+#include "nsDocShell.h"
+#include "nsDocShellEditorData.h"
+#include "nsDocShellLoadState.h"
+#include "nsDocShellLoadTypes.h"
+#include "nsIContentSecurityPolicy.h"
+#include "nsIContentViewer.h"
+#include "nsIDocShellTreeItem.h"
+#include "nsIInputStream.h"
+#include "nsILayoutHistoryState.h"
+#include "nsIMutableArray.h"
+#include "nsIStructuredCloneContainer.h"
+#include "nsIURI.h"
+#include "nsSHEntryShared.h"
+#include "nsSHistory.h"
+
+#include "mozilla/Logging.h"
+#include "nsIReferrerInfo.h"
+
+extern mozilla::LazyLogModule gPageCacheLog;
+
+static uint32_t gEntryID = 0;
+
+nsSHEntry::nsSHEntry()
+    : mShared(new nsSHEntryShared()),
+      mLoadType(0),
+      mID(++gEntryID),  // SessionStore has special handling for 0 values.
+      mScrollPositionX(0),
+      mScrollPositionY(0),
+      mLoadReplace(false),
+      mURIWasModified(false),
+      mIsSrcdocEntry(false),
+      mScrollRestorationIsManual(false),
+      mLoadedInThisProcess(false),
+      mPersist(true),
+      mHasUserInteraction(false),
+      mHasUserActivation(false) {}
+
+nsSHEntry::nsSHEntry(const nsSHEntry& aOther)
+    : mShared(aOther.mShared),
+      mURI(aOther.mURI),
+      mOriginalURI(aOther.mOriginalURI),
+      mResultPrincipalURI(aOther.mResultPrincipalURI),
+      mUnstrippedURI(aOther.mUnstrippedURI),
+      mReferrerInfo(aOther.mReferrerInfo),
+      mTitle(aOther.mTitle),
+      mPostData(aOther.mPostData),
+      mLoadType(0),  // XXX why not copy?
+      mID(aOther.mID),
+      mScrollPositionX(0),  // XXX why not copy?
+      mScrollPositionY(0),  // XXX why not copy?
+      mParent(aOther.mParent),
+      mStateData(aOther.mStateData),
+      mSrcdocData(aOther.mSrcdocData),
+      mBaseURI(aOther.mBaseURI),
+      mLoadReplace(aOther.mLoadReplace),
+      mURIWasModified(aOther.mURIWasModified),
+      mIsSrcdocEntry(aOther.mIsSrcdocEntry),
+      mScrollRestorationIsManual(false),
+      mLoadedInThisProcess(aOther.mLoadedInThisProcess),
+      mPersist(aOther.mPersist),
+      mHasUserInteraction(false),
+      mHasUserActivation(aOther.mHasUserActivation) {}
+
+nsSHEntry::~nsSHEntry() {
+  // Null out the mParent pointers on all our kids.
+  for (nsISHEntry* entry : mChildren) {
+    if (entry) {
+      entry->SetParent(nullptr);
+    }
+  }
+}
+
+NS_IMPL_ISUPPORTS(nsSHEntry, nsISHEntry, nsISupportsWeakReference)
+
+NS_IMETHODIMP
+nsSHEntry::SetScrollPosition(int32_t aX, int32_t aY) {
+  mScrollPositionX = aX;
+  mScrollPositionY = aY;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetScrollPosition(int32_t* aX, int32_t* aY) {
+  *aX = mScrollPositionX;
+  *aY = mScrollPositionY;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetURIWasModified(bool* aOut) {
+  *aOut = mURIWasModified;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetURIWasModified(bool aIn) {
+  mURIWasModified = aIn;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetURI(nsIURI** aURI) {
+  *aURI = mURI;
+  NS_IF_ADDREF(*aURI);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetURI(nsIURI* aURI) {
+  mURI = aURI;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetOriginalURI(nsIURI** aOriginalURI) {
+  *aOriginalURI = mOriginalURI;
+  NS_IF_ADDREF(*aOriginalURI);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetOriginalURI(nsIURI* aOriginalURI) {
+  mOriginalURI = aOriginalURI;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetResultPrincipalURI(nsIURI** aResultPrincipalURI) {
+  *aResultPrincipalURI = mResultPrincipalURI;
+  NS_IF_ADDREF(*aResultPrincipalURI);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetResultPrincipalURI(nsIURI* aResultPrincipalURI) {
+  mResultPrincipalURI = aResultPrincipalURI;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetUnstrippedURI(nsIURI** aUnstrippedURI) {
+  *aUnstrippedURI = mUnstrippedURI;
+  NS_IF_ADDREF(*aUnstrippedURI);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetUnstrippedURI(nsIURI* aUnstrippedURI) {
+  mUnstrippedURI = aUnstrippedURI;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetLoadReplace(bool* aLoadReplace) {
+  *aLoadReplace = mLoadReplace;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetLoadReplace(bool aLoadReplace) {
+  mLoadReplace = aLoadReplace;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetReferrerInfo(nsIReferrerInfo** aReferrerInfo) {
+  *aReferrerInfo = mReferrerInfo;
+  NS_IF_ADDREF(*aReferrerInfo);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetReferrerInfo(nsIReferrerInfo* aReferrerInfo) {
+  mReferrerInfo = aReferrerInfo;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetSticky(bool aSticky) {
+  mShared->mSticky = aSticky;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetSticky(bool* aSticky) {
+  *aSticky = mShared->mSticky;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetTitle(nsAString& aTitle) {
+  // Check for empty title...
+  if (mTitle.IsEmpty() && mURI) {
+    // Default title is the URL.
+    nsAutoCString spec;
+    if (NS_SUCCEEDED(mURI->GetSpec(spec))) {
+      AppendUTF8toUTF16(spec, mTitle);
+    }
+  }
+
+  aTitle = mTitle;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetTitle(const nsAString& aTitle) {
+  mTitle = aTitle;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetName(nsAString& aName) {
+  aName = mName;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetName(const nsAString& aName) {
+  mName = aName;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetPostData(nsIInputStream** aResult) {
+  *aResult = mPostData;
+  NS_IF_ADDREF(*aResult);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetPostData(nsIInputStream* aPostData) {
+  mPostData = aPostData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetHasPostData(bool* aResult) {
+  *aResult = !!mPostData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetLayoutHistoryState(nsILayoutHistoryState** aResult) {
+  *aResult = mShared->mLayoutHistoryState;
+  NS_IF_ADDREF(*aResult);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetLayoutHistoryState(nsILayoutHistoryState* aState) {
+  mShared->mLayoutHistoryState = aState;
+  if (mShared->mLayoutHistoryState) {
+    mShared->mLayoutHistoryState->SetScrollPositionOnly(
+        !mShared->mSaveLayoutState);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::InitLayoutHistoryState(nsILayoutHistoryState** aState) {
+  if (!mShared->mLayoutHistoryState) {
+    nsCOMPtr<nsILayoutHistoryState> historyState;
+    historyState = NS_NewLayoutHistoryState();
+    SetLayoutHistoryState(historyState);
+  }
+
+  nsCOMPtr<nsILayoutHistoryState> state = GetLayoutHistoryState();
+  state.forget(aState);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetLoadType(uint32_t* aResult) {
+  *aResult = mLoadType;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetLoadType(uint32_t aLoadType) {
+  mLoadType = aLoadType;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetID(uint32_t* aResult) {
+  *aResult = mID;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetID(uint32_t aID) {
+  mID = aID;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetIsSubFrame(bool* aFlag) {
+  *aFlag = mShared->mIsFrameNavigation;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetIsSubFrame(bool aFlag) {
+  mShared->mIsFrameNavigation = aFlag;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetHasUserInteraction(bool* aFlag) {
+  // The back button and menulist deal with root/top-level
+  // session history entries, thus we annotate only the root entry.
+  if (!mParent) {
+    *aFlag = mHasUserInteraction;
+  } else {
+    nsCOMPtr<nsISHEntry> root = nsSHistory::GetRootSHEntry(this);
+    root->GetHasUserInteraction(aFlag);
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetHasUserInteraction(bool aFlag) {
+  // The back button and menulist deal with root/top-level
+  // session history entries, thus we annotate only the root entry.
+  if (!mParent) {
+    mHasUserInteraction = aFlag;
+  } else {
+    nsCOMPtr<nsISHEntry> root = nsSHistory::GetRootSHEntry(this);
+    root->SetHasUserInteraction(aFlag);
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetHasUserActivation(bool* aFlag) {
+  *aFlag = mHasUserActivation;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetHasUserActivation(bool aFlag) {
+  mHasUserActivation = aFlag;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetCacheKey(uint32_t* aResult) {
+  *aResult = mShared->mCacheKey;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetCacheKey(uint32_t aCacheKey) {
+  mShared->mCacheKey = aCacheKey;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetContentType(nsACString& aContentType) {
+  aContentType = mShared->mContentType;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetContentType(const nsACString& aContentType) {
+  mShared->mContentType = aContentType;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::Create(
+    nsIURI* aURI, const nsAString& aTitle, nsIInputStream* aInputStream,
+    uint32_t aCacheKey, const nsACString& aContentType,
+    nsIPrincipal* aTriggeringPrincipal, nsIPrincipal* aPrincipalToInherit,
+    nsIPrincipal* aPartitionedPrincipalToInherit,
+    nsIContentSecurityPolicy* aCsp, const nsID& aDocShellID,
+    bool aDynamicCreation, nsIURI* aOriginalURI, nsIURI* aResultPrincipalURI,
+    nsIURI* aUnstrippedURI, bool aLoadReplace, nsIReferrerInfo* aReferrerInfo,
+    const nsAString& aSrcdocData, bool aSrcdocEntry, nsIURI* aBaseURI,
+    bool aSaveLayoutState, bool aExpired, bool aUserActivation) {
+  MOZ_ASSERT(
+      aTriggeringPrincipal,
+      "need a valid triggeringPrincipal to create a session history entry");
+
+  mURI = aURI;
+  mTitle = aTitle;
+  mPostData = aInputStream;
+
+  // Set the LoadType by default to loadHistory during creation
+  mLoadType = LOAD_HISTORY;
+
+  mShared->mCacheKey = aCacheKey;
+  mShared->mContentType = aContentType;
+  mShared->mTriggeringPrincipal = aTriggeringPrincipal;
+  mShared->mPrincipalToInherit = aPrincipalToInherit;
+  mShared->mPartitionedPrincipalToInherit = aPartitionedPrincipalToInherit;
+  mShared->mCsp = aCsp;
+  mShared->mDocShellID = aDocShellID;
+  mShared->mDynamicallyCreated = aDynamicCreation;
+
+  // By default all entries are set false for subframe flag.
+  // nsDocShell::CloneAndReplace() which creates entries for
+  // all subframe navigations, sets the flag to true.
+  mShared->mIsFrameNavigation = false;
+
+  mHasUserInteraction = false;
+
+  mShared->mExpired = aExpired;
+
+  mIsSrcdocEntry = aSrcdocEntry;
+  mSrcdocData = aSrcdocData;
+
+  mBaseURI = aBaseURI;
+
+  mLoadedInThisProcess = true;
+
+  mOriginalURI = aOriginalURI;
+  mResultPrincipalURI = aResultPrincipalURI;
+  mUnstrippedURI = aUnstrippedURI;
+  mLoadReplace = aLoadReplace;
+  mReferrerInfo = aReferrerInfo;
+
+  mHasUserActivation = aUserActivation;
+
+  mShared->mLayoutHistoryState = nullptr;
+
+  mShared->mSaveLayoutState = aSaveLayoutState;
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetParent(nsISHEntry** aResult) {
+  nsCOMPtr<nsISHEntry> parent = do_QueryReferent(mParent);
+  parent.forget(aResult);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetParent(nsISHEntry* aParent) {
+  mParent = do_GetWeakReference(aParent);
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(void)
+nsSHEntry::SetViewerBounds(const nsIntRect& aBounds) {
+  mShared->mViewerBounds = aBounds;
+}
+
+NS_IMETHODIMP_(void)
+nsSHEntry::GetViewerBounds(nsIntRect& aBounds) {
+  aBounds = mShared->mViewerBounds;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetTriggeringPrincipal(nsIPrincipal** aTriggeringPrincipal) {
+  NS_IF_ADDREF(*aTriggeringPrincipal = mShared->mTriggeringPrincipal);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetTriggeringPrincipal(nsIPrincipal* aTriggeringPrincipal) {
+  mShared->mTriggeringPrincipal = aTriggeringPrincipal;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetPrincipalToInherit(nsIPrincipal** aPrincipalToInherit) {
+  NS_IF_ADDREF(*aPrincipalToInherit = mShared->mPrincipalToInherit);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetPrincipalToInherit(nsIPrincipal* aPrincipalToInherit) {
+  mShared->mPrincipalToInherit = aPrincipalToInherit;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetPartitionedPrincipalToInherit(
+    nsIPrincipal** aPartitionedPrincipalToInherit) {
+  NS_IF_ADDREF(*aPartitionedPrincipalToInherit =
+                   mShared->mPartitionedPrincipalToInherit);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetPartitionedPrincipalToInherit(
+    nsIPrincipal* aPartitionedPrincipalToInherit) {
+  mShared->mPartitionedPrincipalToInherit = aPartitionedPrincipalToInherit;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetCsp(nsIContentSecurityPolicy** aCsp) {
+  NS_IF_ADDREF(*aCsp = mShared->mCsp);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetCsp(nsIContentSecurityPolicy* aCsp) {
+  mShared->mCsp = aCsp;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::AdoptBFCacheEntry(nsISHEntry* aEntry) {
+  nsSHEntryShared* shared = static_cast<nsSHEntry*>(aEntry)->mShared;
+  NS_ENSURE_STATE(shared);
+
+  mShared = shared;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SharesDocumentWith(nsISHEntry* aEntry, bool* aOut) {
+  NS_ENSURE_ARG_POINTER(aOut);
+
+  *aOut = mShared == static_cast<nsSHEntry*>(aEntry)->mShared;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetIsSrcdocEntry(bool* aIsSrcdocEntry) {
+  *aIsSrcdocEntry = mIsSrcdocEntry;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetSrcdocData(nsAString& aSrcdocData) {
+  aSrcdocData = mSrcdocData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetSrcdocData(const nsAString& aSrcdocData) {
+  mSrcdocData = aSrcdocData;
+  mIsSrcdocEntry = true;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetBaseURI(nsIURI** aBaseURI) {
+  *aBaseURI = mBaseURI;
+  NS_IF_ADDREF(*aBaseURI);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetBaseURI(nsIURI* aBaseURI) {
+  mBaseURI = aBaseURI;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetScrollRestorationIsManual(bool* aIsManual) {
+  *aIsManual = mScrollRestorationIsManual;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetScrollRestorationIsManual(bool aIsManual) {
+  mScrollRestorationIsManual = aIsManual;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetLoadedInThisProcess(bool* aLoadedInThisProcess) {
+  *aLoadedInThisProcess = mLoadedInThisProcess;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetChildCount(int32_t* aCount) {
+  *aCount = mChildren.Count();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::AddChild(nsISHEntry* aChild, int32_t aOffset,
+                    bool aUseRemoteSubframes) {
+  if (aChild) {
+    NS_ENSURE_SUCCESS(aChild->SetParent(this), NS_ERROR_FAILURE);
+  }
+
+  if (aOffset < 0) {
+    mChildren.AppendObject(aChild);
+    return NS_OK;
+  }
+
+  //
+  // Bug 52670: Ensure children are added in order.
+  //
+  //  Later frames in the child list may load faster and get appended
+  //  before earlier frames, causing session history to be scrambled.
+  //  By growing the list here, they are added to the right position.
+  //
+  //  Assert that aOffset will not be so high as to grow us a lot.
+  //
+  NS_ASSERTION(aOffset < (mChildren.Count() + 1023), "Large frames array!\n");
+
+  bool newChildIsDyn = aChild ? aChild->IsDynamicallyAdded() : false;
+
+  // If the new child is dynamically added, try to add it to aOffset, but if
+  // there are non-dynamically added children, the child must be after those.
+  if (newChildIsDyn) {
+    int32_t lastNonDyn = aOffset - 1;
+    for (int32_t i = aOffset; i < mChildren.Count(); ++i) {
+      nsISHEntry* entry = mChildren[i];
+      if (entry) {
+        if (entry->IsDynamicallyAdded()) {
+          break;
+        } else {
+          lastNonDyn = i;
+        }
+      }
+    }
+    // InsertObjectAt allows only appending one object.
+    // If aOffset is larger than Count(), we must first manually
+    // set the capacity.
+    if (aOffset > mChildren.Count()) {
+      mChildren.SetCount(aOffset);
+    }
+    if (!mChildren.InsertObjectAt(aChild, lastNonDyn + 1)) {
+      NS_WARNING("Adding a child failed!");
+      aChild->SetParent(nullptr);
+      return NS_ERROR_FAILURE;
+    }
+  } else {
+    // If the new child isn't dynamically added, it should be set to aOffset.
+    // If there are dynamically added children before that, those must be
+    // moved to be after aOffset.
+    if (mChildren.Count() > 0) {
+      int32_t start = std::min(mChildren.Count() - 1, aOffset);
+      int32_t dynEntryIndex = -1;
+      nsISHEntry* dynEntry = nullptr;
+      for (int32_t i = start; i >= 0; --i) {
+        nsISHEntry* entry = mChildren[i];
+        if (entry) {
+          if (entry->IsDynamicallyAdded()) {
+            dynEntryIndex = i;
+            dynEntry = entry;
+          } else {
+            break;
+          }
+        }
+      }
+
+      if (dynEntry) {
+        nsCOMArray<nsISHEntry> tmp;
+        tmp.SetCount(aOffset - dynEntryIndex + 1);
+        mChildren.InsertObjectsAt(tmp, dynEntryIndex);
+        NS_ASSERTION(mChildren[aOffset + 1] == dynEntry, "Whaat?");
+      }
+    }
+
+    // Make sure there isn't anything at aOffset.
+    if (aOffset < mChildren.Count()) {
+      nsISHEntry* oldChild = mChildren[aOffset];
+      if (oldChild && oldChild != aChild) {
+        // Under Fission, this can happen when a network-created iframe starts
+        // out in-process, moves out-of-process, and then switches back. At that
+        // point, we'll create a new network-created DocShell at the same index
+        // where we already have an entry for the original network-created
+        // DocShell.
+        //
+        // This should ideally stop being an issue once the Fission-aware
+        // session history rewrite is complete.
+        NS_ASSERTION(
+            aUseRemoteSubframes,
+            "Adding a child where we already have a child? This may misbehave");
+        oldChild->SetParent(nullptr);
+      }
+    }
+
+    mChildren.ReplaceObjectAt(aChild, aOffset);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::RemoveChild(nsISHEntry* aChild) {
+  NS_ENSURE_TRUE(aChild, NS_ERROR_FAILURE);
+  bool childRemoved = false;
+  if (aChild->IsDynamicallyAdded()) {
+    childRemoved = mChildren.RemoveObject(aChild);
+  } else {
+    int32_t index = mChildren.IndexOfObject(aChild);
+    if (index >= 0) {
+      // Other alive non-dynamic child docshells still keep mChildOffset,
+      // so we don't want to change the indices here.
+      mChildren.ReplaceObjectAt(nullptr, index);
+      childRemoved = true;
+    }
+  }
+  if (childRemoved) {
+    aChild->SetParent(nullptr);
+
+    // reduce the child count, i.e. remove empty children at the end
+    for (int32_t i = mChildren.Count() - 1; i >= 0 && !mChildren[i]; --i) {
+      if (!mChildren.RemoveObjectAt(i)) {
+        break;
+      }
+    }
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetChildAt(int32_t aIndex, nsISHEntry** aResult) {
+  if (aIndex >= 0 && aIndex < mChildren.Count()) {
+    *aResult = mChildren[aIndex];
+    // yes, mChildren can have holes in it.  AddChild's offset parameter makes
+    // that possible.
+    NS_IF_ADDREF(*aResult);
+  } else {
+    *aResult = nullptr;
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(void)
+nsSHEntry::GetChildSHEntryIfHasNoDynamicallyAddedChild(int32_t aChildOffset,
+                                                       nsISHEntry** aChild) {
+  *aChild = nullptr;
+
+  bool dynamicallyAddedChild = false;
+  HasDynamicallyAddedChild(&dynamicallyAddedChild);
+  if (dynamicallyAddedChild) {
+    return;
+  }
+
+  // If the user did a shift-reload on this frameset page,
+  // we don't want to load the subframes from history.
+  if (IsForceReloadType(mLoadType) || mLoadType == LOAD_REFRESH) {
+    return;
+  }
+
+  /* Before looking for the subframe's url, check
+   * the expiration status of the parent. If the parent
+   * has expired from cache, then subframes will not be
+   * loaded from history in certain situations.
+   * If the user pressed reload and the parent frame has expired
+   *  from cache, we do not want to load the child frame from history.
+   */
+  if (mShared->mExpired && (mLoadType == LOAD_RELOAD_NORMAL)) {
+    // The parent has expired. Return null.
+    *aChild = nullptr;
+    return;
+  }
+  // Get the child subframe from session history.
