--- a/layout/base/DisplayItemClip.cpp
+++ b/layout/base/DisplayItemClip.cpp
@@ -37,16 +37,33 @@ DisplayItemClip::DisplayItemClip(const D
       static_cast<nsDisplayClipRoundedRect*>(item)->GetRadii(rr->mRadii);
     }
   }
 
   // FIXME: Optimize away excess rounded rectangles due to the new addition.
 }
 
 void
+DisplayItemClip::IntersectWith(const DisplayItemClip& aOther)
+{
+  if (!aOther.mHaveClipRect) {
+    return;
+  }
+  if (!mHaveClipRect) {
+    *this = aOther;
+    return;
+  }
+  if (!mClipRect.IntersectRect(mClipRect, aOther.mClipRect)) {
+    mRoundedClipRects.Clear();
+    return;
+  }
+  mRoundedClipRects.AppendElements(aOther.mRoundedClipRects);
+}
+
+void
 DisplayItemClip::ApplyTo(gfxContext* aContext,
                          nsPresContext* aPresContext,
                          uint32_t aBegin, uint32_t aEnd)
 {
   int32_t A2D = aPresContext->AppUnitsPerDevPixel();
   ApplyRectTo(aContext, A2D);
   ApplyRoundedRectsTo(aContext, A2D, aBegin, aEnd);
 }

--- a/layout/base/DisplayItemClip.h
+++ b/layout/base/DisplayItemClip.h
@@ -53,16 +53,18 @@ public:
   };
 
   // Constructs a DisplayItemClip that does no clipping at all.
   DisplayItemClip() : mHaveClipRect(false) {}
 
   // Construct as the intersection of aOther and aClipItem.
   DisplayItemClip(const DisplayItemClip& aOther, nsDisplayItem* aClipItem);
 
+  void IntersectWith(const DisplayItemClip& aOther);
+
   // Apply this |DisplayItemClip| to the given gfxContext.  Any saving of state
   // or clearing of other clips must be done by the caller.
   // See aBegin/aEnd note on ApplyRoundedRectsTo.
   void ApplyTo(gfxContext* aContext, nsPresContext* aPresContext,
                uint32_t aBegin = 0, uint32_t aEnd = UINT32_MAX);
 
   void ApplyRectTo(gfxContext* aContext, int32_t A2D) const;
   // Applies the rounded rects in this Clip to aContext
@@ -124,18 +126,18 @@ public:
     return mHaveClipRect == aOther.mHaveClipRect &&
            (!mHaveClipRect || mClipRect.IsEqualInterior(aOther.mClipRect)) &&
            mRoundedClipRects == aOther.mRoundedClipRects;
   }
   bool operator!=(const DisplayItemClip& aOther) const {
     return !(*this == aOther);
   }
 
-  bool HasClip() { return mHaveClipRect; }
-  const nsRect& GetClipRect()
+  bool HasClip() const { return mHaveClipRect; }
+  const nsRect& GetClipRect() const
   {
     NS_ASSERTION(HasClip(), "No clip rect!");
     return mClipRect;
   }
 
   /**
    * Find the largest N such that the first N rounded rects in 'this' are
    * equal to the first N rounded rects in aOther, and N <= aMax.

new file mode 100644
--- /dev/null
+++ b/layout/base/DisplayListClipState.cpp
@@ -0,0 +1,35 @@
+/* -*- Mode: C++; tab-width: 20; 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 "DisplayListClipState.h"
+
+#include "nsDisplayList.h"
+
+namespace mozilla {
+
+const DisplayItemClip*
+DisplayListClipState::GetCurrentCombinedClip(nsDisplayListBuilder* aBuilder)
+{
+  if (mCurrentCombinedClip) {
+    return mCurrentCombinedClip;
+  }
+  if (!mClipContentDescendants && !mClipContainingBlockDescendants) {
+    return nullptr;
+  }
+  void* mem = aBuilder->Allocate(sizeof(DisplayItemClip));
+  if (mClipContentDescendants) {
+    mCurrentCombinedClip =
+      new (mem) DisplayItemClip(*mClipContentDescendants);
+    if (mClipContainingBlockDescendants) {
+      mCurrentCombinedClip->IntersectWith(*mClipContainingBlockDescendants);
+    }
+  } else {
+    mCurrentCombinedClip =
+      new (mem) DisplayItemClip(*mClipContainingBlockDescendants);
+  }
+  return mCurrentCombinedClip;
+}
+
+}

