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diff --git a/ipc/mscom/ApartmentRegion.h b/ipc/mscom/ApartmentRegion.h
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+++ b/ipc/mscom/ApartmentRegion.h
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+/* -*- 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_mscom_ApartmentRegion_h
+#define mozilla_mscom_ApartmentRegion_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/mscom/COMWrappers.h"
+
+namespace mozilla::mscom {
+
+// This runtime-dynamic apartment class is used in ProcessRuntime.cpp, to
+// initialize the process's main thread. Do not use it in new contexts if at all
+// possible; instead, prefer ApartmentRegionT, below.
+//
+// For backwards compatibility, this class does not yet automatically disable
+// OLE1/DDE, although there is believed to be no code relying on it.
+//
+// (TODO: phase out all uses of CoInitialize without `COINIT_DISABLE_OLE1DDE`?)
+class MOZ_NON_TEMPORARY_CLASS ApartmentRegion final {
+ public:
+  /**
+   * This constructor is to be used when we want to instantiate the object but
+   * we do not yet know which type of apartment we want. Call Init() to
+   * complete initialization.
+   */
+  constexpr ApartmentRegion() : mInitResult(CO_E_NOTINITIALIZED) {}
+
+  explicit ApartmentRegion(COINIT aAptType)
+      : mInitResult(wrapped::CoInitializeEx(nullptr, aAptType)) {
+    // If this fires then we're probably mixing apartments on the same thread
+    MOZ_ASSERT(IsValid());
+  }
+
+  ~ApartmentRegion() {
+    if (IsValid()) {
+      wrapped::CoUninitialize();
+    }
+  }
+
+  explicit operator bool() const { return IsValid(); }
+
+  bool IsValidOutermost() const { return mInitResult == S_OK; }
+
+  bool IsValid() const { return SUCCEEDED(mInitResult); }
+
+  bool Init(COINIT aAptType) {
+    MOZ_ASSERT(mInitResult == CO_E_NOTINITIALIZED);
+    mInitResult = wrapped::CoInitializeEx(nullptr, aAptType);
+    MOZ_ASSERT(IsValid());
+    return IsValid();
+  }
+
+  HRESULT GetHResult() const { return mInitResult; }
+
+  ApartmentRegion(const ApartmentRegion&) = delete;
+  ApartmentRegion& operator=(const ApartmentRegion&) = delete;
+  ApartmentRegion(ApartmentRegion&&) = delete;
+  ApartmentRegion& operator=(ApartmentRegion&&) = delete;
+
+ private:
+  HRESULT mInitResult;
+};
+
+template <COINIT AptType, bool UseOLE1 = false>
+class MOZ_NON_TEMPORARY_CLASS ApartmentRegionT final {
+  static COINIT ActualType() {
+    static_assert(
+        !((AptType & COINIT_DISABLE_OLE1DDE) == 0 && UseOLE1),
+        "only one of `UseOLE1` and `COINIT_DISABLE_OLE1DDE` permitted");
+    if (UseOLE1) return AptType;
+    return static_cast<COINIT>(AptType | COINIT_DISABLE_OLE1DDE);
+  }
+
+ public:
+  ApartmentRegionT() : mAptRgn(ActualType()) {}
+
+  ~ApartmentRegionT() = default;
+
+  explicit operator bool() const { return mAptRgn.IsValid(); }
+
+  bool IsValidOutermost() const { return mAptRgn.IsValidOutermost(); }
+
+  bool IsValid() const { return mAptRgn.IsValid(); }
+
+  HRESULT GetHResult() const { return mAptRgn.GetHResult(); }
+
+  ApartmentRegionT(const ApartmentRegionT&) = delete;
+  ApartmentRegionT& operator=(const ApartmentRegionT&) = delete;
+  ApartmentRegionT(ApartmentRegionT&&) = delete;
+  ApartmentRegionT& operator=(ApartmentRegionT&&) = delete;
+
+ private:
+  ApartmentRegion mAptRgn;
+};
+
+using STARegion = ApartmentRegionT<COINIT_APARTMENTTHREADED>;
+using MTARegion = ApartmentRegionT<COINIT_MULTITHREADED>;
+
+}  // namespace mozilla::mscom
+
+#endif  // mozilla_mscom_ApartmentRegion_h