+  GetChildAt(aChildOffset, aChild);
+  if (*aChild) {
+    // Set the parent's Load Type on the child
+    (*aChild)->SetLoadType(mLoadType);
+  }
+}
+
+NS_IMETHODIMP
+nsSHEntry::ReplaceChild(nsISHEntry* aNewEntry) {
+  NS_ENSURE_STATE(aNewEntry);
+
+  nsID docshellID;
+  aNewEntry->GetDocshellID(docshellID);
+
+  for (int32_t i = 0; i < mChildren.Count(); ++i) {
+    if (mChildren[i]) {
+      nsID childDocshellID;
+      nsresult rv = mChildren[i]->GetDocshellID(childDocshellID);
+      NS_ENSURE_SUCCESS(rv, rv);
+      if (docshellID == childDocshellID) {
+        mChildren[i]->SetParent(nullptr);
+        mChildren.ReplaceObjectAt(aNewEntry, i);
+        return aNewEntry->SetParent(this);
+      }
+    }
+  }
+  return NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP_(void) nsSHEntry::ClearEntry() {
+  int32_t childCount = GetChildCount();
+  // Remove all children of this entry
+  for (int32_t i = childCount - 1; i >= 0; i--) {
+    nsCOMPtr<nsISHEntry> child;
+    GetChildAt(i, getter_AddRefs(child));
+    RemoveChild(child);
+  }
+  AbandonBFCacheEntry();
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetStateData(nsIStructuredCloneContainer** aContainer) {
+  NS_IF_ADDREF(*aContainer = mStateData);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetStateData(nsIStructuredCloneContainer* aContainer) {
+  mStateData = aContainer;
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(bool)
+nsSHEntry::IsDynamicallyAdded() { return mShared->mDynamicallyCreated; }
+
+NS_IMETHODIMP
+nsSHEntry::HasDynamicallyAddedChild(bool* aAdded) {
+  *aAdded = false;
+  for (int32_t i = 0; i < mChildren.Count(); ++i) {
+    nsISHEntry* entry = mChildren[i];
+    if (entry) {
+      *aAdded = entry->IsDynamicallyAdded();
+      if (*aAdded) {
+        break;
+      }
+    }
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetDocshellID(nsID& aID) {
+  aID = mShared->mDocShellID;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetDocshellID(const nsID& aID) {
+  mShared->mDocShellID = aID;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetLastTouched(uint32_t* aLastTouched) {
+  *aLastTouched = mShared->mLastTouched;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetLastTouched(uint32_t aLastTouched) {
+  mShared->mLastTouched = aLastTouched;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetShistory(nsISHistory** aSHistory) {
+  nsCOMPtr<nsISHistory> shistory(do_QueryReferent(mShared->mSHistory));
+  shistory.forget(aSHistory);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetShistory(nsISHistory* aSHistory) {
+  nsWeakPtr shistory = do_GetWeakReference(aSHistory);
+  // mSHistory can not be changed once it's set
+  MOZ_ASSERT(!mShared->mSHistory || (mShared->mSHistory == shistory));
+  mShared->mSHistory = shistory;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetLoadTypeAsHistory() {
+  // Set the LoadType by default to loadHistory during creation
+  mLoadType = LOAD_HISTORY;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetPersist(bool* aPersist) {
+  *aPersist = mPersist;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetPersist(bool aPersist) {
+  mPersist = aPersist;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::CreateLoadInfo(nsDocShellLoadState** aLoadState) {
+  nsCOMPtr<nsIURI> uri = GetURI();
+  RefPtr<nsDocShellLoadState> loadState(new nsDocShellLoadState(uri));
+
+  nsCOMPtr<nsIURI> originalURI = GetOriginalURI();
+  loadState->SetOriginalURI(originalURI);
+
+  mozilla::Maybe<nsCOMPtr<nsIURI>> emplacedResultPrincipalURI;
+  nsCOMPtr<nsIURI> resultPrincipalURI = GetResultPrincipalURI();
+  emplacedResultPrincipalURI.emplace(std::move(resultPrincipalURI));
+  loadState->SetMaybeResultPrincipalURI(emplacedResultPrincipalURI);
+
+  nsCOMPtr<nsIURI> unstrippedURI = GetUnstrippedURI();
+  loadState->SetUnstrippedURI(unstrippedURI);
+
+  loadState->SetLoadReplace(GetLoadReplace());
+  nsCOMPtr<nsIInputStream> postData = GetPostData();
+  loadState->SetPostDataStream(postData);
+
+  nsAutoCString contentType;
+  GetContentType(contentType);
+  loadState->SetTypeHint(contentType);
+
+  nsCOMPtr<nsIPrincipal> triggeringPrincipal = GetTriggeringPrincipal();
+  loadState->SetTriggeringPrincipal(triggeringPrincipal);
+  nsCOMPtr<nsIPrincipal> principalToInherit = GetPrincipalToInherit();
+  loadState->SetPrincipalToInherit(principalToInherit);
+  nsCOMPtr<nsIPrincipal> partitionedPrincipalToInherit =
+      GetPartitionedPrincipalToInherit();
+  loadState->SetPartitionedPrincipalToInherit(partitionedPrincipalToInherit);
+  nsCOMPtr<nsIContentSecurityPolicy> csp = GetCsp();
+  loadState->SetCsp(csp);
+  nsCOMPtr<nsIReferrerInfo> referrerInfo = GetReferrerInfo();
+  loadState->SetReferrerInfo(referrerInfo);
+
+  // Do not inherit principal from document (security-critical!);
+  uint32_t flags = nsDocShell::InternalLoad::INTERNAL_LOAD_FLAGS_NONE;
+
+  // Passing nullptr as aSourceDocShell gives the same behaviour as before
+  // aSourceDocShell was introduced. According to spec we should be passing
+  // the source browsing context that was used when the history entry was
+  // first created. bug 947716 has been created to address this issue.
+  nsAutoString srcdoc;
+  nsCOMPtr<nsIURI> baseURI;
+  if (GetIsSrcdocEntry()) {
+    GetSrcdocData(srcdoc);
+    baseURI = GetBaseURI();
+    flags |= nsDocShell::InternalLoad::INTERNAL_LOAD_FLAGS_IS_SRCDOC;
+  } else {
+    srcdoc = VoidString();
+  }
+  loadState->SetSrcdocData(srcdoc);
+  loadState->SetBaseURI(baseURI);
+  loadState->SetInternalLoadFlags(flags);
+
+  loadState->SetFirstParty(true);
+
+  loadState->SetHasValidUserGestureActivation(GetHasUserActivation());
+
+  loadState->SetSHEntry(this);
+
+  // When we create a load state from the history entry we already know if
+  // https-first was able to upgrade the request from http to https. There is no
+  // point in re-retrying to upgrade.
+  loadState->SetIsExemptFromHTTPSOnlyMode(true);
+
+  loadState.forget(aLoadState);
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(void)
+nsSHEntry::SyncTreesForSubframeNavigation(
+    nsISHEntry* aEntry, mozilla::dom::BrowsingContext* aTopBC,
+    mozilla::dom::BrowsingContext* aIgnoreBC) {
+  // XXX Keep this in sync with
+  // SessionHistoryEntry::SyncTreesForSubframeNavigation
+  //
+  // We need to sync up the browsing context and session history trees for
+  // subframe navigation.  If the load was in a subframe, we forward up to
+  // the top browsing context, which will then recursively sync up all browsing
+  // contexts to their corresponding entries in the new session history tree. If
+  // we don't do this, then we can cache a content viewer on the wrong cloned
+  // entry, and subsequently restore it at the wrong time.
+  nsCOMPtr<nsISHEntry> newRootEntry = nsSHistory::GetRootSHEntry(aEntry);
+  if (newRootEntry) {
+    // newRootEntry is now the new root entry.
+    // Find the old root entry as well.
+
+    // Need a strong ref. on |oldRootEntry| so it isn't destroyed when
+    // SetChildHistoryEntry() does SwapHistoryEntries() (bug 304639).
+    nsCOMPtr<nsISHEntry> oldRootEntry = nsSHistory::GetRootSHEntry(this);
+
+    if (oldRootEntry) {
+      nsSHistory::SwapEntriesData data = {aIgnoreBC, newRootEntry, nullptr};
+      nsSHistory::SetChildHistoryEntry(oldRootEntry, aTopBC, 0, &data);
+    }
+  }
+}
+
+void nsSHEntry::EvictContentViewer() {
+  nsCOMPtr<nsIContentViewer> viewer = GetContentViewer();
+  if (viewer) {
+    mShared->NotifyListenersContentViewerEvicted();
+    // Drop the presentation state before destroying the viewer, so that
+    // document teardown is able to correctly persist the state.
+    SetContentViewer(nullptr);
+    SyncPresentationState();
+    viewer->Destroy();
+  }
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetContentViewer(nsIContentViewer* aViewer) {
+  return GetState()->SetContentViewer(aViewer);
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetContentViewer(nsIContentViewer** aResult) {
+  *aResult = GetState()->mContentViewer;
+  NS_IF_ADDREF(*aResult);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetIsInBFCache(bool* aResult) {
+  *aResult = !!GetState()->mContentViewer;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::Clone(nsISHEntry** aResult) {
+  nsCOMPtr<nsISHEntry> entry = new nsSHEntry(*this);
+  entry.forget(aResult);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetSaveLayoutStateFlag(bool* aFlag) {
+  *aFlag = mShared->mSaveLayoutState;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetSaveLayoutStateFlag(bool aFlag) {
+  mShared->mSaveLayoutState = aFlag;
+  if (mShared->mLayoutHistoryState) {
+    mShared->mLayoutHistoryState->SetScrollPositionOnly(!aFlag);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetWindowState(nsISupports* aState) {
+  GetState()->mWindowState = aState;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetWindowState(nsISupports** aState) {
+  NS_IF_ADDREF(*aState = GetState()->mWindowState);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetRefreshURIList(nsIMutableArray** aList) {
+  NS_IF_ADDREF(*aList = GetState()->mRefreshURIList);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetRefreshURIList(nsIMutableArray* aList) {
+  GetState()->mRefreshURIList = aList;
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(void)
+nsSHEntry::AddChildShell(nsIDocShellTreeItem* aShell) {
+  MOZ_ASSERT(aShell, "Null child shell added to history entry");
+  GetState()->mChildShells.AppendObject(aShell);
+}
+
+NS_IMETHODIMP
+nsSHEntry::ChildShellAt(int32_t aIndex, nsIDocShellTreeItem** aShell) {
+  NS_IF_ADDREF(*aShell = GetState()->mChildShells.SafeObjectAt(aIndex));
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(void)
+nsSHEntry::ClearChildShells() { GetState()->mChildShells.Clear(); }
+
+NS_IMETHODIMP_(void)
+nsSHEntry::SyncPresentationState() { GetState()->SyncPresentationState(); }
+
+nsDocShellEditorData* nsSHEntry::ForgetEditorData() {
+  // XXX jlebar Check how this is used.
+  return GetState()->mEditorData.release();
+}
+
+void nsSHEntry::SetEditorData(nsDocShellEditorData* aData) {
+  NS_ASSERTION(!(aData && GetState()->mEditorData),
+               "We're going to overwrite an owning ref!");
+  if (GetState()->mEditorData != aData) {
+    GetState()->mEditorData = mozilla::WrapUnique(aData);
+  }
+}
+
+bool nsSHEntry::HasDetachedEditor() {
+  return GetState()->mEditorData != nullptr;
+}
+
+bool nsSHEntry::HasBFCacheEntry(
+    mozilla::dom::SHEntrySharedParentState* aEntry) {
+  return GetState() == aEntry;
+}
+
+NS_IMETHODIMP
+nsSHEntry::AbandonBFCacheEntry() {
+  mShared = GetState()->Duplicate();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetBfcacheID(uint64_t* aBFCacheID) {
+  *aBFCacheID = mShared->GetId();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::GetWireframe(JSContext* aCx, JS::MutableHandle<JS::Value> aOut) {
+  aOut.set(JS::NullValue());
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHEntry::SetWireframe(JSContext* aCx, JS::Handle<JS::Value> aArg) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
diff --git a/docshell/shistory/nsSHEntry.h b/docshell/shistory/nsSHEntry.h
new file mode 100644
index 0000000000..eaa3cf1ad4
--- /dev/null
+++ b/docshell/shistory/nsSHEntry.h
@@ -0,0 +1,72 @@
+/* -*- 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 nsSHEntry_h
+#define nsSHEntry_h
+
+#include "nsCOMArray.h"
+#include "nsCOMPtr.h"
+#include "nsISHEntry.h"
+#include "nsString.h"
+#include "nsWeakReference.h"
+#include "mozilla/Attributes.h"
+
+class nsSHEntryShared;
+class nsIInputStream;
+class nsIURI;
+class nsIReferrerInfo;
+
+class nsSHEntry : public nsISHEntry, public nsSupportsWeakReference {
+ public:
+  nsSHEntry();
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISHENTRY
+
+  virtual void EvictContentViewer();
+
+  static nsresult Startup();
+  static void Shutdown();
+
+  nsSHEntryShared* GetState() { return mShared; }
+
+ protected:
+  explicit nsSHEntry(const nsSHEntry& aOther);
+  virtual ~nsSHEntry();
+
+  // We share the state in here with other SHEntries which correspond to the
+  // same document.
+  RefPtr<nsSHEntryShared> mShared;
+
+  // See nsSHEntry.idl for comments on these members.
+  nsCOMPtr<nsIURI> mURI;
+  nsCOMPtr<nsIURI> mOriginalURI;
+  nsCOMPtr<nsIURI> mResultPrincipalURI;
+  nsCOMPtr<nsIURI> mUnstrippedURI;
+  nsCOMPtr<nsIReferrerInfo> mReferrerInfo;
+  nsString mTitle;
+  nsString mName;
+  nsCOMPtr<nsIInputStream> mPostData;
+  uint32_t mLoadType;
+  uint32_t mID;
+  int32_t mScrollPositionX;
+  int32_t mScrollPositionY;
+  nsWeakPtr mParent;
+  nsCOMArray<nsISHEntry> mChildren;
+  nsCOMPtr<nsIStructuredCloneContainer> mStateData;
+  nsString mSrcdocData;
+  nsCOMPtr<nsIURI> mBaseURI;
+  bool mLoadReplace;
+  bool mURIWasModified;
+  bool mIsSrcdocEntry;
+  bool mScrollRestorationIsManual;
+  bool mLoadedInThisProcess;
+  bool mPersist;
+  bool mHasUserInteraction;
+  bool mHasUserActivation;
+};
+
+#endif /* nsSHEntry_h */
diff --git a/docshell/shistory/nsSHEntryShared.cpp b/docshell/shistory/nsSHEntryShared.cpp
new file mode 100644
index 0000000000..9b8bc3936d
--- /dev/null
+++ b/docshell/shistory/nsSHEntryShared.cpp
@@ -0,0 +1,343 @@
+/* -*- 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 "nsSHEntryShared.h"
+
+#include "nsArray.h"
+#include "nsContentUtils.h"
+#include "nsDocShellEditorData.h"
+#include "nsIContentViewer.h"
+#include "nsISHistory.h"
+#include "mozilla/dom/Document.h"
+#include "nsILayoutHistoryState.h"
+#include "nsIWebNavigation.h"
+#include "nsSHistory.h"
+#include "nsThreadUtils.h"
+#include "nsFrameLoader.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Preferences.h"
+
+namespace dom = mozilla::dom;
+
+namespace {
+uint64_t gSHEntrySharedID = 0;
+nsTHashMap<nsUint64HashKey, mozilla::dom::SHEntrySharedParentState*>*
+    sIdToSharedState = nullptr;
+}  // namespace
+
+namespace mozilla {
+namespace dom {
+
+/* static */
+uint64_t SHEntrySharedState::GenerateId() {
+  return nsContentUtils::GenerateProcessSpecificId(++gSHEntrySharedID);
+}
+
+/* static */
+SHEntrySharedParentState* SHEntrySharedParentState::Lookup(uint64_t aId) {
+  MOZ_ASSERT(aId != 0);
+
+  return sIdToSharedState ? sIdToSharedState->Get(aId) : nullptr;
+}
+
+static void AddSHEntrySharedParentState(
+    SHEntrySharedParentState* aSharedState) {
+  MOZ_ASSERT(aSharedState->mId != 0);
+
+  if (!sIdToSharedState) {
+    sIdToSharedState =
+        new nsTHashMap<nsUint64HashKey, SHEntrySharedParentState*>();
+  }
+  sIdToSharedState->InsertOrUpdate(aSharedState->mId, aSharedState);
+}
+
+SHEntrySharedParentState::SHEntrySharedParentState() {
+  AddSHEntrySharedParentState(this);
+}
+
+SHEntrySharedParentState::SHEntrySharedParentState(
+    nsIPrincipal* aTriggeringPrincipal, nsIPrincipal* aPrincipalToInherit,
+    nsIPrincipal* aPartitionedPrincipalToInherit,
+    nsIContentSecurityPolicy* aCsp, const nsACString& aContentType)
+    : SHEntrySharedState(aTriggeringPrincipal, aPrincipalToInherit,
+                         aPartitionedPrincipalToInherit, aCsp, aContentType) {
+  AddSHEntrySharedParentState(this);
+}
+
+SHEntrySharedParentState::~SHEntrySharedParentState() {
+  MOZ_ASSERT(mId != 0);
+
+  RefPtr<nsFrameLoader> loader = mFrameLoader;
+  SetFrameLoader(nullptr);
+  if (loader) {
+    if (NS_FAILED(NS_DispatchToCurrentThread(NS_NewRunnableFunction(
+            "SHEntrySharedParentState::~SHEntrySharedParentState",
+            [loader]() -> void { loader->AsyncDestroy(); })))) {
+      // Trigger AsyncDestroy immediately during shutdown.
+      loader->AsyncDestroy();
+    }
+  }
+
+  sIdToSharedState->Remove(mId);
+  if (sIdToSharedState->IsEmpty()) {
+    delete sIdToSharedState;
+    sIdToSharedState = nullptr;
+  }
+}
+
+void SHEntrySharedParentState::ChangeId(uint64_t aId) {
+  MOZ_ASSERT(aId != 0);
+
+  sIdToSharedState->Remove(mId);
+  mId = aId;
+  sIdToSharedState->InsertOrUpdate(mId, this);
+}
+
+void SHEntrySharedParentState::CopyFrom(SHEntrySharedParentState* aEntry) {
+  mDocShellID = aEntry->mDocShellID;
+  mTriggeringPrincipal = aEntry->mTriggeringPrincipal;
+  mPrincipalToInherit = aEntry->mPrincipalToInherit;
+  mPartitionedPrincipalToInherit = aEntry->mPartitionedPrincipalToInherit;
+  mCsp = aEntry->mCsp;
+  mSaveLayoutState = aEntry->mSaveLayoutState;
+  mContentType.Assign(aEntry->mContentType);
+  mIsFrameNavigation = aEntry->mIsFrameNavigation;
+  mSticky = aEntry->mSticky;
+  mDynamicallyCreated = aEntry->mDynamicallyCreated;
+  mCacheKey = aEntry->mCacheKey;
+  mLastTouched = aEntry->mLastTouched;
+}
+
+void dom::SHEntrySharedParentState::NotifyListenersContentViewerEvicted() {
+  if (nsCOMPtr<nsISHistory> shistory = do_QueryReferent(mSHistory)) {
+    RefPtr<nsSHistory> nsshistory = static_cast<nsSHistory*>(shistory.get());
+    nsshistory->NotifyListenersContentViewerEvicted(1);
+  }
+}
+
+void SHEntrySharedChildState::CopyFrom(SHEntrySharedChildState* aEntry) {
+  mChildShells.AppendObjects(aEntry->mChildShells);
+}
+
+void SHEntrySharedParentState::SetFrameLoader(nsFrameLoader* aFrameLoader) {
+  // If expiration tracker is removing this object, IsTracked() returns false.
+  if (GetExpirationState()->IsTracked() && mFrameLoader) {
+    if (nsCOMPtr<nsISHistory> shistory = do_QueryReferent(mSHistory)) {
+      shistory->RemoveFromExpirationTracker(this);
+    }
+  }
+
+  mFrameLoader = aFrameLoader;
+
+  if (mFrameLoader) {
+    if (nsCOMPtr<nsISHistory> shistory = do_QueryReferent(mSHistory)) {
+      shistory->AddToExpirationTracker(this);
+    }
+  }
+}
+
+nsFrameLoader* SHEntrySharedParentState::GetFrameLoader() {
+  return mFrameLoader;
+}
+
+}  // namespace dom
+}  // namespace mozilla
+
+void nsSHEntryShared::Shutdown() {}
+
+nsSHEntryShared::~nsSHEntryShared() {
+  // The destruction can be caused by either the entry is removed from session
+  // history and no one holds the reference, or the whole session history is on
+  // destruction. We want to ensure that we invoke
+  // shistory->RemoveFromExpirationTracker for the former case.
+  RemoveFromExpirationTracker();
+
+  // Calling RemoveDynEntriesForBFCacheEntry on destruction is unnecessary since
+  // there couldn't be any SHEntry holding this shared entry, and we noticed
+  // that calling RemoveDynEntriesForBFCacheEntry in the middle of
+  // nsSHistory::Release can cause a crash, so set mSHistory to null explicitly
+  // before RemoveFromBFCacheSync.
