new file mode 100644
--- /dev/null
+++ b/gfx/src/PingPongRegion.h
@@ -0,0 +1,63 @@
+/* -*- 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/. */
+
+#ifndef PingPongRegion_h__
+#define PingPongRegion_h__
+
+/* This class uses a pair of regions and swaps between them while
+ * accumulating to avoid the heap allocations associated with
+ * modifying a region in place.
+ *
+ * It is sizeof(T)*2 + sizeof(T*) and can use end up using
+ * approximately double the amount of memory as using single
+ * region so use it sparingly.
+ */
+
+template <typename T>
+class PingPongRegion
+{
+  typedef typename T::RectType RectType;
+public:
+  PingPongRegion()
+  {
+    rgn = &rgn1;
+  }
+
+  void SubOut(const RectType& aOther)
+  {
+    T* nextRgn = nextRegion();
+    nextRgn->Sub(*rgn, aOther);
+    rgn = nextRgn;
+  }
+
+  void OrWith(const RectType& aOther)
+  {
+    T* nextRgn = nextRegion();
+    nextRgn->Or(*rgn, aOther);
+    rgn = nextRgn;
+  }
+
+  T& Region()
+  {
+    return *rgn;
+  }
+
+private:
+
+  T* nextRegion()
+  {
+    if (rgn == &rgn1) {
+      return &rgn2;
+    } else {
+      return &rgn1;
+    }
+  }
+
+  T* rgn;
+  T rgn1;
+  T rgn2;
+};
+
+#endif

--- a/gfx/src/moz.build
+++ b/gfx/src/moz.build
@@ -32,16 +32,17 @@ EXPORTS += [
     'nsPoint.h',
     'nsRect.h',
     'nsRegion.h',
     'nsRegionFwd.h',
     'nsRenderingContext.h',
     'nsSize.h',
     'nsThemeConstants.h',
     'nsTransform2D.h',
+    'PingPongRegion.h',
     'RegionBuilder.h',
 ]
 
 EXPORTS.mozilla += [
     'AppUnits.h',
 ]
 
 if CONFIG['MOZ_X11']:

--- a/gfx/tests/gtest/TestRegion.cpp
+++ b/gfx/tests/gtest/TestRegion.cpp
@@ -1,15 +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 <algorithm>
 
+#include "PingPongRegion.h"
 #include "gtest/gtest.h"
 #include "gtest/MozGTestBench.h"
 #include "nsRect.h"
 #include "nsRegion.h"
 #include "RegionBuilder.h"
 
 using namespace std;
 
@@ -558,49 +559,105 @@ TEST(Gfx, RegionVisitEdges) {
     r.Or(r, nsRect(115, 49, 99, 1));
     r.Or(r, nsRect(12, 50, 11, 5));
     r.Or(r, nsRect(25, 50, 28, 5));
     r.Or(r, nsRect(115, 50, 99, 5));
     r.Or(r, nsRect(115, 55, 99, 12));
 
     TestVisit(r);
   }
+}
 
+TEST(Gfx, PingPongRegion) {
+  nsRect rects[] = {
+    nsRect(4, 1, 61, 49),
+    nsRect(115, 1, 99, 49),
+    nsRect(115, 49, 99, 1),
+    nsRect(12, 50, 11, 5),
+    nsRect(25, 50, 28, 5),
+    nsRect(115, 50, 99, 5),
+    nsRect(115, 55, 99, 12),
+  };
+
+  // Test accumulations of various sizes to make sure
+  // the ping-pong behavior of PingPongRegion is working.
+  for (size_t size = 0; size < mozilla::ArrayLength(rects); size++) {
+    // bug 1130978.
+    nsRegion r;
+    PingPongRegion<nsRegion> ar;
+    for (size_t i = 0; i < size; i++) {
+      r.Or(r, rects[i]);
+      ar.OrWith(rects[i]);
+      EXPECT_TRUE(ar.Region().IsEqual(r));
+    }
+
+    for (size_t i = 0; i < size; i++) {
+      ar.SubOut(rects[i]);
+      r.SubOut(rects[i]);
+      EXPECT_TRUE(ar.Region().IsEqual(r));
+    }
+  }
 }
 
 MOZ_GTEST_BENCH(GfxBench, RegionOr, []{
   const int size = 5000;
+
   nsRegion r;
   for (int i = 0; i < size; i++) {
     r = r.Or(r, nsRect(i, i, i + 10, i + 10));
   }
+
+  nsIntRegion rInt;
+  for (int i = 0; i < size; i++) {
+    rInt = rInt.Or(rInt, nsIntRect(i, i, i + 10, i + 10));
+  }
 });
 
 MOZ_GTEST_BENCH(GfxBench, RegionAnd, []{
   const int size = 5000;
   nsRegion r(nsRect(0, 0, size, size));
   for (int i = 0; i < size; i++) {
     nsRegion rMissingPixel(nsRect(0, 0, size, size));
     rMissingPixel = rMissingPixel.Sub(rMissingPixel, nsRect(i, i, 1, 1));
     r = r.And(r, rMissingPixel);
   }
 });
 
-void TestExec() {
+void BenchRegionBuilderOr() {
   const int size = 5000;
 
   RegionBuilder<nsRegion> r;
   for (int i = 0; i < size; i++) {
     r.Or(nsRect(i, i, i + 10, i + 10));
   }
   r.ToRegion();
 
   RegionBuilder<nsIntRegion> rInt;
   for (int i = 0; i < size; i++) {
     rInt.Or(nsIntRect(i, i, i + 10, i + 10));
   }
   rInt.ToRegion();
 }
 
 MOZ_GTEST_BENCH(GfxBench, RegionBuilderOr, []{
-  TestExec();
+  BenchRegionBuilderOr();
 });
 
+void BenchPingPongRegionOr() {
+  const int size = 5000;
+
+  PingPongRegion<nsRegion> r;
+  for (int i = 0; i < size; i++) {
+    r.OrWith(nsRect(i, i, i + 10, i + 10));
+  }
+  r.Region();
+
+  PingPongRegion<nsIntRegion> rInt;
+  for (int i = 0; i < size; i++) {
+    rInt.OrWith(nsIntRect(i, i, i + 10, i + 10));
+  }
+  rInt.Region();
+}
+
+MOZ_GTEST_BENCH(GfxBench, PingPongRegionOr, []{
+  BenchPingPongRegionOr();
+});
+