new file mode 100644
--- /dev/null
+++ b/layout/base/DisplayListClipState.h
@@ -0,0 +1,76 @@
+/* -*- Mode: C++; tab-width: 20; 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 DISPLAYLISTCLIPSTATE_H_
+#define DISPLAYLISTCLIPSTATE_H_
+
+#include "DisplayItemClip.h"
+
+class nsIFrame;
+class nsDisplayListBuilder;
+
+namespace mozilla {
+
+/**
+ * All clip coordinates are in appunits relative to the reference frame
+ * for the display item we're building.
+ */
+class DisplayListClipState {
+public:
+  DisplayListClipState()
+    : mClipContentDescendants(nullptr)
+    , mClipContainingBlockDescendants(nullptr)
+    , mCurrentCombinedClip(nullptr)
+  {}
+
+  const DisplayItemClip* GetCurrentCombinedClip(nsDisplayListBuilder* aBuilder);
+
+  const DisplayItemClip* GetClipForContainingBlockDescendants()
+  {
+    return mClipContainingBlockDescendants;
+  }
+
+  const DisplayItemClip* GetClipForContentDescendants()
+  {
+    return mClipContentDescendants;
+  }
+
+  void SetClipForContainingBlockDescendants(const DisplayItemClip* aClip)
+  {
+    mClipContainingBlockDescendants = aClip;
+    mCurrentCombinedClip = nullptr;
+  }
+  void SetClipForContentDescendants(const DisplayItemClip* aClip)
+  {
+    mClipContentDescendants = aClip;
+    mCurrentCombinedClip = nullptr;
+  }
+
+private:
+  /**
+   * All content descendants (i.e. following placeholder frames to their
+   * out-of-flows if necessary) should be clipped by mClipContentDescendants.
+   * Null if no clipping applies.
+   */
+  const DisplayItemClip* mClipContentDescendants;
+  /**
+   * All containing-block descendants (i.e. frame descendants) should be
+   * clipped by mClipContainingBlockDescendants.
+   * Null if no clipping applies.
+   */
+  const DisplayItemClip* mClipContainingBlockDescendants;
+  /**
+   * The intersection of mClipContentDescendants and
+   * mClipContainingBlockDescendants.
+   * Allocated in the nsDisplayListBuilder arena. Null if none has been
+   * allocated or both mClipContentDescendants and mClipContainingBlockDescendants
+   * are null.
+   */
+  const DisplayItemClip* mCurrentCombinedClip;
+};
+
+}
+
+#endif /* DISPLAYLISTCLIPSTATE_H_ */

--- a/layout/base/Makefile.in
+++ b/layout/base/Makefile.in
@@ -16,16 +16,18 @@ ifndef _MSC_VER
 FAIL_ON_WARNINGS = 1
 endif # !_MSC_VER
 
 
 
 EXPORTS_NAMESPACES = mozilla
 
 EXPORTS		= \
+		DisplayItemClip.h \
+		DisplayListClipState.h \
 		FrameLayerBuilder.h \
 		FramePropertyTable.h \
 		nsArenaMemoryStats.h \
 		nsBidi.h \
 		nsBidiPresUtils.h \
 		nsCaret.h \
 		nsCSSFrameConstructor.h \
 		nsChangeHint.h \
@@ -54,16 +56,17 @@ EXPORTS		= \
 		$(NULL)
 
 EXPORTS_mozilla = \
   PaintTracker.h \
   $(NULL)
 
 CPPSRCS		= \
 		DisplayItemClip.cpp \
+		DisplayListClipState.cpp \
 		FrameLayerBuilder.cpp \
 		FramePropertyTable.cpp \
 		MaskLayerImageCache.cpp \
 		nsCSSColorUtils.cpp \
 		nsCSSFrameConstructor.cpp \
 		nsCSSRendering.cpp \
 		nsCSSRenderingBorders.cpp \
 		nsCaret.cpp \