+  mSHistory = nullptr;
+  if (mContentViewer) {
+    RemoveFromBFCacheSync();
+  }
+}
+
+NS_IMPL_QUERY_INTERFACE(nsSHEntryShared, nsIBFCacheEntry, nsIMutationObserver)
+NS_IMPL_ADDREF_INHERITED(nsSHEntryShared, dom::SHEntrySharedParentState)
+NS_IMPL_RELEASE_INHERITED(nsSHEntryShared, dom::SHEntrySharedParentState)
+
+already_AddRefed<nsSHEntryShared> nsSHEntryShared::Duplicate() {
+  RefPtr<nsSHEntryShared> newEntry = new nsSHEntryShared();
+
+  newEntry->dom::SHEntrySharedParentState::CopyFrom(this);
+  newEntry->dom::SHEntrySharedChildState::CopyFrom(this);
+
+  return newEntry.forget();
+}
+
+void nsSHEntryShared::RemoveFromExpirationTracker() {
+  nsCOMPtr<nsISHistory> shistory = do_QueryReferent(mSHistory);
+  if (shistory && GetExpirationState()->IsTracked()) {
+    shistory->RemoveFromExpirationTracker(this);
+  }
+}
+
+void nsSHEntryShared::SyncPresentationState() {
+  if (mContentViewer && mWindowState) {
+    // If we have a content viewer and a window state, we should be ok.
+    return;
+  }
+
+  DropPresentationState();
+}
+
+void nsSHEntryShared::DropPresentationState() {
+  RefPtr<nsSHEntryShared> kungFuDeathGrip = this;
+
+  if (mDocument) {
+    mDocument->SetBFCacheEntry(nullptr);
+    mDocument->RemoveMutationObserver(this);
+    mDocument = nullptr;
+  }
+  if (mContentViewer) {
+    mContentViewer->ClearHistoryEntry();
+  }
+
+  RemoveFromExpirationTracker();
+  mContentViewer = nullptr;
+  mSticky = true;
+  mWindowState = nullptr;
+  mViewerBounds.SetRect(0, 0, 0, 0);
+  mChildShells.Clear();
+  mRefreshURIList = nullptr;
+  mEditorData = nullptr;
+}
+
+nsresult nsSHEntryShared::SetContentViewer(nsIContentViewer* aViewer) {
+  MOZ_ASSERT(!aViewer || !mContentViewer,
+             "SHEntryShared already contains viewer");
+
+  if (mContentViewer || !aViewer) {
+    DropPresentationState();
+  }
+
+  // If we're setting mContentViewer to null, state should already be cleared
+  // in the DropPresentationState() call above; If we're setting it to a
+  // non-null content viewer, the entry shouldn't have been tracked either.
+  MOZ_ASSERT(!GetExpirationState()->IsTracked());
+  mContentViewer = aViewer;
+
+  if (mContentViewer) {
+    // mSHistory is only set for root entries, but in general bfcache only
+    // applies to root entries as well. BFCache for subframe navigation has been
+    // disabled since 2005 in bug 304860.
+    if (nsCOMPtr<nsISHistory> shistory = do_QueryReferent(mSHistory)) {
+      shistory->AddToExpirationTracker(this);
+    }
+
+    // Store observed document in strong pointer in case it is removed from
+    // the contentviewer
+    mDocument = mContentViewer->GetDocument();
+    if (mDocument) {
+      mDocument->SetBFCacheEntry(this);
+      mDocument->AddMutationObserver(this);
+    }
+  }
+
+  return NS_OK;
+}
+
+nsresult nsSHEntryShared::RemoveFromBFCacheSync() {
+  MOZ_ASSERT(mContentViewer && mDocument, "we're not in the bfcache!");
+
+  // The call to DropPresentationState could drop the last reference, so hold
+  // |this| until RemoveDynEntriesForBFCacheEntry finishes.
+  RefPtr<nsSHEntryShared> kungFuDeathGrip = this;
+
+  // DropPresentationState would clear mContentViewer.
+  nsCOMPtr<nsIContentViewer> viewer = mContentViewer;
+  DropPresentationState();
+
+  if (viewer) {
+    viewer->Destroy();
+  }
+
+  // Now that we've dropped the viewer, we have to clear associated dynamic
+  // subframe entries.
+  nsCOMPtr<nsISHistory> shistory = do_QueryReferent(mSHistory);
+  if (shistory) {
+    shistory->RemoveDynEntriesForBFCacheEntry(this);
+  }
+
+  return NS_OK;
+}
+
+nsresult nsSHEntryShared::RemoveFromBFCacheAsync() {
+  MOZ_ASSERT(mContentViewer && mDocument, "we're not in the bfcache!");
+
+  // Check it again to play safe in release builds.
+  if (!mDocument) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  // DropPresentationState would clear mContentViewer & mDocument. Capture and
+  // release the references asynchronously so that the document doesn't get
+  // nuked mid-mutation.
+  nsCOMPtr<nsIContentViewer> viewer = mContentViewer;
+  RefPtr<dom::Document> document = mDocument;
+  RefPtr<nsSHEntryShared> self = this;
+  nsresult rv = mDocument->Dispatch(
+      mozilla::TaskCategory::Other,
+      NS_NewRunnableFunction(
+          "nsSHEntryShared::RemoveFromBFCacheAsync",
+          [self, viewer, document]() {
+            if (viewer) {
+              viewer->Destroy();
+            }
+
+            nsCOMPtr<nsISHistory> shistory = do_QueryReferent(self->mSHistory);
+            if (shistory) {
+              shistory->RemoveDynEntriesForBFCacheEntry(self);
+            }
+          }));
+
+  if (NS_FAILED(rv)) {
+    NS_WARNING("Failed to dispatch RemoveFromBFCacheAsync runnable.");
+  } else {
+    // Drop presentation. Only do this if we succeeded in posting the event
+    // since otherwise the document could be torn down mid-mutation, causing
+    // crashes.
+    DropPresentationState();
+  }
+
+  return NS_OK;
+}
+
+void nsSHEntryShared::CharacterDataChanged(nsIContent* aContent,
+                                           const CharacterDataChangeInfo&) {
+  RemoveFromBFCacheAsync();
+}
+
+void nsSHEntryShared::AttributeChanged(dom::Element* aElement,
+                                       int32_t aNameSpaceID, nsAtom* aAttribute,
+                                       int32_t aModType,
+                                       const nsAttrValue* aOldValue) {
+  RemoveFromBFCacheAsync();
+}
+
+void nsSHEntryShared::ContentAppended(nsIContent* aFirstNewContent) {
+  RemoveFromBFCacheAsync();
+}
+
+void nsSHEntryShared::ContentInserted(nsIContent* aChild) {
+  RemoveFromBFCacheAsync();
+}
+
+void nsSHEntryShared::ContentRemoved(nsIContent* aChild,
+                                     nsIContent* aPreviousSibling) {
+  RemoveFromBFCacheAsync();
+}
diff --git a/docshell/shistory/nsSHEntryShared.h b/docshell/shistory/nsSHEntryShared.h
new file mode 100644
index 0000000000..6bb6344202
--- /dev/null
+++ b/docshell/shistory/nsSHEntryShared.h
@@ -0,0 +1,219 @@
+/* -*- 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 nsSHEntryShared_h__
+#define nsSHEntryShared_h__
+
+#include "nsCOMArray.h"
+#include "nsCOMPtr.h"
+#include "nsExpirationTracker.h"
+#include "nsIBFCacheEntry.h"
+#include "nsIWeakReferenceUtils.h"
+#include "nsRect.h"
+#include "nsString.h"
+#include "nsStubMutationObserver.h"
+
+#include "mozilla/Attributes.h"
+#include "mozilla/UniquePtr.h"
+
+class nsSHEntry;
+class nsISHEntry;
+class nsISHistory;
+class nsIContentSecurityPolicy;
+class nsIContentViewer;
+class nsIDocShellTreeItem;
+class nsILayoutHistoryState;
+class nsIPrincipal;
+class nsDocShellEditorData;
+class nsFrameLoader;
+class nsIMutableArray;
+class nsSHistory;
+
+// A document may have multiple SHEntries, either due to hash navigations or
+// calls to history.pushState.  SHEntries corresponding to the same document
+// share many members; in particular, they share state related to the
+// back/forward cache.
+//
+// The classes defined here are the vehicle for this sharing.
+//
+// Some of the state can only be stored in the process where we did the actual
+// load, because that's where the objects live (eg. the content viewer).
+
+namespace mozilla {
+namespace dom {
+class Document;
+
+/**
+ * SHEntrySharedState holds shared state both in the child process and in the
+ * parent process.
+ */
+struct SHEntrySharedState {
+  SHEntrySharedState() : mId(GenerateId()) {}
+  SHEntrySharedState(const SHEntrySharedState& aState) = default;
+  SHEntrySharedState(nsIPrincipal* aTriggeringPrincipal,
+                     nsIPrincipal* aPrincipalToInherit,
+                     nsIPrincipal* aPartitionedPrincipalToInherit,
+                     nsIContentSecurityPolicy* aCsp,
+                     const nsACString& aContentType)
+      : mId(GenerateId()),
+        mTriggeringPrincipal(aTriggeringPrincipal),
+        mPrincipalToInherit(aPrincipalToInherit),
+        mPartitionedPrincipalToInherit(aPartitionedPrincipalToInherit),
+        mCsp(aCsp),
+        mContentType(aContentType) {}
+
+  // These members aren't copied by SHEntrySharedParentState::CopyFrom() because
+  // they're specific to a particular content viewer.
+  uint64_t mId = 0;
+
+  // These members are copied by SHEntrySharedParentState::CopyFrom(). If you
+  // add a member here, be sure to update the CopyFrom() implementation.
+  nsCOMPtr<nsIPrincipal> mTriggeringPrincipal;
+  nsCOMPtr<nsIPrincipal> mPrincipalToInherit;
+  nsCOMPtr<nsIPrincipal> mPartitionedPrincipalToInherit;
+  nsCOMPtr<nsIContentSecurityPolicy> mCsp;
+  nsCString mContentType;
+  // Child side updates layout history state when page is being unloaded or
+  // moved to bfcache.
+  nsCOMPtr<nsILayoutHistoryState> mLayoutHistoryState;
+  uint32_t mCacheKey = 0;
+  bool mIsFrameNavigation = false;
+  bool mSaveLayoutState = true;
+
+ protected:
+  static uint64_t GenerateId();
+};
+
+/**
+ * SHEntrySharedParentState holds the shared state that can live in the parent
+ * process.
+ */
+class SHEntrySharedParentState : public SHEntrySharedState {
+ public:
+  friend class SessionHistoryInfo;
+
+  uint64_t GetId() const { return mId; }
+  void ChangeId(uint64_t aId);
+
+  void SetFrameLoader(nsFrameLoader* aFrameLoader);
+
+  nsFrameLoader* GetFrameLoader();
+
+  void NotifyListenersContentViewerEvicted();
+
+  nsExpirationState* GetExpirationState() { return &mExpirationState; }
+
+  SHEntrySharedParentState();
+  SHEntrySharedParentState(nsIPrincipal* aTriggeringPrincipal,
+                           nsIPrincipal* aPrincipalToInherit,
+                           nsIPrincipal* aPartitionedPrincipalToInherit,
+                           nsIContentSecurityPolicy* aCsp,
+                           const nsACString& aContentType);
+
+  // This returns the existing SHEntrySharedParentState that was registered for
+  // aId, if one exists.
+  static SHEntrySharedParentState* Lookup(uint64_t aId);
+
+ protected:
+  virtual ~SHEntrySharedParentState();
+  NS_INLINE_DECL_VIRTUAL_REFCOUNTING_WITH_DESTROY(SHEntrySharedParentState,
+                                                  Destroy())
+
+  virtual void Destroy() { delete this; }
+
+  void CopyFrom(SHEntrySharedParentState* aSource);
+
+  // These members are copied by SHEntrySharedParentState::CopyFrom(). If you
+  // add a member here, be sure to update the CopyFrom() implementation.
+  nsID mDocShellID{};
+
+  nsIntRect mViewerBounds{0, 0, 0, 0};
+
+  uint32_t mLastTouched = 0;
+
+  // These members aren't copied by SHEntrySharedParentState::CopyFrom() because
+  // they're specific to a particular content viewer.
+  nsWeakPtr mSHistory;
+
+  RefPtr<nsFrameLoader> mFrameLoader;
+
+  nsExpirationState mExpirationState;
+
+  bool mSticky = true;
+  bool mDynamicallyCreated = false;
+
+  // This flag is about necko cache, not bfcache.
+  bool mExpired = false;
+};
+
+/**
+ * SHEntrySharedChildState holds the shared state that needs to live in the
+ * process where the document was loaded.
+ */
+class SHEntrySharedChildState {
+ protected:
+  void CopyFrom(SHEntrySharedChildState* aSource);
+
+ public:
+  // These members are copied by SHEntrySharedChildState::CopyFrom(). If you
+  // add a member here, be sure to update the CopyFrom() implementation.
+  nsCOMArray<nsIDocShellTreeItem> mChildShells;
+
+  // These members aren't copied by SHEntrySharedChildState::CopyFrom() because
+  // they're specific to a particular content viewer.
+  nsCOMPtr<nsIContentViewer> mContentViewer;
+  RefPtr<mozilla::dom::Document> mDocument;
+  nsCOMPtr<nsISupports> mWindowState;
+  // FIXME Move to parent?
+  nsCOMPtr<nsIMutableArray> mRefreshURIList;
+  UniquePtr<nsDocShellEditorData> mEditorData;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+/**
+ * nsSHEntryShared holds the shared state if the session history is not stored
+ * in the parent process, or if the load itself happens in the parent process.
+ * Note, since nsSHEntryShared inherits both SHEntrySharedParentState and
+ * SHEntrySharedChildState and those have some same member variables,
+ * the ones from SHEntrySharedParentState should be used.
+ */
+class nsSHEntryShared final : public nsIBFCacheEntry,
+                              public nsStubMutationObserver,
+                              public mozilla::dom::SHEntrySharedParentState,
+                              public mozilla::dom::SHEntrySharedChildState {
+ public:
+  static void EnsureHistoryTracker();
+  static void Shutdown();
+
+  using SHEntrySharedParentState::SHEntrySharedParentState;
+
+  already_AddRefed<nsSHEntryShared> Duplicate();
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIBFCACHEENTRY
+
+  // The nsIMutationObserver bits we actually care about.
+  NS_DECL_NSIMUTATIONOBSERVER_CHARACTERDATACHANGED
+  NS_DECL_NSIMUTATIONOBSERVER_ATTRIBUTECHANGED
+  NS_DECL_NSIMUTATIONOBSERVER_CONTENTAPPENDED
+  NS_DECL_NSIMUTATIONOBSERVER_CONTENTINSERTED
+  NS_DECL_NSIMUTATIONOBSERVER_CONTENTREMOVED
+
+ private:
+  ~nsSHEntryShared();
+
+  friend class nsSHEntry;
+
+  void RemoveFromExpirationTracker();
+  void SyncPresentationState();
+  void DropPresentationState();
+
+  nsresult SetContentViewer(nsIContentViewer* aViewer);
+};
+
+#endif
diff --git a/docshell/shistory/nsSHistory.cpp b/docshell/shistory/nsSHistory.cpp
new file mode 100644
index 0000000000..214646dde9
--- /dev/null
+++ b/docshell/shistory/nsSHistory.cpp
@@ -0,0 +1,2410 @@
+/* -*- 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 "nsSHistory.h"
+
+#include <algorithm>
+
+#include "nsContentUtils.h"
+#include "nsCOMArray.h"
+#include "nsComponentManagerUtils.h"
+#include "nsDocShell.h"
+#include "nsFrameLoaderOwner.h"
+#include "nsHashKeys.h"
+#include "nsIContentViewer.h"
+#include "nsIDocShell.h"
+#include "nsDocShellLoadState.h"
+#include "nsIDocShellTreeItem.h"
+#include "nsILayoutHistoryState.h"
+#include "nsIObserverService.h"
+#include "nsISHEntry.h"
+#include "nsISHistoryListener.h"
+#include "nsIURI.h"
+#include "nsIXULRuntime.h"
+#include "nsNetUtil.h"
+#include "nsTHashMap.h"
+#include "nsSHEntry.h"
+#include "SessionHistoryEntry.h"
+#include "nsTArray.h"
+#include "prsystem.h"
+
+#include "mozilla/Attributes.h"
+#include "mozilla/dom/BrowsingContextGroup.h"
+#include "mozilla/dom/CanonicalBrowsingContext.h"
+#include "mozilla/dom/ContentParent.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/RemoteWebProgressRequest.h"
+#include "mozilla/dom/WindowGlobalParent.h"
+#include "mozilla/LinkedList.h"
+#include "mozilla/MathAlgorithms.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/ProcessPriorityManager.h"
+#include "mozilla/Services.h"
+#include "mozilla/StaticPrefs_fission.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/dom/CanonicalBrowsingContext.h"
+#include "nsIWebNavigation.h"
+#include "nsDocShellLoadTypes.h"
+#include "base/process.h"
+
+using namespace mozilla;
+using namespace mozilla::dom;
+
+#define PREF_SHISTORY_SIZE "browser.sessionhistory.max_entries"
+#define PREF_SHISTORY_MAX_TOTAL_VIEWERS \
+  "browser.sessionhistory.max_total_viewers"
+#define CONTENT_VIEWER_TIMEOUT_SECONDS \
+  "browser.sessionhistory.contentViewerTimeout"
+// Observe fission.bfcacheInParent so that BFCache can be enabled/disabled when
+// the pref is changed.
+#define PREF_FISSION_BFCACHEINPARENT "fission.bfcacheInParent"
+
+// Default this to time out unused content viewers after 30 minutes
+#define CONTENT_VIEWER_TIMEOUT_SECONDS_DEFAULT (30 * 60)
+
+static const char* kObservedPrefs[] = {PREF_SHISTORY_SIZE,
+                                       PREF_SHISTORY_MAX_TOTAL_VIEWERS,
+                                       PREF_FISSION_BFCACHEINPARENT, nullptr};
+
+static int32_t gHistoryMaxSize = 50;
+
+// List of all SHistory objects, used for content viewer cache eviction.
+// When being destroyed, this helper removes everything from the list to avoid
+// assertions when we leak.
+struct ListHelper {
+#ifdef DEBUG
+  ~ListHelper() { mList.clear(); }
+#endif  // DEBUG
+
+  LinkedList<nsSHistory> mList;
+};
+
+static ListHelper gSHistoryList;
+// Max viewers allowed total, across all SHistory objects - negative default
+// means we will calculate how many viewers to cache based on total memory
+int32_t nsSHistory::sHistoryMaxTotalViewers = -1;
+
+// A counter that is used to be able to know the order in which
+// entries were touched, so that we can evict older entries first.
+static uint32_t gTouchCounter = 0;
+
+extern mozilla::LazyLogModule gSHLog;
+
+LazyLogModule gSHistoryLog("nsSHistory");
+
+#define LOG(format) MOZ_LOG(gSHistoryLog, mozilla::LogLevel::Debug, format)
+
+extern mozilla::LazyLogModule gPageCacheLog;
+extern mozilla::LazyLogModule gSHIPBFCacheLog;
+
+// This macro makes it easier to print a log message which includes a URI's
+// spec.  Example use:
+//
+//  nsIURI *uri = [...];
+//  LOG_SPEC(("The URI is %s.", _spec), uri);
+//
+#define LOG_SPEC(format, uri)                        \
+  PR_BEGIN_MACRO                                     \
+  if (MOZ_LOG_TEST(gSHistoryLog, LogLevel::Debug)) { \
+    nsAutoCString _specStr("(null)"_ns);             \
+    if (uri) {                                       \
+      _specStr = uri->GetSpecOrDefault();            \
+    }                                                \
+    const char* _spec = _specStr.get();              \
+    LOG(format);                                     \
+  }                                                  \
+  PR_END_MACRO
+
+// This macro makes it easy to log a message including an SHEntry's URI.
+// For example:
+//
+//  nsCOMPtr<nsISHEntry> shentry = [...];
+//  LOG_SHENTRY_SPEC(("shentry %p has uri %s.", shentry.get(), _spec), shentry);
+//
+#define LOG_SHENTRY_SPEC(format, shentry)            \
+  PR_BEGIN_MACRO                                     \
+  if (MOZ_LOG_TEST(gSHistoryLog, LogLevel::Debug)) { \
+    nsCOMPtr<nsIURI> uri = shentry->GetURI();        \
+    LOG_SPEC(format, uri);                           \
+  }                                                  \
+  PR_END_MACRO
+
+// Calls a F on all registered session history listeners.
+template <typename F>
+static void NotifyListeners(nsAutoTObserverArray<nsWeakPtr, 2>& aListeners,
+                            F&& f) {
+  for (const nsWeakPtr& weakPtr : aListeners.EndLimitedRange()) {
+    nsCOMPtr<nsISHistoryListener> listener = do_QueryReferent(weakPtr);
+    if (listener) {
+      f(listener);
+    }
+  }
+}
+
+class MOZ_STACK_CLASS SHistoryChangeNotifier {
+ public:
+  explicit SHistoryChangeNotifier(nsSHistory* aHistory) {
+    // If we're already in an update, the outermost change notifier will
+    // update browsing context in the destructor.
