--- a/gfx/layers/LayersLogging.cpp
+++ b/gfx/layers/LayersLogging.cpp
@@ -103,16 +103,33 @@ AppendToString(std::stringstream& aStrea
   aStream << pfx;
   aStream << nsPrintfCString(
     "(x=%d, y=%d, w=%d, h=%d)",
     r.x, r.y, r.width, r.height).get();
   aStream << sfx;
 }
 
 void
+AppendToString(std::stringstream& aStream, const nsRegion& r,
+               const char* pfx, const char* sfx)
+{
+  aStream << pfx;
+
+  nsRegionRectIterator it(r);
+  aStream << "< ";
+  while (const nsRect* sr = it.Next()) {
+    AppendToString(aStream, *sr);
+    aStream << "; ";
+  }
+  aStream << ">";
+
+  aStream << sfx;
+}
+
+void
 AppendToString(std::stringstream& aStream, const nsIntRegion& r,
                const char* pfx, const char* sfx)
 {
   aStream << pfx;
 
   nsIntRegionRectIterator it(r);
   aStream << "< ";
   while (const nsIntRect* sr = it.Next()) {

--- a/gfx/layers/LayersLogging.h
+++ b/gfx/layers/LayersLogging.h
@@ -8,17 +8,17 @@
 
 #include "FrameMetrics.h"               // for FrameMetrics, etc
 #include "GraphicsFilter.h"             // for GraphicsFilter
 #include "mozilla/gfx/Point.h"          // for IntSize, etc
 #include "mozilla/gfx/Types.h"          // for Filter, SurfaceFormat
 #include "mozilla/layers/CompositorTypes.h"  // for TextureFlags
 #include "nsAString.h"
 #include "nsPrintfCString.h"            // for nsPrintfCString
-#include "nsRegion.h"                   // for nsIntRegion
+#include "nsRegion.h"                   // for nsRegion, nsIntRegion
 #include "nscore.h"                     // for nsACString, etc
 
 struct gfxRGBA;
 struct nsIntPoint;
 struct nsIntRect;
 struct nsIntSize;
 
 namespace mozilla {
@@ -89,16 +89,20 @@ AppendToString(std::stringstream& aStrea
   aStream << pfx;
   aStream << nsPrintfCString(
     "(x=%d, y=%d, w=%d, h=%d)",
     r.x, r.y, r.width, r.height).get();
   aStream << sfx;
 }
 
 void
+AppendToString(std::stringstream& aStream, const nsRegion& r,
+               const char* pfx="", const char* sfx="");
+
+void
 AppendToString(std::stringstream& aStream, const nsIntRegion& r,
                const char* pfx="", const char* sfx="");
 
 void
 AppendToString(std::stringstream& aStream, const EventRegions& e,
                const char* pfx="", const char* sfx="");
 
 void

--- a/layout/base/nsDisplayList.cpp
+++ b/layout/base/nsDisplayList.cpp
@@ -2983,16 +2983,32 @@ nsDisplayLayerEventRegions::AddFrame(nsD
 }
 
 void
 nsDisplayLayerEventRegions::AddInactiveScrollPort(const nsRect& aRect)
 {
   mDispatchToContentHitRegion.Or(mDispatchToContentHitRegion, aRect);
 }
 
+#ifdef MOZ_DUMP_PAINTING
+void
+nsDisplayLayerEventRegions::WriteDebugInfo(std::stringstream& aStream)
+{
+  if (!mHitRegion.IsEmpty()) {
+    AppendToString(aStream, mHitRegion, " (hitRegion ", ")");
+  }
+  if (!mMaybeHitRegion.IsEmpty()) {
+    AppendToString(aStream, mMaybeHitRegion, " (maybeHitRegion ", ")");
+  }
+  if (!mDispatchToContentHitRegion.IsEmpty()) {
+    AppendToString(aStream, mDispatchToContentHitRegion, " (dispatchToContentRegion ", ")");
+  }
+}
+#endif
+
 nsDisplayCaret::nsDisplayCaret(nsDisplayListBuilder* aBuilder,
                                nsIFrame* aCaretFrame)
   : nsDisplayItem(aBuilder, aCaretFrame)
   , mCaret(aBuilder->GetCaret())
   , mBounds(aBuilder->GetCaretRect() + ToReferenceFrame())
 {
   MOZ_COUNT_CTOR(nsDisplayCaret);
 }

--- a/layout/base/nsDisplayList.h
+++ b/layout/base/nsDisplayList.h
@@ -2624,16 +2624,20 @@ public:
   // dispatch-to-content region, to ensure that APZ lets content create a
   // displayport.
   void AddInactiveScrollPort(const nsRect& aRect);
 
   const nsRegion& HitRegion() { return mHitRegion; }
   const nsRegion& MaybeHitRegion() { return mMaybeHitRegion; }
   const nsRegion& DispatchToContentHitRegion() { return mDispatchToContentHitRegion; }
 
+#ifdef MOZ_DUMP_PAINTING
+  virtual void WriteDebugInfo(std::stringstream& aStream) MOZ_OVERRIDE;
+#endif
+
 private:
   // Relative to aFrame's reference frame.
   // These are the points that are definitely in the hit region.
   nsRegion mHitRegion;
   // These are points that may or may not be in the hit region. Only main-thread
   // event handling can tell for sure (e.g. because complex shapes are present).
   nsRegion mMaybeHitRegion;
   // These are points that need to be dispatched to the content thread for