+    if (!aHistory->HasOngoingUpdate()) {
+      aHistory->SetHasOngoingUpdate(true);
+      mSHistory = aHistory;
+    }
+  }
+
+  ~SHistoryChangeNotifier() {
+    if (mSHistory) {
+      MOZ_ASSERT(mSHistory->HasOngoingUpdate());
+      mSHistory->SetHasOngoingUpdate(false);
+
+      RefPtr<BrowsingContext> rootBC = mSHistory->GetBrowsingContext();
+      if (mozilla::SessionHistoryInParent() && rootBC) {
+        rootBC->Canonical()->HistoryCommitIndexAndLength();
+      }
+    }
+  }
+
+  RefPtr<nsSHistory> mSHistory;
+};
+
+enum HistCmd { HIST_CMD_GOTOINDEX, HIST_CMD_RELOAD };
+
+class nsSHistoryObserver final : public nsIObserver {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIOBSERVER
+
+  nsSHistoryObserver() {}
+
+  static void PrefChanged(const char* aPref, void* aSelf);
+  void PrefChanged(const char* aPref);
+
+ protected:
+  ~nsSHistoryObserver() {}
+};
+
+StaticRefPtr<nsSHistoryObserver> gObserver;
+
+NS_IMPL_ISUPPORTS(nsSHistoryObserver, nsIObserver)
+
+// static
+void nsSHistoryObserver::PrefChanged(const char* aPref, void* aSelf) {
+  static_cast<nsSHistoryObserver*>(aSelf)->PrefChanged(aPref);
+}
+
+void nsSHistoryObserver::PrefChanged(const char* aPref) {
+  nsSHistory::UpdatePrefs();
+  nsSHistory::GloballyEvictContentViewers();
+}
+
+NS_IMETHODIMP
+nsSHistoryObserver::Observe(nsISupports* aSubject, const char* aTopic,
+                            const char16_t* aData) {
+  if (!strcmp(aTopic, "cacheservice:empty-cache") ||
+      !strcmp(aTopic, "memory-pressure")) {
+    nsSHistory::GloballyEvictAllContentViewers();
+  }
+
+  return NS_OK;
+}
+
+void nsSHistory::EvictContentViewerForEntry(nsISHEntry* aEntry) {
+  nsCOMPtr<nsIContentViewer> viewer = aEntry->GetContentViewer();
+  if (viewer) {
+    LOG_SHENTRY_SPEC(("Evicting content viewer 0x%p for "
+                      "owning SHEntry 0x%p at %s.",
+                      viewer.get(), aEntry, _spec),
+                     aEntry);
+
+    // Drop the presentation state before destroying the viewer, so that
+    // document teardown is able to correctly persist the state.
+    NotifyListenersContentViewerEvicted(1);
+    aEntry->SetContentViewer(nullptr);
+    aEntry->SyncPresentationState();
+    viewer->Destroy();
+  } else if (nsCOMPtr<SessionHistoryEntry> she = do_QueryInterface(aEntry)) {
+    if (RefPtr<nsFrameLoader> frameLoader = she->GetFrameLoader()) {
+      nsCOMPtr<nsFrameLoaderOwner> owner =
+          do_QueryInterface(frameLoader->GetOwnerContent());
+      RefPtr<nsFrameLoader> currentFrameLoader;
+      if (owner) {
+        currentFrameLoader = owner->GetFrameLoader();
+      }
+
+      // Only destroy non-current frameloader when evicting from the bfcache.
+      if (currentFrameLoader != frameLoader) {
+        MOZ_LOG(gSHIPBFCacheLog, LogLevel::Debug,
+                ("nsSHistory::EvictContentViewerForEntry "
+                 "destroying an nsFrameLoader."));
+        NotifyListenersContentViewerEvicted(1);
+        she->SetFrameLoader(nullptr);
+        frameLoader->Destroy();
+      }
+    }
+  }
+
+  // When dropping bfcache, we have to remove associated dynamic entries as
+  // well.
+  int32_t index = GetIndexOfEntry(aEntry);
+  if (index != -1) {
+    RemoveDynEntries(index, aEntry);
+  }
+}
+
+nsSHistory::nsSHistory(BrowsingContext* aRootBC)
+    : mRootBC(aRootBC->Id()),
+      mHasOngoingUpdate(false),
+      mIndex(-1),
+      mRequestedIndex(-1),
+      mRootDocShellID(aRootBC->GetHistoryID()) {
+  static bool sCalledStartup = false;
+  if (!sCalledStartup) {
+    Startup();
+    sCalledStartup = true;
+  }
+
+  // Add this new SHistory object to the list
+  gSHistoryList.mList.insertBack(this);
+
+  // Init mHistoryTracker on setting mRootBC so we can bind its event
+  // target to the tabGroup.
+  mHistoryTracker = mozilla::MakeUnique<HistoryTracker>(
+      this,
+      mozilla::Preferences::GetUint(CONTENT_VIEWER_TIMEOUT_SECONDS,
+                                    CONTENT_VIEWER_TIMEOUT_SECONDS_DEFAULT),
+      GetCurrentSerialEventTarget());
+}
+
+nsSHistory::~nsSHistory() {
+  // Clear mEntries explicitly here so that the destructor of the entries
+  // can still access nsSHistory in a reasonable way.
+  mEntries.Clear();
+}
+
+NS_IMPL_ADDREF(nsSHistory)
+NS_IMPL_RELEASE(nsSHistory)
+
+NS_INTERFACE_MAP_BEGIN(nsSHistory)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsISHistory)
+  NS_INTERFACE_MAP_ENTRY(nsISHistory)
+  NS_INTERFACE_MAP_ENTRY(nsISupportsWeakReference)
+NS_INTERFACE_MAP_END
+
+// static
+uint32_t nsSHistory::CalcMaxTotalViewers() {
+// This value allows tweaking how fast the allowed amount of content viewers
+// grows with increasing amounts of memory. Larger values mean slower growth.
+#ifdef ANDROID
+#  define MAX_TOTAL_VIEWERS_BIAS 15.9
+#else
+#  define MAX_TOTAL_VIEWERS_BIAS 14
+#endif
+
+  // Calculate an estimate of how many ContentViewers we should cache based
+  // on RAM.  This assumes that the average ContentViewer is 4MB (conservative)
+  // and caps the max at 8 ContentViewers
+  //
+  // TODO: Should we split the cache memory betw. ContentViewer caching and
+  // nsCacheService?
+  //
+  // RAM    | ContentViewers | on Android
+  // -------------------------------------
+  // 32   Mb       0                0
+  // 64   Mb       1                0
+  // 128  Mb       2                0
+  // 256  Mb       3                1
+  // 512  Mb       5                2
+  // 768  Mb       6                2
+  // 1024 Mb       8                3
+  // 2048 Mb       8                5
+  // 3072 Mb       8                7
+  // 4096 Mb       8                8
+  uint64_t bytes = PR_GetPhysicalMemorySize();
+
+  if (bytes == 0) {
+    return 0;
+  }
+
+  // Conversion from unsigned int64_t to double doesn't work on all platforms.
+  // We need to truncate the value at INT64_MAX to make sure we don't
+  // overflow.
+  if (bytes > INT64_MAX) {
+    bytes = INT64_MAX;
+  }
+
+  double kBytesD = (double)(bytes >> 10);
+
+  // This is essentially the same calculation as for nsCacheService,
+  // except that we divide the final memory calculation by 4, since
+  // we assume each ContentViewer takes on average 4MB
+  uint32_t viewers = 0;
+  double x = std::log(kBytesD) / std::log(2.0) - MAX_TOTAL_VIEWERS_BIAS;
+  if (x > 0) {
+    viewers = (uint32_t)(x * x - x + 2.001);  // add .001 for rounding
+    viewers /= 4;
+  }
+
+  // Cap it off at 8 max
+  if (viewers > 8) {
+    viewers = 8;
+  }
+  return viewers;
+}
+
+// static
+void nsSHistory::UpdatePrefs() {
+  Preferences::GetInt(PREF_SHISTORY_SIZE, &gHistoryMaxSize);
+  if (mozilla::SessionHistoryInParent() && !mozilla::BFCacheInParent()) {
+    sHistoryMaxTotalViewers = 0;
+    return;
+  }
+
+  Preferences::GetInt(PREF_SHISTORY_MAX_TOTAL_VIEWERS,
+                      &sHistoryMaxTotalViewers);
+  // If the pref is negative, that means we calculate how many viewers
+  // we think we should cache, based on total memory
+  if (sHistoryMaxTotalViewers < 0) {
+    sHistoryMaxTotalViewers = CalcMaxTotalViewers();
+  }
+}
+
+// static
+nsresult nsSHistory::Startup() {
+  UpdatePrefs();
+
+  // The goal of this is to unbreak users who have inadvertently set their
+  // session history size to less than the default value.
+  int32_t defaultHistoryMaxSize =
+      Preferences::GetInt(PREF_SHISTORY_SIZE, 50, PrefValueKind::Default);
+  if (gHistoryMaxSize < defaultHistoryMaxSize) {
+    gHistoryMaxSize = defaultHistoryMaxSize;
+  }
+
+  // Allow the user to override the max total number of cached viewers,
+  // but keep the per SHistory cached viewer limit constant
+  if (!gObserver) {
+    gObserver = new nsSHistoryObserver();
+    Preferences::RegisterCallbacks(nsSHistoryObserver::PrefChanged,
+                                   kObservedPrefs, gObserver.get());
+
+    nsCOMPtr<nsIObserverService> obsSvc =
+        mozilla::services::GetObserverService();
+    if (obsSvc) {
+      // Observe empty-cache notifications so tahat clearing the disk/memory
+      // cache will also evict all content viewers.
+      obsSvc->AddObserver(gObserver, "cacheservice:empty-cache", false);
+
+      // Same for memory-pressure notifications
+      obsSvc->AddObserver(gObserver, "memory-pressure", false);
+    }
+  }
+
+  return NS_OK;
+}
+
+// static
+void nsSHistory::Shutdown() {
+  if (gObserver) {
+    Preferences::UnregisterCallbacks(nsSHistoryObserver::PrefChanged,
+                                     kObservedPrefs, gObserver.get());
+
+    nsCOMPtr<nsIObserverService> obsSvc =
+        mozilla::services::GetObserverService();
+    if (obsSvc) {
+      obsSvc->RemoveObserver(gObserver, "cacheservice:empty-cache");
+      obsSvc->RemoveObserver(gObserver, "memory-pressure");
+    }
+    gObserver = nullptr;
+  }
+}
+
+// static
+already_AddRefed<nsISHEntry> nsSHistory::GetRootSHEntry(nsISHEntry* aEntry) {
+  nsCOMPtr<nsISHEntry> rootEntry = aEntry;
+  nsCOMPtr<nsISHEntry> result = nullptr;
+  while (rootEntry) {
+    result = rootEntry;
+    rootEntry = result->GetParent();
+  }
+
+  return result.forget();
+}
+
+// static
+nsresult nsSHistory::WalkHistoryEntries(nsISHEntry* aRootEntry,
+                                        BrowsingContext* aBC,
+                                        WalkHistoryEntriesFunc aCallback,
+                                        void* aData) {
+  NS_ENSURE_TRUE(aRootEntry, NS_ERROR_FAILURE);
+
+  int32_t childCount = aRootEntry->GetChildCount();
+  for (int32_t i = 0; i < childCount; i++) {
+    nsCOMPtr<nsISHEntry> childEntry;
+    aRootEntry->GetChildAt(i, getter_AddRefs(childEntry));
+    if (!childEntry) {
+      // childEntry can be null for valid reasons, for example if the
+      // docshell at index i never loaded anything useful.
+      // Remember to clone also nulls in the child array (bug 464064).
+      aCallback(nullptr, nullptr, i, aData);
+      continue;
+    }
+
+    BrowsingContext* childBC = nullptr;
+    if (aBC) {
+      for (BrowsingContext* child : aBC->Children()) {
+        // If the SH pref is on and we are in the parent process, update
+        // canonical BC directly
+        bool foundChild = false;
+        if (mozilla::SessionHistoryInParent() && XRE_IsParentProcess()) {
+          if (child->Canonical()->HasHistoryEntry(childEntry)) {
+            childBC = child;
+            foundChild = true;
+          }
+        }
+
+        nsDocShell* docshell = static_cast<nsDocShell*>(child->GetDocShell());
+        if (docshell && docshell->HasHistoryEntry(childEntry)) {
+          childBC = docshell->GetBrowsingContext();
+          foundChild = true;
+        }
+
+        // XXX Simplify this once the old and new session history
+        // implementations don't run at the same time.
+        if (foundChild) {
+          break;
+        }
+      }
+    }
+
+    nsresult rv = aCallback(childEntry, childBC, i, aData);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  return NS_OK;
+}
+
+// callback data for WalkHistoryEntries
+struct MOZ_STACK_CLASS CloneAndReplaceData {
+  CloneAndReplaceData(uint32_t aCloneID, nsISHEntry* aReplaceEntry,
+                      bool aCloneChildren, nsISHEntry* aDestTreeParent)
+      : cloneID(aCloneID),
+        cloneChildren(aCloneChildren),
+        replaceEntry(aReplaceEntry),
+        destTreeParent(aDestTreeParent) {}
+
+  uint32_t cloneID;
+  bool cloneChildren;
+  nsISHEntry* replaceEntry;
+  nsISHEntry* destTreeParent;
+  nsCOMPtr<nsISHEntry> resultEntry;
+};
+
+nsresult nsSHistory::CloneAndReplaceChild(nsISHEntry* aEntry,
+                                          BrowsingContext* aOwnerBC,
+                                          int32_t aChildIndex, void* aData) {
+  nsCOMPtr<nsISHEntry> dest;
+
+  CloneAndReplaceData* data = static_cast<CloneAndReplaceData*>(aData);
+  uint32_t cloneID = data->cloneID;
+  nsISHEntry* replaceEntry = data->replaceEntry;
+
+  if (!aEntry) {
+    if (data->destTreeParent) {
+      data->destTreeParent->AddChild(nullptr, aChildIndex);
+    }
+    return NS_OK;
+  }
+
+  uint32_t srcID = aEntry->GetID();
+
+  nsresult rv = NS_OK;
+  if (srcID == cloneID) {
+    // Replace the entry
+    dest = replaceEntry;
+  } else {
+    // Clone the SHEntry...
+    rv = aEntry->Clone(getter_AddRefs(dest));
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+  dest->SetIsSubFrame(true);
+
+  if (srcID != cloneID || data->cloneChildren) {
+    // Walk the children
+    CloneAndReplaceData childData(cloneID, replaceEntry, data->cloneChildren,
+                                  dest);
+    rv = nsSHistory::WalkHistoryEntries(aEntry, aOwnerBC, CloneAndReplaceChild,
+                                        &childData);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  if (srcID != cloneID && aOwnerBC) {
+    nsSHistory::HandleEntriesToSwapInDocShell(aOwnerBC, aEntry, dest);
+  }
+
+  if (data->destTreeParent) {
+    data->destTreeParent->AddChild(dest, aChildIndex);
+  }
+  data->resultEntry = dest;
+  return rv;
+}
+
+// static
+nsresult nsSHistory::CloneAndReplace(
+    nsISHEntry* aSrcEntry, BrowsingContext* aOwnerBC, uint32_t aCloneID,
+    nsISHEntry* aReplaceEntry, bool aCloneChildren, nsISHEntry** aDestEntry) {
+  NS_ENSURE_ARG_POINTER(aDestEntry);
+  NS_ENSURE_TRUE(aReplaceEntry, NS_ERROR_FAILURE);
+  CloneAndReplaceData data(aCloneID, aReplaceEntry, aCloneChildren, nullptr);
+  nsresult rv = CloneAndReplaceChild(aSrcEntry, aOwnerBC, 0, &data);
+  data.resultEntry.swap(*aDestEntry);
+  return rv;
+}
+
+// static
+void nsSHistory::WalkContiguousEntries(
+    nsISHEntry* aEntry, const std::function<void(nsISHEntry*)>& aCallback) {
+  MOZ_ASSERT(aEntry);
+
+  nsCOMPtr<nsISHistory> shistory = aEntry->GetShistory();
+  if (!shistory) {
+    // If there is no session history in the entry, it means this is not a root
+    // entry. So, we can return from here.
+    return;
+  }
+
+  int32_t index = shistory->GetIndexOfEntry(aEntry);
+  int32_t count = shistory->GetCount();
+
+  nsCOMPtr<nsIURI> targetURI = aEntry->GetURI();
+
+  // First, call the callback on the input entry.
+  aCallback(aEntry);
+
+  // Walk backward to find the entries that have the same origin as the
+  // input entry.
+  for (int32_t i = index - 1; i >= 0; i--) {
+    RefPtr<nsISHEntry> entry;
+    shistory->GetEntryAtIndex(i, getter_AddRefs(entry));
+    if (entry) {
+      nsCOMPtr<nsIURI> uri = entry->GetURI();
+      if (NS_FAILED(nsContentUtils::GetSecurityManager()->CheckSameOriginURI(
+              targetURI, uri, false, false))) {
+        break;
+      }
+
+      aCallback(entry);
+    }
+  }
+
+  // Then, Walk forward.
+  for (int32_t i = index + 1; i < count; i++) {
+    RefPtr<nsISHEntry> entry;
+    shistory->GetEntryAtIndex(i, getter_AddRefs(entry));
+    if (entry) {
+      nsCOMPtr<nsIURI> uri = entry->GetURI();
+      if (NS_FAILED(nsContentUtils::GetSecurityManager()->CheckSameOriginURI(
+              targetURI, uri, false, false))) {
+        break;
+      }
+
+      aCallback(entry);
+    }
+  }
+}
+
+NS_IMETHODIMP
+nsSHistory::AddChildSHEntryHelper(nsISHEntry* aCloneRef, nsISHEntry* aNewEntry,
+                                  BrowsingContext* aRootBC,
+                                  bool aCloneChildren) {
+  MOZ_ASSERT(aRootBC->IsTop());
+
+  /* You are currently in the rootDocShell.
+   * You will get here when a subframe has a new url
+   * to load and you have walked up the tree all the
+   * way to the top to clone the current SHEntry hierarchy
+   * and replace the subframe where a new url was loaded with
+   * a new entry.
+   */
+  nsCOMPtr<nsISHEntry> child;
+  nsCOMPtr<nsISHEntry> currentHE;
+  int32_t index = mIndex;
+  if (index < 0) {
+    return NS_ERROR_FAILURE;
+  }
+
+  GetEntryAtIndex(index, getter_AddRefs(currentHE));
+  NS_ENSURE_TRUE(currentHE, NS_ERROR_FAILURE);
+
+  nsresult rv = NS_OK;
+  uint32_t cloneID = aCloneRef->GetID();
+  rv = nsSHistory::CloneAndReplace(currentHE, aRootBC, cloneID, aNewEntry,
+                                   aCloneChildren, getter_AddRefs(child));
+
+  if (NS_SUCCEEDED(rv)) {
+    rv = AddEntry(child, true);
+    if (NS_SUCCEEDED(rv)) {
+      child->SetDocshellID(aRootBC->GetHistoryID());
+    }
+  }
+
+  return rv;
+}
+
+nsresult nsSHistory::SetChildHistoryEntry(nsISHEntry* aEntry,
+                                          BrowsingContext* aBC,
+                                          int32_t aEntryIndex, void* aData) {
+  SwapEntriesData* data = static_cast<SwapEntriesData*>(aData);
+  if (!aBC || aBC == data->ignoreBC) {
+    return NS_OK;
+  }
+
+  nsISHEntry* destTreeRoot = data->destTreeRoot;
+
+  nsCOMPtr<nsISHEntry> destEntry;
+
+  if (data->destTreeParent) {
+    // aEntry is a clone of some child of destTreeParent, but since the
+    // trees aren't necessarily in sync, we'll have to locate it.
+    // Note that we could set aShell's entry to null if we don't find a
+    // corresponding entry under destTreeParent.
+
+    uint32_t targetID = aEntry->GetID();
+
+    // First look at the given index, since this is the common case.
+    nsCOMPtr<nsISHEntry> entry;
+    data->destTreeParent->GetChildAt(aEntryIndex, getter_AddRefs(entry));
+    if (entry && entry->GetID() == targetID) {
+      destEntry.swap(entry);
+    } else {
+      int32_t childCount;
+      data->destTreeParent->GetChildCount(&childCount);
+      for (int32_t i = 0; i < childCount; ++i) {
+        data->destTreeParent->GetChildAt(i, getter_AddRefs(entry));
+        if (!entry) {
+          continue;
+        }
+
+        if (entry->GetID() == targetID) {
+          destEntry.swap(entry);
+          break;
+        }
+      }
+    }
+  } else {
+    destEntry = destTreeRoot;
+  }
+
+  nsSHistory::HandleEntriesToSwapInDocShell(aBC, aEntry, destEntry);
+  // Now handle the children of aEntry.
+  SwapEntriesData childData = {data->ignoreBC, destTreeRoot, destEntry};
+  return nsSHistory::WalkHistoryEntries(aEntry, aBC, SetChildHistoryEntry,
+                                        &childData);
+}
+
+// static
+void nsSHistory::HandleEntriesToSwapInDocShell(
+    mozilla::dom::BrowsingContext* aBC, nsISHEntry* aOldEntry,
+    nsISHEntry* aNewEntry) {
+  bool shPref = mozilla::SessionHistoryInParent();
+  if (aBC->IsInProcess() || !shPref) {
+    nsDocShell* docshell = static_cast<nsDocShell*>(aBC->GetDocShell());
+    if (docshell) {
+      docshell->SwapHistoryEntries(aOldEntry, aNewEntry);
+    }
+  } else {
+    // FIXME Bug 1633988: Need to update entries?
+  }
+
+  // XXX Simplify this once the old and new session history implementations
+  // don't run at the same time.
+  if (shPref && XRE_IsParentProcess()) {
+    aBC->Canonical()->SwapHistoryEntries(aOldEntry, aNewEntry);
+  }
+}
+
+void nsSHistory::UpdateRootBrowsingContextState(BrowsingContext* aRootBC) {
+  if (aRootBC && aRootBC->EverAttached()) {
+    bool sameDocument = IsEmptyOrHasEntriesForSingleTopLevelPage();
+    if (sameDocument != aRootBC->GetIsSingleToplevelInHistory()) {
+      // If the browsing context is discarded then its session history is
+      // invalid and will go away.
+      Unused << aRootBC->SetIsSingleToplevelInHistory(sameDocument);
+    }
+  }
+}
+
+NS_IMETHODIMP
+nsSHistory::AddToRootSessionHistory(bool aCloneChildren, nsISHEntry* aOSHE,
+                                    BrowsingContext* aRootBC,
+                                    nsISHEntry* aEntry, uint32_t aLoadType,
+                                    bool aShouldPersist,
+                                    Maybe<int32_t>* aPreviousEntryIndex,
+                                    Maybe<int32_t>* aLoadedEntryIndex) {
+  MOZ_ASSERT(aRootBC->IsTop());
+
+  nsresult rv = NS_OK;
+
+  // If we need to clone our children onto the new session
+  // history entry, do so now.
+  if (aCloneChildren && aOSHE) {
+    uint32_t cloneID = aOSHE->GetID();
+    nsCOMPtr<nsISHEntry> newEntry;
+    nsSHistory::CloneAndReplace(aOSHE, aRootBC, cloneID, aEntry, true,
+                                getter_AddRefs(newEntry));
+    NS_ASSERTION(aEntry == newEntry,
+                 "The new session history should be in the new entry");
+  }
+  // This is the root docshell
+  bool addToSHistory = !LOAD_TYPE_HAS_FLAGS(
+      aLoadType, nsIWebNavigation::LOAD_FLAGS_REPLACE_HISTORY);
+  if (!addToSHistory) {
+    // Replace current entry in session history; If the requested index is
+    // valid, it indicates the loading was triggered by a history load, and
+    // we should replace the entry at requested index instead.
+    int32_t index = GetIndexForReplace();
+
+    // Replace the current entry with the new entry
+    if (index >= 0) {
+      rv = ReplaceEntry(index, aEntry);
+    } else {
+      // If we're trying to replace an inexistant shistory entry, append.
+      addToSHistory = true;
+    }
+  }
+  if (addToSHistory) {
+    // Add to session history
+    *aPreviousEntryIndex = Some(mIndex);
+    rv = AddEntry(aEntry, aShouldPersist);
+    *aLoadedEntryIndex = Some(mIndex);
+    MOZ_LOG(gPageCacheLog, LogLevel::Verbose,
+            ("Previous index: %d, Loaded index: %d",
+             aPreviousEntryIndex->value(), aLoadedEntryIndex->value()));
+  }
+  if (NS_SUCCEEDED(rv)) {
+    aEntry->SetDocshellID(aRootBC->GetHistoryID());
+  }
+  return rv;
+}
+
+/* Add an entry to the History list at mIndex and
+ * increment the index to point to the new entry
+ */
+NS_IMETHODIMP
+nsSHistory::AddEntry(nsISHEntry* aSHEntry, bool aPersist) {
+  NS_ENSURE_ARG(aSHEntry);
+
+  nsCOMPtr<nsISHistory> shistoryOfEntry = aSHEntry->GetShistory();
+  if (shistoryOfEntry && shistoryOfEntry != this) {
+    NS_WARNING(
+        "The entry has been associated to another nsISHistory instance. "
+        "Try nsISHEntry.clone() and nsISHEntry.abandonBFCacheEntry() "
+        "first if you're copying an entry from another nsISHistory.");
+    return NS_ERROR_FAILURE;
+  }
+
+  aSHEntry->SetShistory(this);
+
+  // If we have a root docshell, update the docshell id of the root shentry to
+  // match the id of that docshell
+  RefPtr<BrowsingContext> rootBC = GetBrowsingContext();
+  if (rootBC) {
+    aSHEntry->SetDocshellID(mRootDocShellID);
+  }
+
+  if (mIndex >= 0) {
+    MOZ_ASSERT(mIndex < Length(), "Index out of range!");
+    if (mIndex >= Length()) {
+      return NS_ERROR_FAILURE;
+    }
+
+    if (mEntries[mIndex] && !mEntries[mIndex]->GetPersist()) {
+      NotifyListeners(mListeners, [](auto l) { l->OnHistoryReplaceEntry(); });
+      aSHEntry->SetPersist(aPersist);
+      mEntries[mIndex] = aSHEntry;
+      UpdateRootBrowsingContextState();
+      return NS_OK;
+    }
+  }
+  SHistoryChangeNotifier change(this);
+
+  int32_t truncating = Length() - 1 - mIndex;
+  if (truncating > 0) {
+    NotifyListeners(mListeners,
+                    [truncating](auto l) { l->OnHistoryTruncate(truncating); });
+  }
+
+  nsCOMPtr<nsIURI> uri = aSHEntry->GetURI();
+  NotifyListeners(mListeners,
+                  [&uri, this](auto l) { l->OnHistoryNewEntry(uri, mIndex); });
+
+  // Remove all entries after the current one, add the new one, and set the
+  // new one as the current one.
+  MOZ_ASSERT(mIndex >= -1);
+  aSHEntry->SetPersist(aPersist);
+  mEntries.TruncateLength(mIndex + 1);
+  mEntries.AppendElement(aSHEntry);
+  mIndex++;
+  if (mIndex > 0) {
+    UpdateEntryLength(mEntries[mIndex - 1], mEntries[mIndex], false);
+  }
+
+  // Purge History list if it is too long
+  if (gHistoryMaxSize >= 0 && Length() > gHistoryMaxSize) {
+    PurgeHistory(Length() - gHistoryMaxSize);
+  }
+
+  UpdateRootBrowsingContextState();
+
+  return NS_OK;
+}
+
+void nsSHistory::NotifyOnHistoryReplaceEntry() {
+  NotifyListeners(mListeners, [](auto l) { l->OnHistoryReplaceEntry(); });
+}
+
+/* Get size of the history list */
+NS_IMETHODIMP
+nsSHistory::GetCount(int32_t* aResult) {
+  MOZ_ASSERT(aResult, "null out param?");
+  *aResult = Length();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHistory::GetIndex(int32_t* aResult) {
+  MOZ_ASSERT(aResult, "null out param?");
+  *aResult = mIndex;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHistory::SetIndex(int32_t aIndex) {
+  if (aIndex < 0 || aIndex >= Length()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  mIndex = aIndex;
+  return NS_OK;
+}
+
+/* Get the requestedIndex */
+NS_IMETHODIMP
+nsSHistory::GetRequestedIndex(int32_t* aResult) {
+  MOZ_ASSERT(aResult, "null out param?");
+  *aResult = mRequestedIndex;
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(void)
+nsSHistory::InternalSetRequestedIndex(int32_t aRequestedIndex) {
+  MOZ_ASSERT(aRequestedIndex >= -1 && aRequestedIndex < Length());
+  mRequestedIndex = aRequestedIndex;
+}
+
+NS_IMETHODIMP
+nsSHistory::GetEntryAtIndex(int32_t aIndex, nsISHEntry** aResult) {
+  NS_ENSURE_ARG_POINTER(aResult);
+
+  if (aIndex < 0 || aIndex >= Length()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  *aResult = mEntries[aIndex];
+  NS_ADDREF(*aResult);
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(int32_t)
+nsSHistory::GetIndexOfEntry(nsISHEntry* aSHEntry) {
+  for (int32_t i = 0; i < Length(); i++) {
+    if (aSHEntry == mEntries[i]) {
+      return i;
+    }
+  }
+
+  return -1;
+}
+
+static void LogEntry(nsISHEntry* aEntry, int32_t aIndex, int32_t aTotal,
+                     const nsCString& aPrefix, bool aIsCurrent) {
+  if (!aEntry) {
+    MOZ_LOG(gSHLog, LogLevel::Debug,
+            (" %s+- %i SH Entry null\n", aPrefix.get(), aIndex));
+    return;
+  }
+
+  nsCOMPtr<nsIURI> uri = aEntry->GetURI();
+  nsAutoString title, name;
+  aEntry->GetTitle(title);
+  aEntry->GetName(name);
+
+  SHEntrySharedParentState* shared;
+  if (mozilla::SessionHistoryInParent()) {
+    shared = static_cast<SessionHistoryEntry*>(aEntry)->SharedInfo();
+  } else {
+    shared = static_cast<nsSHEntry*>(aEntry)->GetState();
+  }
+
+  nsID docShellId;
+  aEntry->GetDocshellID(docShellId);
+
+  int32_t childCount = aEntry->GetChildCount();
+
+  MOZ_LOG(gSHLog, LogLevel::Debug,
+          ("%s%s+- %i SH Entry %p %" PRIu64 " %s\n", aIsCurrent ? ">" : " ",
+           aPrefix.get(), aIndex, aEntry, shared->GetId(),
+           nsIDToCString(docShellId).get()));
+
+  nsCString prefix(aPrefix);
+  if (aIndex < aTotal - 1) {
+    prefix.AppendLiteral("|   ");
+  } else {
+    prefix.AppendLiteral("    ");
+  }
+
+  MOZ_LOG(gSHLog, LogLevel::Debug,
+          (" %s%s  URL = %s\n", prefix.get(), childCount > 0 ? "|" : " ",
+           uri->GetSpecOrDefault().get()));
+  MOZ_LOG(gSHLog, LogLevel::Debug,
+          (" %s%s  Title = %s\n", prefix.get(), childCount > 0 ? "|" : " ",
+           NS_LossyConvertUTF16toASCII(title).get()));
+  MOZ_LOG(gSHLog, LogLevel::Debug,
+          (" %s%s  Name = %s\n", prefix.get(), childCount > 0 ? "|" : " ",
+           NS_LossyConvertUTF16toASCII(name).get()));
+  MOZ_LOG(
+      gSHLog, LogLevel::Debug,
+      (" %s%s  Is in BFCache = %s\n", prefix.get(), childCount > 0 ? "|" : " ",
+       aEntry->GetIsInBFCache() ? "true" : "false"));
+
+  nsCOMPtr<nsISHEntry> prevChild;
+  for (int32_t i = 0; i < childCount; ++i) {
+    nsCOMPtr<nsISHEntry> child;
+    aEntry->GetChildAt(i, getter_AddRefs(child));
+    LogEntry(child, i, childCount, prefix, false);
+    child.swap(prevChild);
+  }
+}
+
+void nsSHistory::LogHistory() {
+  if (!MOZ_LOG_TEST(gSHLog, LogLevel::Debug)) {
+    return;
+  }
+
+  MOZ_LOG(gSHLog, LogLevel::Debug, ("nsSHistory %p\n", this));
+  int32_t length = Length();
+  for (int32_t i = 0; i < length; i++) {
+    LogEntry(mEntries[i], i, length, EmptyCString(), i == mIndex);
+  }
+}
+
+void nsSHistory::WindowIndices(int32_t aIndex, int32_t* aOutStartIndex,
+                               int32_t* aOutEndIndex) {
+  *aOutStartIndex = std::max(0, aIndex - nsSHistory::VIEWER_WINDOW);
+  *aOutEndIndex = std::min(Length() - 1, aIndex + nsSHistory::VIEWER_WINDOW);
+}
+
+static void MarkAsInitialEntry(
+    SessionHistoryEntry* aEntry,
+    nsTHashMap<nsIDHashKey, SessionHistoryEntry*>& aHashtable) {
+  if (!aEntry->BCHistoryLength().Modified()) {
+    ++(aEntry->BCHistoryLength());
+  }
+  aHashtable.InsertOrUpdate(aEntry->DocshellID(), aEntry);
+  for (const RefPtr<SessionHistoryEntry>& entry : aEntry->Children()) {
+    if (entry) {
+      MarkAsInitialEntry(entry, aHashtable);
+    }
+  }
+}
+
+static void ClearEntries(SessionHistoryEntry* aEntry) {
+  aEntry->ClearBCHistoryLength();
+  for (const RefPtr<SessionHistoryEntry>& entry : aEntry->Children()) {
+    if (entry) {
+      ClearEntries(entry);
+    }
+  }
+}
+
+NS_IMETHODIMP
+nsSHistory::PurgeHistory(int32_t aNumEntries) {
+  if (Length() <= 0 || aNumEntries <= 0) {
+    return NS_ERROR_FAILURE;
+  }
+
+  SHistoryChangeNotifier change(this);
+
+  aNumEntries = std::min(aNumEntries, Length());
+
+  NotifyListeners(mListeners,
+                  [aNumEntries](auto l) { l->OnHistoryPurge(aNumEntries); });
+
+  // Set all the entries hanging of the first entry that we keep
+  // (mEntries[aNumEntries]) as being created as the result of a load
+  // (so contributing one to their BCHistoryLength).
+  nsTHashMap<nsIDHashKey, SessionHistoryEntry*> docshellIDToEntry;
+  if (aNumEntries != Length()) {
+    nsCOMPtr<SessionHistoryEntry> she =
+        do_QueryInterface(mEntries[aNumEntries]);
+    if (she) {
+      MarkAsInitialEntry(she, docshellIDToEntry);
+    }
+  }
+
+  // Reset the BCHistoryLength of all the entries that we're removing to a new
+  // counter with value 0 while decreasing their contribution to a shared
+  // BCHistoryLength. The end result is that they don't contribute to the
+  // BCHistoryLength of any other entry anymore.
+  for (int32_t i = 0; i < aNumEntries; ++i) {
+    nsCOMPtr<SessionHistoryEntry> she = do_QueryInterface(mEntries[i]);
+    if (she) {
+      ClearEntries(she);
+    }
+  }
+
+  RefPtr<BrowsingContext> rootBC = GetBrowsingContext();
+  if (rootBC) {
+    rootBC->PreOrderWalk([&docshellIDToEntry](BrowsingContext* aBC) {
+      SessionHistoryEntry* entry = docshellIDToEntry.Get(aBC->GetHistoryID());
+      Unused << aBC->SetHistoryEntryCount(
+          entry ? uint32_t(entry->BCHistoryLength()) : 0);
+    });
+  }
+
+  // Remove the first `aNumEntries` entries.
+  mEntries.RemoveElementsAt(0, aNumEntries);
+
+  // Adjust the indices, but don't let them go below -1.
+  mIndex -= aNumEntries;
+  mIndex = std::max(mIndex, -1);
+  mRequestedIndex -= aNumEntries;
+  mRequestedIndex = std::max(mRequestedIndex, -1);
+
+  if (rootBC && rootBC->GetDocShell()) {
+    rootBC->GetDocShell()->HistoryPurged(aNumEntries);
+  }
+
+  UpdateRootBrowsingContextState(rootBC);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHistory::AddSHistoryListener(nsISHistoryListener* aListener) {
+  NS_ENSURE_ARG_POINTER(aListener);
+
+  // Check if the listener supports Weak Reference. This is a must.
+  // This listener functionality is used by embedders and we want to
+  // have the right ownership with who ever listens to SHistory
+  nsWeakPtr listener = do_GetWeakReference(aListener);
+  if (!listener) {
+    return NS_ERROR_FAILURE;
+  }
+
+  mListeners.AppendElementUnlessExists(listener);
+  return NS_OK;
+}
+
+void nsSHistory::NotifyListenersContentViewerEvicted(uint32_t aNumEvicted) {
+  NotifyListeners(mListeners, [aNumEvicted](auto l) {
+    l->OnContentViewerEvicted(aNumEvicted);
+  });
+}
+
+NS_IMETHODIMP
+nsSHistory::RemoveSHistoryListener(nsISHistoryListener* aListener) {
+  // Make sure the listener that wants to be removed is the
+  // one we have in store.
+  nsWeakPtr listener = do_GetWeakReference(aListener);
+  mListeners.RemoveElement(listener);
+  return NS_OK;
+}
+
+/* Replace an entry in the History list at a particular index.
+ * Do not update index or count.
+ */
+NS_IMETHODIMP
+nsSHistory::ReplaceEntry(int32_t aIndex, nsISHEntry* aReplaceEntry) {
+  NS_ENSURE_ARG(aReplaceEntry);
+
+  if (aIndex < 0 || aIndex >= Length()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  nsCOMPtr<nsISHistory> shistoryOfEntry = aReplaceEntry->GetShistory();
+  if (shistoryOfEntry && shistoryOfEntry != this) {
+    NS_WARNING(
+        "The entry has been associated to another nsISHistory instance. "
+        "Try nsISHEntry.clone() and nsISHEntry.abandonBFCacheEntry() "
+        "first if you're copying an entry from another nsISHistory.");
+    return NS_ERROR_FAILURE;
+  }
+
+  aReplaceEntry->SetShistory(this);
+
+  NotifyListeners(mListeners, [](auto l) { l->OnHistoryReplaceEntry(); });
+
+  aReplaceEntry->SetPersist(true);
+  mEntries[aIndex] = aReplaceEntry;
+
+  UpdateRootBrowsingContextState();
+
+  return NS_OK;
+}
+
+// Calls OnHistoryReload on all registered session history listeners.
+// Listeners may return 'false' to cancel an action so make sure that we
+// set the return value to 'false' if one of the listeners wants to cancel.
+NS_IMETHODIMP
+nsSHistory::NotifyOnHistoryReload(bool* aCanReload) {
+  *aCanReload = true;
+
+  for (const nsWeakPtr& weakPtr : mListeners.EndLimitedRange()) {
+    nsCOMPtr<nsISHistoryListener> listener = do_QueryReferent(weakPtr);
+    if (listener) {
+      bool retval = true;
+
+      if (NS_SUCCEEDED(listener->OnHistoryReload(&retval)) && !retval) {
+        *aCanReload = false;
+      }
+    }
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHistory::EvictOutOfRangeContentViewers(int32_t aIndex) {
+  MOZ_LOG(gSHIPBFCacheLog, LogLevel::Debug,
+          ("nsSHistory::EvictOutOfRangeContentViewers %i", aIndex));
+
+  // Check our per SHistory object limit in the currently navigated SHistory
+  EvictOutOfRangeWindowContentViewers(aIndex);
+  // Check our total limit across all SHistory objects
+  GloballyEvictContentViewers();
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(void)
+nsSHistory::EvictContentViewersOrReplaceEntry(nsISHEntry* aNewSHEntry,
+                                              bool aReplace) {
+  if (!aReplace) {
+    int32_t curIndex;
+    GetIndex(&curIndex);
+    if (curIndex > -1) {
+      EvictOutOfRangeContentViewers(curIndex);
+    }
+  } else {
+    nsCOMPtr<nsISHEntry> rootSHEntry = nsSHistory::GetRootSHEntry(aNewSHEntry);
+
+    int32_t index = GetIndexOfEntry(rootSHEntry);
+    if (index > -1) {
+      ReplaceEntry(index, rootSHEntry);
+    }
+  }
+}
+
+NS_IMETHODIMP
+nsSHistory::EvictAllContentViewers() {
+  // XXXbz we don't actually do a good job of evicting things as we should, so
+  // we might have viewers quite far from mIndex.  So just evict everything.
+  for (int32_t i = 0; i < Length(); i++) {
+    EvictContentViewerForEntry(mEntries[i]);
+  }
+
+  return NS_OK;
+}
+
+static void FinishRestore(CanonicalBrowsingContext* aBrowsingContext,
+                          nsDocShellLoadState* aLoadState,
+                          SessionHistoryEntry* aEntry,
+                          nsFrameLoader* aFrameLoader, bool aCanSave) {
+  MOZ_ASSERT(aEntry);
+  MOZ_ASSERT(aFrameLoader);
+
+  aEntry->SetFrameLoader(nullptr);
+
+  nsCOMPtr<nsISHistory> shistory = aEntry->GetShistory();
+  int32_t indexOfHistoryLoad =
+      shistory ? shistory->GetIndexOfEntry(aEntry) : -1;
+
+  nsCOMPtr<nsFrameLoaderOwner> frameLoaderOwner =
+      do_QueryInterface(aBrowsingContext->GetEmbedderElement());
+  if (frameLoaderOwner && aFrameLoader->GetMaybePendingBrowsingContext() &&
+      indexOfHistoryLoad >= 0) {
+    RefPtr<BrowsingContextWebProgress> webProgress =
+        aBrowsingContext->GetWebProgress();
+    if (webProgress) {
+      // Synthesize a STATE_START WebProgress state change event from here
+      // in order to ensure emitting it on the BrowsingContext we navigate
+      // *from* instead of the BrowsingContext we navigate *to*. This will fire
+      // before and the next one will be ignored by BrowsingContextWebProgress:
+      // https://searchfox.org/mozilla-central/rev/77f0b36028b2368e342c982ea47609040b399d89/docshell/base/BrowsingContextWebProgress.cpp#196-203
+      nsCOMPtr<nsIURI> nextURI = aEntry->GetURI();
+      nsCOMPtr<nsIURI> nextOriginalURI = aEntry->GetOriginalURI();
+      nsCOMPtr<nsIRequest> request = MakeAndAddRef<RemoteWebProgressRequest>(
+          nextURI, nextOriginalURI ? nextOriginalURI : nextURI,
+          ""_ns /* aMatchedList */);
+      webProgress->OnStateChange(webProgress, request,
+                                 nsIWebProgressListener::STATE_START |
+                                     nsIWebProgressListener::STATE_IS_DOCUMENT |
+                                     nsIWebProgressListener::STATE_IS_REQUEST |
+                                     nsIWebProgressListener::STATE_IS_WINDOW |
+                                     nsIWebProgressListener::STATE_IS_NETWORK,
+                                 NS_OK);
+    }
+
+    RefPtr<CanonicalBrowsingContext> loadingBC =
+        aFrameLoader->GetMaybePendingBrowsingContext()->Canonical();
+    RefPtr<nsFrameLoader> currentFrameLoader =
+        frameLoaderOwner->GetFrameLoader();
+    // The current page can be bfcached, store the
+    // nsFrameLoader in the current SessionHistoryEntry.
+    RefPtr<SessionHistoryEntry> currentSHEntry =
+        aBrowsingContext->GetActiveSessionHistoryEntry();
+    if (currentSHEntry) {
+      // Update layout history state now, before we change the IsInBFCache flag
+      // and the active session history entry.
+      aBrowsingContext->SynchronizeLayoutHistoryState();
+
+      if (aCanSave) {
+        currentSHEntry->SetFrameLoader(currentFrameLoader);
+        Unused << aBrowsingContext->SetIsInBFCache(true);
+      }
+    }
+
+    if (aBrowsingContext->IsActive()) {
+      loadingBC->PreOrderWalk([&](BrowsingContext* aContext) {
+        if (BrowserParent* bp = aContext->Canonical()->GetBrowserParent()) {
+          ProcessPriorityManager::BrowserPriorityChanged(bp, true);
+        }
+      });
+    }
+
+    if (aEntry) {
+      aEntry->SetWireframe(Nothing());
+    }
+
+    // ReplacedBy will swap the entry back.
+    aBrowsingContext->SetActiveSessionHistoryEntry(aEntry);
+    loadingBC->SetActiveSessionHistoryEntry(nullptr);
+    NavigationIsolationOptions options;
+    aBrowsingContext->ReplacedBy(loadingBC, options);
+
+    // Assuming we still have the session history, update the index.
+    if (loadingBC->GetSessionHistory()) {
+      shistory->InternalSetRequestedIndex(indexOfHistoryLoad);
+      shistory->UpdateIndex();
+    }
+    loadingBC->HistoryCommitIndexAndLength();
+
+    // ResetSHEntryHasUserInteractionCache(); ?
+    // browser.navigation.requireUserInteraction is still
+    // disabled everywhere.
+
+    frameLoaderOwner->RestoreFrameLoaderFromBFCache(aFrameLoader);
+    // EvictOutOfRangeContentViewers is called here explicitly to
+    // possibly evict the now in the bfcache document.
+    // HistoryCommitIndexAndLength might not have evicted that before the
+    // FrameLoader swap.
+    shistory->EvictOutOfRangeContentViewers(indexOfHistoryLoad);
+
+    // The old page can't be stored in the bfcache,
+    // destroy the nsFrameLoader.
+    if (!aCanSave && currentFrameLoader) {
+      currentFrameLoader->Destroy();
+    }
+
+    Unused << loadingBC->SetIsInBFCache(false);
+
+    // We need to call this after we've restored the page from BFCache (see
+    // SetIsInBFCache(false) above), so that the page is not frozen anymore and
+    // the right focus events are fired.
+    frameLoaderOwner->UpdateFocusAndMouseEnterStateAfterFrameLoaderChange();
+
+    return;
+  }
+
+  aFrameLoader->Destroy();
+
+  // Fall back to do a normal load.
+  aBrowsingContext->LoadURI(aLoadState, false);
+}
+
+/* static */
+void nsSHistory::LoadURIOrBFCache(LoadEntryResult& aLoadEntry) {
+  if (mozilla::BFCacheInParent() && aLoadEntry.mBrowsingContext->IsTop()) {
+    MOZ_ASSERT(XRE_IsParentProcess());
+    RefPtr<nsDocShellLoadState> loadState = aLoadEntry.mLoadState;
+    RefPtr<CanonicalBrowsingContext> canonicalBC =
+        aLoadEntry.mBrowsingContext->Canonical();
+    nsCOMPtr<SessionHistoryEntry> she = do_QueryInterface(loadState->SHEntry());
+    nsCOMPtr<SessionHistoryEntry> currentShe =
+        canonicalBC->GetActiveSessionHistoryEntry();
+    MOZ_ASSERT(she);
+    RefPtr<nsFrameLoader> frameLoader = she->GetFrameLoader();
+    if (frameLoader &&
+        (!currentShe || (she->SharedInfo() != currentShe->SharedInfo() &&
+                         !currentShe->GetFrameLoader()))) {
+      bool canSave = (!currentShe || currentShe->GetSaveLayoutStateFlag()) &&
+                     canonicalBC->AllowedInBFCache(Nothing(), nullptr);
+
+      MOZ_LOG(gSHIPBFCacheLog, LogLevel::Debug,
+              ("nsSHistory::LoadURIOrBFCache "
+               "saving presentation=%i",
+               canSave));
+
+      nsCOMPtr<nsFrameLoaderOwner> frameLoaderOwner =
+          do_QueryInterface(canonicalBC->GetEmbedderElement());
+      if (frameLoaderOwner) {
+        RefPtr<nsFrameLoader> currentFrameLoader =
+            frameLoaderOwner->GetFrameLoader();
+        if (currentFrameLoader &&
+            currentFrameLoader->GetMaybePendingBrowsingContext()) {
+          if (WindowGlobalParent* wgp =
+                  currentFrameLoader->GetMaybePendingBrowsingContext()
+                      ->Canonical()
+                      ->GetCurrentWindowGlobal()) {
+            wgp->PermitUnload([canonicalBC, loadState, she, frameLoader,
+                               currentFrameLoader, canSave](bool aAllow) {
+              if (aAllow && !canonicalBC->IsReplaced()) {
+                FinishRestore(canonicalBC, loadState, she, frameLoader,
+                              canSave && canonicalBC->AllowedInBFCache(
+                                             Nothing(), nullptr));
+              } else if (currentFrameLoader->GetMaybePendingBrowsingContext()) {
+                nsISHistory* shistory =
+                    currentFrameLoader->GetMaybePendingBrowsingContext()
+                        ->Canonical()
+                        ->GetSessionHistory();
+                if (shistory) {
+                  shistory->InternalSetRequestedIndex(-1);
+                }
+              }
+            });
+            return;
+          }
+        }
+      }
+
+      FinishRestore(canonicalBC, loadState, she, frameLoader, canSave);
+      return;
+    }
+    if (frameLoader) {
+      she->SetFrameLoader(nullptr);
+      frameLoader->Destroy();
+    }
+  }
+
+  RefPtr<BrowsingContext> bc = aLoadEntry.mBrowsingContext;
+  RefPtr<nsDocShellLoadState> loadState = aLoadEntry.mLoadState;
+  bc->LoadURI(loadState, false);
+}
+
+/* static */
+void nsSHistory::LoadURIs(nsTArray<LoadEntryResult>& aLoadResults) {
+  for (LoadEntryResult& loadEntry : aLoadResults) {
+    LoadURIOrBFCache(loadEntry);
+  }
+}
+
+NS_IMETHODIMP
+nsSHistory::Reload(uint32_t aReloadFlags) {
+  nsTArray<LoadEntryResult> loadResults;
+  nsresult rv = Reload(aReloadFlags, loadResults);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (loadResults.IsEmpty()) {
+    return NS_OK;
+  }
+
+  LoadURIs(loadResults);
+  return NS_OK;
+}
+
+nsresult nsSHistory::Reload(uint32_t aReloadFlags,
+                            nsTArray<LoadEntryResult>& aLoadResults) {
+  MOZ_ASSERT(aLoadResults.IsEmpty());
+
+  uint32_t loadType;
+  if (aReloadFlags & nsIWebNavigation::LOAD_FLAGS_BYPASS_PROXY &&
+      aReloadFlags & nsIWebNavigation::LOAD_FLAGS_BYPASS_CACHE) {
+    loadType = LOAD_RELOAD_BYPASS_PROXY_AND_CACHE;
+  } else if (aReloadFlags & nsIWebNavigation::LOAD_FLAGS_BYPASS_PROXY) {
+    loadType = LOAD_RELOAD_BYPASS_PROXY;
+  } else if (aReloadFlags & nsIWebNavigation::LOAD_FLAGS_BYPASS_CACHE) {
+    loadType = LOAD_RELOAD_BYPASS_CACHE;
+  } else if (aReloadFlags & nsIWebNavigation::LOAD_FLAGS_CHARSET_CHANGE) {
+    loadType = LOAD_RELOAD_CHARSET_CHANGE;
+  } else {
+    loadType = LOAD_RELOAD_NORMAL;
+  }
+
+  // We are reloading. Send Reload notifications.
+  // nsDocShellLoadFlagType is not public, where as nsIWebNavigation
+  // is public. So send the reload notifications with the
+  // nsIWebNavigation flags.
+  bool canNavigate = true;
+  MOZ_ALWAYS_SUCCEEDS(NotifyOnHistoryReload(&canNavigate));
+  if (!canNavigate) {
+    return NS_OK;
+  }
+
+  nsresult rv = LoadEntry(
+      mIndex, loadType, HIST_CMD_RELOAD, aLoadResults, /* aSameEpoch */ false,
+      /* aLoadCurrentEntry */ true,
+      aReloadFlags & nsIWebNavigation::LOAD_FLAGS_USER_ACTIVATION);
+  if (NS_FAILED(rv)) {
+    aLoadResults.Clear();
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHistory::ReloadCurrentEntry() {
+  nsTArray<LoadEntryResult> loadResults;
+  nsresult rv = ReloadCurrentEntry(loadResults);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  LoadURIs(loadResults);
+  return NS_OK;
+}
+
+nsresult nsSHistory::ReloadCurrentEntry(
+    nsTArray<LoadEntryResult>& aLoadResults) {
+  // Notify listeners
+  NotifyListeners(mListeners, [](auto l) { l->OnHistoryGotoIndex(); });
+
+  return LoadEntry(mIndex, LOAD_HISTORY, HIST_CMD_RELOAD, aLoadResults,
+                   /* aSameEpoch */ false, /* aLoadCurrentEntry */ true,
+                   /* aUserActivation */ false);
+}
+
+void nsSHistory::EvictOutOfRangeWindowContentViewers(int32_t aIndex) {
+  // XXX rename method to EvictContentViewersExceptAroundIndex, or something.
+
+  // We need to release all content viewers that are no longer in the range
+  //
+  //  aIndex - VIEWER_WINDOW to aIndex + VIEWER_WINDOW
+  //
+  // to ensure that this SHistory object isn't responsible for more than
+  // VIEWER_WINDOW content viewers.  But our job is complicated by the
+  // fact that two entries which are related by either hash navigations or
+  // history.pushState will have the same content viewer.
+  //
+  // To illustrate the issue, suppose VIEWER_WINDOW = 3 and we have four
+  // linked entries in our history.  Suppose we then add a new content
+  // viewer and call into this function.  So the history looks like:
+  //
+  //   A A A A B
+  //     +     *
+  //
+  // where the letters are content viewers and + and * denote the beginning and
+  // end of the range aIndex +/- VIEWER_WINDOW.
+  //
+  // Although one copy of the content viewer A exists outside the range, we
+  // don't want to evict A, because it has other copies in range!
+  //
+  // We therefore adjust our eviction strategy to read:
+  //
+  //   Evict each content viewer outside the range aIndex -/+
+  //   VIEWER_WINDOW, unless that content viewer also appears within the
+  //   range.
+  //
+  // (Note that it's entirely legal to have two copies of one content viewer
+  // separated by a different content viewer -- call pushState twice, go back
+  // once, and refresh -- so we can't rely on identical viewers only appearing
+  // adjacent to one another.)
+
+  if (aIndex < 0) {
+    return;
+  }
+  NS_ENSURE_TRUE_VOID(aIndex < Length());
+
+  // Calculate the range that's safe from eviction.
+  int32_t startSafeIndex, endSafeIndex;
+  WindowIndices(aIndex, &startSafeIndex, &endSafeIndex);
+
+  LOG(
+      ("EvictOutOfRangeWindowContentViewers(index=%d), "
+       "Length()=%d. Safe range [%d, %d]",
+       aIndex, Length(), startSafeIndex, endSafeIndex));
+
+  // The content viewers in range aIndex -/+ VIEWER_WINDOW will not be
+  // evicted.  Collect a set of them so we don't accidentally evict one of them
+  // if it appears outside this range.
+  nsCOMArray<nsIContentViewer> safeViewers;
+  nsTArray<RefPtr<nsFrameLoader>> safeFrameLoaders;
+  for (int32_t i = startSafeIndex; i <= endSafeIndex; i++) {
+    nsCOMPtr<nsIContentViewer> viewer = mEntries[i]->GetContentViewer();
+    if (viewer) {
+      safeViewers.AppendObject(viewer);
+    } else if (nsCOMPtr<SessionHistoryEntry> she =
+                   do_QueryInterface(mEntries[i])) {
+      nsFrameLoader* frameLoader = she->GetFrameLoader();
+      if (frameLoader) {
+        safeFrameLoaders.AppendElement(frameLoader);
+      }
+    }
+  }
+
+  // Walk the SHistory list and evict any content viewers that aren't safe.
+  // (It's important that the condition checks Length(), rather than a cached
+  // copy of Length(), because the length might change between iterations.)
+  for (int32_t i = 0; i < Length(); i++) {
+    nsCOMPtr<nsISHEntry> entry = mEntries[i];
+    nsCOMPtr<nsIContentViewer> viewer = entry->GetContentViewer();
+    if (viewer) {
+      if (safeViewers.IndexOf(viewer) == -1) {
+        EvictContentViewerForEntry(entry);
+      }
+    } else if (nsCOMPtr<SessionHistoryEntry> she =
+                   do_QueryInterface(mEntries[i])) {
+      nsFrameLoader* frameLoader = she->GetFrameLoader();
+      if (frameLoader) {
+        if (!safeFrameLoaders.Contains(frameLoader)) {
+          EvictContentViewerForEntry(entry);
+        }
+      }
+    }
+  }
+}
+
+namespace {
+
+class EntryAndDistance {
+ public:
+  EntryAndDistance(nsSHistory* aSHistory, nsISHEntry* aEntry, uint32_t aDist)
+      : mSHistory(aSHistory),
+        mEntry(aEntry),
+        mViewer(aEntry->GetContentViewer()),
+        mLastTouched(mEntry->GetLastTouched()),
+        mDistance(aDist) {
+    nsCOMPtr<SessionHistoryEntry> she = do_QueryInterface(aEntry);
+    if (she) {
+      mFrameLoader = she->GetFrameLoader();
+    }
+    NS_ASSERTION(mViewer || mFrameLoader,
+                 "Entry should have a content viewer or frame loader.");
+  }
+
+  bool operator<(const EntryAndDistance& aOther) const {
+    // Compare distances first, and fall back to last-accessed times.
+    if (aOther.mDistance != this->mDistance) {
+      return this->mDistance < aOther.mDistance;
+    }
+
+    return this->mLastTouched < aOther.mLastTouched;
+  }
+
+  bool operator==(const EntryAndDistance& aOther) const {
+    // This is a little silly; we need == so the default comaprator can be
+    // instantiated, but this function is never actually called when we sort
+    // the list of EntryAndDistance objects.
+    return aOther.mDistance == this->mDistance &&
+           aOther.mLastTouched == this->mLastTouched;
+  }
+
+  RefPtr<nsSHistory> mSHistory;
+  nsCOMPtr<nsISHEntry> mEntry;
+  nsCOMPtr<nsIContentViewer> mViewer;
+  RefPtr<nsFrameLoader> mFrameLoader;
+  uint32_t mLastTouched;
+  int32_t mDistance;
+};
+
+}  // namespace
+
+// static
+void nsSHistory::GloballyEvictContentViewers() {
+  // First, collect from each SHistory object the entries which have a cached
+  // content viewer. Associate with each entry its distance from its SHistory's
+  // current index.
+
+  nsTArray<EntryAndDistance> entries;
+
+  for (auto shist : gSHistoryList.mList) {
+    // Maintain a list of the entries which have viewers and belong to
+    // this particular shist object.  We'll add this list to the global list,
+    // |entries|, eventually.
+    nsTArray<EntryAndDistance> shEntries;
+
+    // Content viewers are likely to exist only within shist->mIndex -/+
+    // VIEWER_WINDOW, so only search within that range.
+    //
+    // A content viewer might exist outside that range due to either:
+    //
+    //   * history.pushState or hash navigations, in which case a copy of the
+    //     content viewer should exist within the range, or
+    //
+    //   * bugs which cause us not to call nsSHistory::EvictContentViewers()
+    //     often enough.  Once we do call EvictContentViewers() for the
+    //     SHistory object in question, we'll do a full search of its history
+    //     and evict the out-of-range content viewers, so we don't bother here.
+    //
+    int32_t startIndex, endIndex;
+    shist->WindowIndices(shist->mIndex, &startIndex, &endIndex);
+    for (int32_t i = startIndex; i <= endIndex; i++) {
+      nsCOMPtr<nsISHEntry> entry = shist->mEntries[i];
+      nsCOMPtr<nsIContentViewer> contentViewer = entry->GetContentViewer();
+
+      bool found = false;
+      bool hasContentViewerOrFrameLoader = false;
+      if (contentViewer) {
+        hasContentViewerOrFrameLoader = true;
+        // Because one content viewer might belong to multiple SHEntries, we
+        // have to search through shEntries to see if we already know
+        // about this content viewer.  If we find the viewer, update its
+        // distance from the SHistory's index and continue.
+        for (uint32_t j = 0; j < shEntries.Length(); j++) {
+          EntryAndDistance& container = shEntries[j];
+          if (container.mViewer == contentViewer) {
+            container.mDistance =
+                std::min(container.mDistance, DeprecatedAbs(i - shist->mIndex));
+            found = true;
+            break;
+          }
+        }
+      } else if (nsCOMPtr<SessionHistoryEntry> she = do_QueryInterface(entry)) {
+        if (RefPtr<nsFrameLoader> frameLoader = she->GetFrameLoader()) {
+          hasContentViewerOrFrameLoader = true;
+          // Similar search as above but using frameloader.
+          for (uint32_t j = 0; j < shEntries.Length(); j++) {
+            EntryAndDistance& container = shEntries[j];
+            if (container.mFrameLoader == frameLoader) {
+              container.mDistance = std::min(container.mDistance,
+                                             DeprecatedAbs(i - shist->mIndex));
+              found = true;
+              break;
+            }
+          }
+        }
+      }
+
+      // If we didn't find a EntryAndDistance for this content viewer /
+      // frameloader, make a new one.
+      if (hasContentViewerOrFrameLoader && !found) {
+        EntryAndDistance container(shist, entry,
+                                   DeprecatedAbs(i - shist->mIndex));
+        shEntries.AppendElement(container);
+      }
+    }
+
+    // We've found all the entries belonging to shist which have viewers.
+    // Add those entries to our global list and move on.
+    entries.AppendElements(shEntries);
+  }
+
+  // We now have collected all cached content viewers.  First check that we
+  // have enough that we actually need to evict some.
+  if ((int32_t)entries.Length() <= sHistoryMaxTotalViewers) {
+    return;
+  }
+
+  // If we need to evict, sort our list of entries and evict the largest
+  // ones.  (We could of course get better algorithmic complexity here by using
+  // a heap or something more clever.  But sHistoryMaxTotalViewers isn't large,
+  // so let's not worry about it.)
+  entries.Sort();
+
+  for (int32_t i = entries.Length() - 1; i >= sHistoryMaxTotalViewers; --i) {
+    (entries[i].mSHistory)->EvictContentViewerForEntry(entries[i].mEntry);
+  }
+}
+
+nsresult nsSHistory::FindEntryForBFCache(SHEntrySharedParentState* aEntry,
+                                         nsISHEntry** aResult,
+                                         int32_t* aResultIndex) {
+  *aResult = nullptr;
+  *aResultIndex = -1;
+
+  int32_t startIndex, endIndex;
+  WindowIndices(mIndex, &startIndex, &endIndex);
+
+  for (int32_t i = startIndex; i <= endIndex; ++i) {
+    nsCOMPtr<nsISHEntry> shEntry = mEntries[i];
+
+    // Does shEntry have the same BFCacheEntry as the argument to this method?
+    if (shEntry->HasBFCacheEntry(aEntry)) {
+      shEntry.forget(aResult);
+      *aResultIndex = i;
+      return NS_OK;
+    }
+  }
+  return NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP_(void)
+nsSHistory::EvictExpiredContentViewerForEntry(
+    SHEntrySharedParentState* aEntry) {
+  int32_t index;
+  nsCOMPtr<nsISHEntry> shEntry;
+  FindEntryForBFCache(aEntry, getter_AddRefs(shEntry), &index);
+
+  if (index == mIndex) {
+    NS_WARNING("How did the current SHEntry expire?");
+  }
+
+  if (shEntry) {
+    EvictContentViewerForEntry(shEntry);
+  }
+}
+
+NS_IMETHODIMP_(void)
+nsSHistory::AddToExpirationTracker(SHEntrySharedParentState* aEntry) {
+  RefPtr<SHEntrySharedParentState> entry = aEntry;
+  if (!mHistoryTracker || !entry) {
+    return;
+  }
+
+  mHistoryTracker->AddObject(entry);
+  return;
+}
+
+NS_IMETHODIMP_(void)
+nsSHistory::RemoveFromExpirationTracker(SHEntrySharedParentState* aEntry) {
+  RefPtr<SHEntrySharedParentState> entry = aEntry;
+  MOZ_ASSERT(mHistoryTracker && !mHistoryTracker->IsEmpty());
+  if (!mHistoryTracker || !entry) {
+    return;
+  }
+
+  mHistoryTracker->RemoveObject(entry);
+}
+
+// Evicts all content viewers in all history objects.  This is very
+// inefficient, because it requires a linear search through all SHistory
+// objects for each viewer to be evicted.  However, this method is called
+// infrequently -- only when the disk or memory cache is cleared.
+
+// static
+void nsSHistory::GloballyEvictAllContentViewers() {
+  int32_t maxViewers = sHistoryMaxTotalViewers;
+  sHistoryMaxTotalViewers = 0;
+  GloballyEvictContentViewers();
+  sHistoryMaxTotalViewers = maxViewers;
+}
+
+void GetDynamicChildren(nsISHEntry* aEntry, nsTArray<nsID>& aDocshellIDs) {
+  int32_t count = aEntry->GetChildCount();
+  for (int32_t i = 0; i < count; ++i) {
+    nsCOMPtr<nsISHEntry> child;
+    aEntry->GetChildAt(i, getter_AddRefs(child));
+    if (child) {
+      if (child->IsDynamicallyAdded()) {
+        child->GetDocshellID(*aDocshellIDs.AppendElement());
+      } else {
+        GetDynamicChildren(child, aDocshellIDs);
+      }
+    }
+  }
+}
+
+bool RemoveFromSessionHistoryEntry(nsISHEntry* aRoot,
+                                   nsTArray<nsID>& aDocshellIDs) {
+  bool didRemove = false;
+  int32_t childCount = aRoot->GetChildCount();
+  for (int32_t i = childCount - 1; i >= 0; --i) {
+    nsCOMPtr<nsISHEntry> child;
+    aRoot->GetChildAt(i, getter_AddRefs(child));
+    if (child) {
+      nsID docshelldID;
+      child->GetDocshellID(docshelldID);
+      if (aDocshellIDs.Contains(docshelldID)) {
+        didRemove = true;
+        aRoot->RemoveChild(child);
+      } else if (RemoveFromSessionHistoryEntry(child, aDocshellIDs)) {
+        didRemove = true;
+      }
+    }
+  }
+  return didRemove;
+}
+
+bool RemoveChildEntries(nsISHistory* aHistory, int32_t aIndex,
+                        nsTArray<nsID>& aEntryIDs) {
+  nsCOMPtr<nsISHEntry> root;
+  aHistory->GetEntryAtIndex(aIndex, getter_AddRefs(root));
+  return root ? RemoveFromSessionHistoryEntry(root, aEntryIDs) : false;
+}
+
+bool IsSameTree(nsISHEntry* aEntry1, nsISHEntry* aEntry2) {
+  if (!aEntry1 && !aEntry2) {
+    return true;
+  }
+  if ((!aEntry1 && aEntry2) || (aEntry1 && !aEntry2)) {
+    return false;
+  }
+  uint32_t id1 = aEntry1->GetID();
+  uint32_t id2 = aEntry2->GetID();
+  if (id1 != id2) {
+    return false;
+  }
+
+  int32_t count1 = aEntry1->GetChildCount();
+  int32_t count2 = aEntry2->GetChildCount();
+  // We allow null entries in the end of the child list.
+  int32_t count = std::max(count1, count2);
+  for (int32_t i = 0; i < count; ++i) {
+    nsCOMPtr<nsISHEntry> child1, child2;
+    aEntry1->GetChildAt(i, getter_AddRefs(child1));
+    aEntry2->GetChildAt(i, getter_AddRefs(child2));
+    if (!IsSameTree(child1, child2)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+bool nsSHistory::RemoveDuplicate(int32_t aIndex, bool aKeepNext) {
+  NS_ASSERTION(aIndex >= 0, "aIndex must be >= 0!");
+  NS_ASSERTION(aIndex != 0 || aKeepNext,
+               "If we're removing index 0 we must be keeping the next");
+  NS_ASSERTION(aIndex != mIndex, "Shouldn't remove mIndex!");
+
+  int32_t compareIndex = aKeepNext ? aIndex + 1 : aIndex - 1;
+
+  nsresult rv;
+  nsCOMPtr<nsISHEntry> root1, root2;
+  rv = GetEntryAtIndex(aIndex, getter_AddRefs(root1));
+  if (NS_FAILED(rv)) {
+    return false;
+  }
+  rv = GetEntryAtIndex(compareIndex, getter_AddRefs(root2));
+  if (NS_FAILED(rv)) {
+    return false;
+  }
+
+  SHistoryChangeNotifier change(this);
+
+  if (IsSameTree(root1, root2)) {
+    if (aIndex < compareIndex) {
+      // If we're removing the entry with the lower index we need to move its
+      // BCHistoryLength to the entry we're keeping. If we're removing the entry
+      // with the higher index then it shouldn't have a modified
+      // BCHistoryLength.
+      UpdateEntryLength(root1, root2, true);
+    }
+    nsCOMPtr<SessionHistoryEntry> she = do_QueryInterface(root1);
+    if (she) {
+      ClearEntries(she);
+    }
+    mEntries.RemoveElementAt(aIndex);
+
+    // FIXME Bug 1546350: Reimplement history listeners.
+    // if (mRootBC && mRootBC->GetDocShell()) {
+    //  static_cast<nsDocShell*>(mRootBC->GetDocShell())
+    //      ->HistoryEntryRemoved(aIndex);
+    //}
+
+    // Adjust our indices to reflect the removed entry.
+    if (mIndex > aIndex) {
+      mIndex = mIndex - 1;
+    }
+
+    // NB: If the entry we are removing is the entry currently
+    // being navigated to (mRequestedIndex) then we adjust the index
+    // only if we're not keeping the next entry (because if we are keeping
+    // the next entry (because the current is a duplicate of the next), then
+    // that entry slides into the spot that we're currently pointing to.
+    // We don't do this adjustment for mIndex because mIndex cannot equal
+    // aIndex.
+
+    // NB: We don't need to guard on mRequestedIndex being nonzero here,
+    // because either they're strictly greater than aIndex which is at least
+    // zero, or they are equal to aIndex in which case aKeepNext must be true
+    // if aIndex is zero.
+    if (mRequestedIndex > aIndex || (mRequestedIndex == aIndex && !aKeepNext)) {
+      mRequestedIndex = mRequestedIndex - 1;
+    }
+
+    return true;
+  }
+  return false;
+}
+
+NS_IMETHODIMP_(void)
+nsSHistory::RemoveEntries(nsTArray<nsID>& aIDs, int32_t aStartIndex) {
+  bool didRemove;
+  RemoveEntries(aIDs, aStartIndex, &didRemove);
+  if (didRemove) {
+    RefPtr<BrowsingContext> rootBC = GetBrowsingContext();
+    if (rootBC && rootBC->GetDocShell()) {
+      rootBC->GetDocShell()->DispatchLocationChangeEvent();
+    }
+  }
+}
+
+void nsSHistory::RemoveEntries(nsTArray<nsID>& aIDs, int32_t aStartIndex,
+                               bool* aDidRemove) {
+  SHistoryChangeNotifier change(this);
+
+  int32_t index = aStartIndex;
+  while (index >= 0 && RemoveChildEntries(this, --index, aIDs)) {
+  }
+  int32_t minIndex = index;
+  index = aStartIndex;
+  while (index >= 0 && RemoveChildEntries(this, index++, aIDs)) {
+  }
+
+  // We need to remove duplicate nsSHEntry trees.
+  *aDidRemove = false;
+  while (index > minIndex) {
+    if (index != mIndex && RemoveDuplicate(index, index < mIndex)) {
+      *aDidRemove = true;
+    }
+    --index;
+  }
+
+  UpdateRootBrowsingContextState();
+}
+
+void nsSHistory::RemoveFrameEntries(nsISHEntry* aEntry) {
+  int32_t count = aEntry->GetChildCount();
+  AutoTArray<nsID, 16> ids;
+  for (int32_t i = 0; i < count; ++i) {
+    nsCOMPtr<nsISHEntry> child;
+    aEntry->GetChildAt(i, getter_AddRefs(child));
+    if (child) {
+      child->GetDocshellID(*ids.AppendElement());
+    }
+  }
+  RemoveEntries(ids, mIndex);
+}
+
+void nsSHistory::RemoveDynEntries(int32_t aIndex, nsISHEntry* aEntry) {
+  // Remove dynamic entries which are at the index and belongs to the container.
+  nsCOMPtr<nsISHEntry> entry(aEntry);
+  if (!entry) {
+    GetEntryAtIndex(aIndex, getter_AddRefs(entry));
+  }
+
+  if (entry) {
+    AutoTArray<nsID, 16> toBeRemovedEntries;
+    GetDynamicChildren(entry, toBeRemovedEntries);
+    if (toBeRemovedEntries.Length()) {
+      RemoveEntries(toBeRemovedEntries, aIndex);
+    }
+  }
+}
+
+void nsSHistory::RemoveDynEntriesForBFCacheEntry(nsIBFCacheEntry* aBFEntry) {
+  int32_t index;
+  nsCOMPtr<nsISHEntry> shEntry;
+  FindEntryForBFCache(static_cast<nsSHEntryShared*>(aBFEntry),
+                      getter_AddRefs(shEntry), &index);
+  if (shEntry) {
+    RemoveDynEntries(index, shEntry);
+  }
+}
+
+NS_IMETHODIMP
+nsSHistory::UpdateIndex() {
+  SHistoryChangeNotifier change(this);
+
+  // Update the actual index with the right value.
+  if (mIndex != mRequestedIndex && mRequestedIndex != -1) {
+    mIndex = mRequestedIndex;
+  }
+
+  mRequestedIndex = -1;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSHistory::GotoIndex(int32_t aIndex, bool aUserActivation) {
+  nsTArray<LoadEntryResult> loadResults;
+  nsresult rv = GotoIndex(aIndex, loadResults, /*aSameEpoch*/ false,
+                          aIndex == mIndex, aUserActivation);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  LoadURIs(loadResults);
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(void)
+nsSHistory::EnsureCorrectEntryAtCurrIndex(nsISHEntry* aEntry) {
+  int index = mRequestedIndex == -1 ? mIndex : mRequestedIndex;
+  if (index > -1 && (mEntries[index] != aEntry)) {
+    ReplaceEntry(index, aEntry);
+  }
+}
+
+nsresult nsSHistory::GotoIndex(int32_t aIndex,
+                               nsTArray<LoadEntryResult>& aLoadResults,
+                               bool aSameEpoch, bool aLoadCurrentEntry,
+                               bool aUserActivation) {
+  return LoadEntry(aIndex, LOAD_HISTORY, HIST_CMD_GOTOINDEX, aLoadResults,
+                   aSameEpoch, aLoadCurrentEntry, aUserActivation);
+}
+
+NS_IMETHODIMP_(bool)
+nsSHistory::HasUserInteractionAtIndex(int32_t aIndex) {
+  nsCOMPtr<nsISHEntry> entry;
+  GetEntryAtIndex(aIndex, getter_AddRefs(entry));
+  if (!entry) {
+    return false;
+  }
+  return entry->GetHasUserInteraction();
+}
+
+nsresult nsSHistory::LoadNextPossibleEntry(
+    int32_t aNewIndex, long aLoadType, uint32_t aHistCmd,
+    nsTArray<LoadEntryResult>& aLoadResults, bool aLoadCurrentEntry,
+    bool aUserActivation) {
+  mRequestedIndex = -1;
+  if (aNewIndex < mIndex) {
+    return LoadEntry(aNewIndex - 1, aLoadType, aHistCmd, aLoadResults,
+                     /*aSameEpoch*/ false, aLoadCurrentEntry, aUserActivation);
+  }
+  if (aNewIndex > mIndex) {
+    return LoadEntry(aNewIndex + 1, aLoadType, aHistCmd, aLoadResults,
+                     /*aSameEpoch*/ false, aLoadCurrentEntry, aUserActivation);
+  }
+  return NS_ERROR_FAILURE;
+}
+
+nsresult nsSHistory::LoadEntry(int32_t aIndex, long aLoadType,
+                               uint32_t aHistCmd,
+                               nsTArray<LoadEntryResult>& aLoadResults,
+                               bool aSameEpoch, bool aLoadCurrentEntry,
+                               bool aUserActivation) {
+  MOZ_LOG(gSHistoryLog, LogLevel::Debug,
+          ("LoadEntry(%d, 0x%lx, %u)", aIndex, aLoadType, aHistCmd));
+  RefPtr<BrowsingContext> rootBC = GetBrowsingContext();
+  if (!rootBC) {
+    return NS_ERROR_FAILURE;
+  }
+
+  if (aIndex < 0 || aIndex >= Length()) {
+    MOZ_LOG(gSHistoryLog, LogLevel::Debug, ("Index out of range"));
+    // The index is out of range.
+    // Clear the requested index in case it had bogus value. This way the next
+    // load succeeds if the offset is reasonable.
+    mRequestedIndex = -1;
+
+    return NS_ERROR_FAILURE;
+  }
+
+  int32_t originalRequestedIndex = mRequestedIndex;
+  int32_t previousRequest = mRequestedIndex > -1 ? mRequestedIndex : mIndex;
+  int32_t requestedOffset = aIndex - previousRequest;
+
+  // This is a normal local history navigation.
+
+  nsCOMPtr<nsISHEntry> prevEntry;
+  nsCOMPtr<nsISHEntry> nextEntry;
+  GetEntryAtIndex(mIndex, getter_AddRefs(prevEntry));
+  GetEntryAtIndex(aIndex, getter_AddRefs(nextEntry));
+  if (!nextEntry || !prevEntry) {
+    mRequestedIndex = -1;
+    return NS_ERROR_FAILURE;
+  }
+
+  if (mozilla::SessionHistoryInParent()) {
+    if (aHistCmd == HIST_CMD_GOTOINDEX) {
+      // https://html.spec.whatwg.org/#history-traversal:
+      // To traverse the history
+      // "If entry has a different Document object than the current entry, then
+      // run the following substeps: Remove any tasks queued by the history
+      // traversal task source..."
+      //
+      // aSameEpoch is true only if the navigations would have been
+      // generated in the same spin of the event loop (i.e. history.go(-2);
+      // history.go(-1)) and from the same content process.
+      if (aSameEpoch) {
+        bool same_doc = false;
+        prevEntry->SharesDocumentWith(nextEntry, &same_doc);
+        if (!same_doc) {
+          MOZ_LOG(
+              gSHistoryLog, LogLevel::Debug,
+              ("Aborting GotoIndex %d - same epoch and not same doc", aIndex));
+          // Ignore this load. Before SessionHistoryInParent, this would
+          // have been dropped in InternalLoad after we filter out SameDoc
+          // loads.
+          return NS_ERROR_FAILURE;
+        }
+      }
+    }
+  }
+  // Keep note of requested history index in mRequestedIndex; after all bailouts
+  mRequestedIndex = aIndex;
+
+  // Remember that this entry is getting loaded at this point in the sequence
+
+  nextEntry->SetLastTouched(++gTouchCounter);
+
+  // Get the uri for the entry we are about to visit
+  nsCOMPtr<nsIURI> nextURI = nextEntry->GetURI();
+
+  MOZ_ASSERT(nextURI, "nextURI can't be null");
+
+  // Send appropriate listener notifications.
+  if (aHistCmd == HIST_CMD_GOTOINDEX) {
+    // We are going somewhere else. This is not reload either
+    NotifyListeners(mListeners, [](auto l) { l->OnHistoryGotoIndex(); });
+  }
+
+  if (mRequestedIndex == mIndex) {
+    // Possibly a reload case
+    InitiateLoad(nextEntry, rootBC, aLoadType, aLoadResults, aLoadCurrentEntry,
+                 aUserActivation, requestedOffset);
+    return NS_OK;
+  }
+
+  // Going back or forward.
+  bool differenceFound = LoadDifferingEntries(
+      prevEntry, nextEntry, rootBC, aLoadType, aLoadResults, aLoadCurrentEntry,
+      aUserActivation, requestedOffset);
+  if (!differenceFound) {
+    // LoadNextPossibleEntry will change the offset by one, and in order
+    // to keep track of the requestedOffset, need to reset mRequestedIndex to
+    // the value it had initially.
+    mRequestedIndex = originalRequestedIndex;
+    // We did not find any differences. Go further in the history.
+    return LoadNextPossibleEntry(aIndex, aLoadType, aHistCmd, aLoadResults,
+                                 aLoadCurrentEntry, aUserActivation);
+  }
+
+  return NS_OK;
+}
+
+bool nsSHistory::LoadDifferingEntries(nsISHEntry* aPrevEntry,
+                                      nsISHEntry* aNextEntry,
+                                      BrowsingContext* aParent, long aLoadType,
+                                      nsTArray<LoadEntryResult>& aLoadResults,
+                                      bool aLoadCurrentEntry,
+                                      bool aUserActivation, int32_t aOffset) {
+  MOZ_ASSERT(aPrevEntry && aNextEntry && aParent);
+
+  uint32_t prevID = aPrevEntry->GetID();
+  uint32_t nextID = aNextEntry->GetID();
+
+  // Check the IDs to verify if the pages are different.
+  if (prevID != nextID) {
+    // Set the Subframe flag if not navigating the root docshell.
+    aNextEntry->SetIsSubFrame(aParent->Id() != mRootBC);
+    InitiateLoad(aNextEntry, aParent, aLoadType, aLoadResults,
+                 aLoadCurrentEntry, aUserActivation, aOffset);
+    return true;
+  }
+
+  // The entries are the same, so compare any child frames
+  int32_t pcnt = aPrevEntry->GetChildCount();
+  int32_t ncnt = aNextEntry->GetChildCount();
+
+  // Create an array for child browsing contexts.
+  nsTArray<RefPtr<BrowsingContext>> browsingContexts;
+  aParent->GetChildren(browsingContexts);
+
+  // Search for something to load next.
+  bool differenceFound = false;
+  for (int32_t i = 0; i < ncnt; ++i) {
+    // First get an entry which may cause a new page to be loaded.
+    nsCOMPtr<nsISHEntry> nChild;
+    aNextEntry->GetChildAt(i, getter_AddRefs(nChild));
+    if (!nChild) {
+      continue;
+    }
+    nsID docshellID;
+    nChild->GetDocshellID(docshellID);
+
+    // Then find the associated docshell.
+    RefPtr<BrowsingContext> bcChild;
+    for (const RefPtr<BrowsingContext>& bc : browsingContexts) {
+      if (bc->GetHistoryID() == docshellID) {
+        bcChild = bc;
+        break;
+      }
+    }
+    if (!bcChild) {
+      continue;
+    }
+
+    // Then look at the previous entries to see if there was
+    // an entry for the docshell.
+    nsCOMPtr<nsISHEntry> pChild;
+    for (int32_t k = 0; k < pcnt; ++k) {
+      nsCOMPtr<nsISHEntry> child;
+      aPrevEntry->GetChildAt(k, getter_AddRefs(child));
+      if (child) {
+        nsID dID;
+        child->GetDocshellID(dID);
+        if (dID == docshellID) {
+          pChild = child;
+          break;
+        }
+      }
+    }
+    if (!pChild) {
+      continue;
+    }
+
+    // Finally recursively call this method.
+    // This will either load a new page to shell or some subshell or
+    // do nothing.
+    if (LoadDifferingEntries(pChild, nChild, bcChild, aLoadType, aLoadResults,
+                             aLoadCurrentEntry, aUserActivation, aOffset)) {
+      differenceFound = true;
+    }
+  }
+  return differenceFound;
+}
+
+void nsSHistory::InitiateLoad(nsISHEntry* aFrameEntry,
+                              BrowsingContext* aFrameBC, long aLoadType,
+                              nsTArray<LoadEntryResult>& aLoadResults,
+                              bool aLoadCurrentEntry, bool aUserActivation,
+                              int32_t aOffset) {
+  MOZ_ASSERT(aFrameBC && aFrameEntry);
+
+  LoadEntryResult* loadResult = aLoadResults.AppendElement();
+  loadResult->mBrowsingContext = aFrameBC;
+
+  nsCOMPtr<nsIURI> newURI = aFrameEntry->GetURI();
+  RefPtr<nsDocShellLoadState> loadState = new nsDocShellLoadState(newURI);
+
+  loadState->SetHasValidUserGestureActivation(aUserActivation);
+
+  // At the time we initiate a history entry load we already know if https-first
+  // was able to upgrade the request from http to https. There is no point in
+  // re-retrying to upgrade.
+  loadState->SetIsExemptFromHTTPSOnlyMode(true);
+
+  /* Set the loadType in the SHEntry too to  what was passed on.
+   * This will be passed on to child subframes later in nsDocShell,
+   * so that proper loadType is maintained through out a frameset
+   */
+  aFrameEntry->SetLoadType(aLoadType);
+
+  loadState->SetLoadType(aLoadType);
+
+  loadState->SetSHEntry(aFrameEntry);
+
+  // If we're loading the current entry we want to treat it as not a
+  // same-document navigation (see nsDocShell::IsSameDocumentNavigation), so
+  // record that here in the LoadingSessionHistoryEntry.
+  bool loadingCurrentEntry;
+  if (mozilla::SessionHistoryInParent()) {
+    loadingCurrentEntry = aLoadCurrentEntry;
+  } else {
+    loadingCurrentEntry =
+        aFrameBC->GetDocShell() &&
+        nsDocShell::Cast(aFrameBC->GetDocShell())->IsOSHE(aFrameEntry);
+  }
+  loadState->SetLoadIsFromSessionHistory(aOffset, loadingCurrentEntry);
+
+  if (mozilla::SessionHistoryInParent()) {
+    nsCOMPtr<SessionHistoryEntry> she = do_QueryInterface(aFrameEntry);
+    aFrameBC->Canonical()->AddLoadingSessionHistoryEntry(
+        loadState->GetLoadingSessionHistoryInfo()->mLoadId, she);
+  }
+
+  nsCOMPtr<nsIURI> originalURI = aFrameEntry->GetOriginalURI();
+  loadState->SetOriginalURI(originalURI);
+
+  loadState->SetLoadReplace(aFrameEntry->GetLoadReplace());
+
+  loadState->SetLoadFlags(nsIWebNavigation::LOAD_FLAGS_NONE);
+  nsCOMPtr<nsIPrincipal> triggeringPrincipal =
+      aFrameEntry->GetTriggeringPrincipal();
+  loadState->SetTriggeringPrincipal(triggeringPrincipal);
+  loadState->SetFirstParty(false);
+  nsCOMPtr<nsIContentSecurityPolicy> csp = aFrameEntry->GetCsp();
+  loadState->SetCsp(csp);
+
+  loadResult->mLoadState = std::move(loadState);
+}
+
+NS_IMETHODIMP
+nsSHistory::CreateEntry(nsISHEntry** aEntry) {
+  nsCOMPtr<nsISHEntry> entry;
+  if (XRE_IsParentProcess() && mozilla::SessionHistoryInParent()) {
+    entry = new SessionHistoryEntry();
+  } else {
+    entry = new nsSHEntry();
+  }
+  entry.forget(aEntry);
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(bool)
+nsSHistory::IsEmptyOrHasEntriesForSingleTopLevelPage() {
+  if (mEntries.IsEmpty()) {
+    return true;
+  }
+
+  nsISHEntry* entry = mEntries[0];
+  size_t length = mEntries.Length();
+  for (size_t i = 1; i < length; ++i) {
+    bool sharesDocument = false;
+    mEntries[i]->SharesDocumentWith(entry, &sharesDocument);
+    if (!sharesDocument) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+static void CollectEntries(
+    nsTHashMap<nsIDHashKey, SessionHistoryEntry*>& aHashtable,
+    SessionHistoryEntry* aEntry) {
+  aHashtable.InsertOrUpdate(aEntry->DocshellID(), aEntry);
+  for (const RefPtr<SessionHistoryEntry>& entry : aEntry->Children()) {
+    if (entry) {
+      CollectEntries(aHashtable, entry);
+    }
+  }
+}
+
+static void UpdateEntryLength(
+    nsTHashMap<nsIDHashKey, SessionHistoryEntry*>& aHashtable,
+    SessionHistoryEntry* aNewEntry, bool aMove) {
+  SessionHistoryEntry* oldEntry = aHashtable.Get(aNewEntry->DocshellID());
+  if (oldEntry) {
+    MOZ_ASSERT(oldEntry->GetID() != aNewEntry->GetID() || !aMove ||
+               !aNewEntry->BCHistoryLength().Modified());
+    aNewEntry->SetBCHistoryLength(oldEntry->BCHistoryLength());
+    if (oldEntry->GetID() != aNewEntry->GetID()) {
+      MOZ_ASSERT(!aMove);
+      // If we have a new id then aNewEntry was created for a new load, so
+      // record that in BCHistoryLength.
+      ++aNewEntry->BCHistoryLength();
+    } else if (aMove) {
+      // We're moving the BCHistoryLength from the old entry to the new entry,
+      // so we need to let the old entry know that it's not contributing to its
+      // BCHistoryLength, and the new one that it does if the old one was
+      // before.
+      aNewEntry->BCHistoryLength().SetModified(
+          oldEntry->BCHistoryLength().Modified());
+      oldEntry->BCHistoryLength().SetModified(false);
+    }
+  }
+
+  for (const RefPtr<SessionHistoryEntry>& entry : aNewEntry->Children()) {
+    if (entry) {
+      UpdateEntryLength(aHashtable, entry, aMove);
+    }
+  }
+}
+
+void nsSHistory::UpdateEntryLength(nsISHEntry* aOldEntry, nsISHEntry* aNewEntry,
+                                   bool aMove) {
+  nsCOMPtr<SessionHistoryEntry> oldSHE = do_QueryInterface(aOldEntry);
+  nsCOMPtr<SessionHistoryEntry> newSHE = do_QueryInterface(aNewEntry);
+
+  if (!oldSHE || !newSHE) {
+    return;
+  }
+
+  nsTHashMap<nsIDHashKey, SessionHistoryEntry*> docshellIDToEntry;
+  CollectEntries(docshellIDToEntry, oldSHE);
+
+  ::UpdateEntryLength(docshellIDToEntry, newSHE, aMove);
+}
diff --git a/docshell/shistory/nsSHistory.h b/docshell/shistory/nsSHistory.h
new file mode 100644
index 0000000000..d029a55c6c
--- /dev/null
+++ b/docshell/shistory/nsSHistory.h
@@ -0,0 +1,343 @@
+/* -*- 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 nsSHistory_h
+#define nsSHistory_h
+
+#include "nsCOMPtr.h"
+#include "nsDocShellLoadState.h"
+#include "nsExpirationTracker.h"
+#include "nsISHistory.h"
+#include "nsSHEntryShared.h"
+#include "nsSimpleEnumerator.h"
+#include "nsTObserverArray.h"
+#include "nsWeakReference.h"
+
+#include "mozilla/dom/ipc/IdType.h"
+#include "mozilla/LinkedList.h"
+#include "mozilla/UniquePtr.h"
+
+class nsIDocShell;
+class nsDocShell;
+class nsSHistoryObserver;
+class nsISHEntry;
+
+namespace mozilla {
+namespace dom {
+class LoadSHEntryResult;
+}
+}  // namespace mozilla
+
+class nsSHistory : public mozilla::LinkedListElement<nsSHistory>,
+                   public nsISHistory,
+                   public nsSupportsWeakReference {
+ public:
+  // The timer based history tracker is used to evict bfcache on expiration.
+  class HistoryTracker final
+      : public nsExpirationTracker<mozilla::dom::SHEntrySharedParentState, 3> {
+   public:
+    explicit HistoryTracker(nsSHistory* aSHistory, uint32_t aTimeout,
+                            nsIEventTarget* aEventTarget)
+        : nsExpirationTracker(1000 * aTimeout / 2, "HistoryTracker",
+                              aEventTarget) {
+      MOZ_ASSERT(aSHistory);
+      mSHistory = aSHistory;
+    }
+
+   protected:
+    virtual void NotifyExpired(
+        mozilla::dom::SHEntrySharedParentState* aObj) override {
+      RemoveObject(aObj);
+      mSHistory->EvictExpiredContentViewerForEntry(aObj);
+    }
+
+   private:
+    // HistoryTracker is owned by nsSHistory; it always outlives HistoryTracker
+    // so it's safe to use raw pointer here.
+    nsSHistory* mSHistory;
+  };
+
+  // Structure used in SetChildHistoryEntry
+  struct SwapEntriesData {
+    mozilla::dom::BrowsingContext*
+        ignoreBC;                // constant; the browsing context to ignore
+    nsISHEntry* destTreeRoot;    // constant; the root of the dest tree
+    nsISHEntry* destTreeParent;  // constant; the node under destTreeRoot
+                                 // whose children will correspond to aEntry
+  };
+
+  explicit nsSHistory(mozilla::dom::BrowsingContext* aRootBC);
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISHISTORY
+
+  // One time initialization method
+  static nsresult Startup();
+  static void Shutdown();
+  static void UpdatePrefs();
+
+  // Max number of total cached content viewers.  If the pref
+  // browser.sessionhistory.max_total_viewers is negative, then
+  // this value is calculated based on the total amount of memory.
+  // Otherwise, it comes straight from the pref.
+  static uint32_t GetMaxTotalViewers() { return sHistoryMaxTotalViewers; }
+
+  // Get the root SHEntry from a given entry.
+  static already_AddRefed<nsISHEntry> GetRootSHEntry(nsISHEntry* aEntry);
+
+  // Callback prototype for WalkHistoryEntries.
+  // `aEntry` is the child history entry, `aBC` is its corresponding browsing
+  // context, `aChildIndex` is the child's index in its parent entry, and
+  // `aData` is the opaque pointer passed to WalkHistoryEntries. Both structs
+  // that are passed as `aData` to this function have a field
+  // `aEntriesToUpdate`, which is an array of entries we need to update in
+  // docshell, if the 'SH in parent' pref is on (which implies that this method
+  // is executed in the parent)
+  typedef nsresult (*WalkHistoryEntriesFunc)(nsISHEntry* aEntry,
+                                             mozilla::dom::BrowsingContext* aBC,
+                                             int32_t aChildIndex, void* aData);
+
+  // Clone a session history tree for subframe navigation.
+  // The tree rooted at |aSrcEntry| will be cloned into |aDestEntry|, except
+  // for the entry with id |aCloneID|, which will be replaced with
+  // |aReplaceEntry|. |aSrcShell| is a (possibly null) docshell which
+  // corresponds to |aSrcEntry| via its mLSHE or mOHE pointers, and will
+  // have that pointer updated to point to the cloned history entry.
+  // If aCloneChildren is true then the children of the entry with id
+  // |aCloneID| will be cloned into |aReplaceEntry|.
+  static nsresult CloneAndReplace(nsISHEntry* aSrcEntry,
+                                  mozilla::dom::BrowsingContext* aOwnerBC,
+                                  uint32_t aCloneID, nsISHEntry* aReplaceEntry,
+                                  bool aCloneChildren, nsISHEntry** aDestEntry);
+
+  // Child-walking callback for CloneAndReplace
+  static nsresult CloneAndReplaceChild(nsISHEntry* aEntry,
+                                       mozilla::dom::BrowsingContext* aOwnerBC,
+                                       int32_t aChildIndex, void* aData);
+
+  // Child-walking callback for SetHistoryEntry
+  static nsresult SetChildHistoryEntry(nsISHEntry* aEntry,
+                                       mozilla::dom::BrowsingContext* aBC,
+                                       int32_t aEntryIndex, void* aData);
+
+  // For each child of aRootEntry, find the corresponding shell which is
+  // a child of aBC, and call aCallback. The opaque pointer aData
+  // is passed to the callback.
+  static nsresult WalkHistoryEntries(nsISHEntry* aRootEntry,
+                                     mozilla::dom::BrowsingContext* aBC,
+                                     WalkHistoryEntriesFunc aCallback,
+                                     void* aData);
+
+  // This function finds all entries that are contiguous and same-origin with
+  // the aEntry. And call the aCallback on them, including the aEntry. This only
+  // works for the root entries. It will do nothing for non-root entries.
+  static void WalkContiguousEntries(
+      nsISHEntry* aEntry, const std::function<void(nsISHEntry*)>& aCallback);
+
+  nsTArray<nsCOMPtr<nsISHEntry>>& Entries() { return mEntries; }
+
+  void NotifyOnHistoryReplaceEntry();
+
+  void RemoveEntries(nsTArray<nsID>& aIDs, int32_t aStartIndex,
+                     bool* aDidRemove);
+
+  // The size of the window of SHEntries which can have alive viewers in the
+  // bfcache around the currently active SHEntry.
+  //
+  // We try to keep viewers for SHEntries between index - VIEWER_WINDOW and
+  // index + VIEWER_WINDOW alive.
+  static const int32_t VIEWER_WINDOW = 3;
+
+  struct LoadEntryResult {
+    RefPtr<mozilla::dom::BrowsingContext> mBrowsingContext;
+    RefPtr<nsDocShellLoadState> mLoadState;
+  };
+
+  static void LoadURIs(nsTArray<LoadEntryResult>& aLoadResults);
+  static void LoadURIOrBFCache(LoadEntryResult& aLoadEntry);
+
+  // If this doesn't return an error then either aLoadResult is set to nothing,
+  // in which case the caller should ignore the load, or it returns a valid
+  // LoadEntryResult in aLoadResult which the caller should use to do the load.
+  nsresult Reload(uint32_t aReloadFlags,
+                  nsTArray<LoadEntryResult>& aLoadResults);
+  nsresult ReloadCurrentEntry(nsTArray<LoadEntryResult>& aLoadResults);
+  nsresult GotoIndex(int32_t aIndex, nsTArray<LoadEntryResult>& aLoadResults,
+                     bool aSameEpoch, bool aLoadCurrentEntry,
+                     bool aUserActivation);
+
+  void WindowIndices(int32_t aIndex, int32_t* aOutStartIndex,
+                     int32_t* aOutEndIndex);
+  void NotifyListenersContentViewerEvicted(uint32_t aNumEvicted);
+
+  int32_t Length() { return int32_t(mEntries.Length()); }
+  int32_t Index() { return mIndex; }
+  already_AddRefed<mozilla::dom::BrowsingContext> GetBrowsingContext() {
+    return mozilla::dom::BrowsingContext::Get(mRootBC);
+  }
+  bool HasOngoingUpdate() { return mHasOngoingUpdate; }
+  void SetHasOngoingUpdate(bool aVal) { mHasOngoingUpdate = aVal; }
+
+  void SetBrowsingContext(mozilla::dom::BrowsingContext* aRootBC) {
+    uint64_t newID = aRootBC ? aRootBC->Id() : 0;
+    if (mRootBC != newID) {
+      mRootBC = newID;
+      UpdateRootBrowsingContextState(aRootBC);
+    }
+  }
+
+  int32_t GetIndexForReplace() {
+    // Replace current entry in session history; If the requested index is
+    // valid, it indicates the loading was triggered by a history load, and
+    // we should replace the entry at requested index instead.
+    return mRequestedIndex == -1 ? mIndex : mRequestedIndex;
+  }
+
+  // Update the root browsing context state when adding, removing or
+  // replacing entries.
+  void UpdateRootBrowsingContextState() {
+    RefPtr<mozilla::dom::BrowsingContext> rootBC(GetBrowsingContext());
+    UpdateRootBrowsingContextState(rootBC);
+  }
+
+  void GetEpoch(uint64_t& aEpoch,
+                mozilla::Maybe<mozilla::dom::ContentParentId>& aId) const {
+    aEpoch = mEpoch;
+    aId = mEpochParentId;
+  }
+  void SetEpoch(uint64_t aEpoch,
+                mozilla::Maybe<mozilla::dom::ContentParentId> aId) {
+    mEpoch = aEpoch;
+    mEpochParentId = aId;
+  }
+
+  void LogHistory();
+
+ protected:
+  virtual ~nsSHistory();
+
+  uint64_t mRootBC;
+
+ private:
+  friend class nsSHistoryObserver;
+
+  void UpdateRootBrowsingContextState(
+      mozilla::dom::BrowsingContext* aBrowsingContext);
+
+  bool LoadDifferingEntries(nsISHEntry* aPrevEntry, nsISHEntry* aNextEntry,
+                            mozilla::dom::BrowsingContext* aParent,
+                            long aLoadType,
+                            nsTArray<LoadEntryResult>& aLoadResults,
+                            bool aLoadCurrentEntry, bool aUserActivation,
+                            int32_t aOffset);
+  void InitiateLoad(nsISHEntry* aFrameEntry,
+                    mozilla::dom::BrowsingContext* aFrameBC, long aLoadType,
+                    nsTArray<LoadEntryResult>& aLoadResult,
+                    bool aLoadCurrentEntry, bool aUserActivation,
+                    int32_t aOffset);
+
+  nsresult LoadEntry(int32_t aIndex, long aLoadType, uint32_t aHistCmd,
+                     nsTArray<LoadEntryResult>& aLoadResults, bool aSameEpoch,
+                     bool aLoadCurrentEntry, bool aUserActivation);
+
+  // Find the history entry for a given bfcache entry. It only looks up between
+  // the range where alive viewers may exist (i.e nsSHistory::VIEWER_WINDOW).
+  nsresult FindEntryForBFCache(mozilla::dom::SHEntrySharedParentState* aEntry,
+                               nsISHEntry** aResult, int32_t* aResultIndex);
+
+  // Evict content viewers in this window which don't lie in the "safe" range
+  // around aIndex.
+  virtual void EvictOutOfRangeWindowContentViewers(int32_t aIndex);
+  void EvictContentViewerForEntry(nsISHEntry* aEntry);
+  static void GloballyEvictContentViewers();
+  static void GloballyEvictAllContentViewers();
+
+  // Calculates a max number of total
+  // content viewers to cache, based on amount of total memory
+  static uint32_t CalcMaxTotalViewers();
+
+  nsresult LoadNextPossibleEntry(int32_t aNewIndex, long aLoadType,
+                                 uint32_t aHistCmd,
+                                 nsTArray<LoadEntryResult>& aLoadResults,
+                                 bool aLoadCurrentEntry, bool aUserActivation);
+
+  // aIndex is the index of the entry which may be removed.
+  // If aKeepNext is true, aIndex is compared to aIndex + 1,
+  // otherwise comparison is done to aIndex - 1.
+  bool RemoveDuplicate(int32_t aIndex, bool aKeepNext);
+
+  // We need to update entries in docshell and browsing context.
+  // If our docshell is located in parent or 'SH in parent' pref is off we can
+  // update it directly, Otherwise, we have two choices. If the browsing context
+  // that owns the docshell is in the same process as the process who called us
+  // over IPC, then we save entries that need to be updated in a list, and once
+  // we have returned from the IPC call, we update the docshell in the child
+  // process. Otherwise, if the browsing context is in a different process, we
+  // do a nested IPC call to that process to update the docshell in that
+  // process.
+  static void HandleEntriesToSwapInDocShell(mozilla::dom::BrowsingContext* aBC,
+                                            nsISHEntry* aOldEntry,
+                                            nsISHEntry* aNewEntry);
+
+  void UpdateEntryLength(nsISHEntry* aOldEntry, nsISHEntry* aNewEntry,
+                         bool aMove);
+
+ protected:
+  bool mHasOngoingUpdate;
+  nsTArray<nsCOMPtr<nsISHEntry>> mEntries;  // entries are never null
+ private:
+  // Track all bfcache entries and evict on expiration.
+  mozilla::UniquePtr<HistoryTracker> mHistoryTracker;
+
+  int32_t mIndex;           // -1 means "no index"
+  int32_t mRequestedIndex;  // -1 means "no requested index"
+
+  // Session History listeners
+  nsAutoTObserverArray<nsWeakPtr, 2> mListeners;
+
+  nsID mRootDocShellID;
+
+  // Max viewers allowed total, across all SHistory objects
+  static int32_t sHistoryMaxTotalViewers;
+
+  // The epoch (and id) tell us what navigations occured within the same
+  // event-loop spin in the child.  We need to know this in order to
+  // implement spec requirements for dropping pending navigations when we
+  // do a history navigation, if it's not same-document.  Content processes
+  // update the epoch via a runnable on each ::Go (including AsyncGo).
+  uint64_t mEpoch = 0;
+  mozilla::Maybe<mozilla::dom::ContentParentId> mEpochParentId;
+};
+
+// CallerWillNotifyHistoryIndexAndLengthChanges is used to prevent
+// SHistoryChangeNotifier to send automatic index and length updates.
+// When that is done, it is up to the caller to explicitly send those updates.
+// This is needed in cases when the update is a reaction to some change in a
+// child process and child process passes a changeId to the parent side.
+class MOZ_STACK_CLASS CallerWillNotifyHistoryIndexAndLengthChanges {
+ public:
+  explicit CallerWillNotifyHistoryIndexAndLengthChanges(
+      nsISHistory* aSHistory) {
+    nsSHistory* shistory = static_cast<nsSHistory*>(aSHistory);
+    if (shistory && !shistory->HasOngoingUpdate()) {
+      shistory->SetHasOngoingUpdate(true);
+      mSHistory = shistory;
+    }
+  }
+
+  ~CallerWillNotifyHistoryIndexAndLengthChanges() {
+    if (mSHistory) {
+      mSHistory->SetHasOngoingUpdate(false);
+    }
+  }
+
+  RefPtr<nsSHistory> mSHistory;
+};
+
+inline nsISupports* ToSupports(nsSHistory* aObj) {
+  return static_cast<nsISHistory*>(aObj);
+}
+
+#endif /* nsSHistory */