diff --git a/gfx/layers/Compositor.h b/gfx/layers/Compositor.h
index 9ed589d387d0..cf7023be972c 100644
--- a/gfx/layers/Compositor.h
+++ b/gfx/layers/Compositor.h
@@ -431,7 +431,6 @@ public:
   // XXX I expect we will want to move mWidget into this class and implement
   // these methods properly.
   virtual nsIWidget* GetWidget() const { return nullptr; }
-  virtual const nsIntSize& GetWidgetSize() = 0;
 
   // Call before and after any rendering not done by this compositor but which
   // might affect the compositor's internal state or the state of any APIs it
diff --git a/gfx/layers/basic/BasicCompositor.cpp b/gfx/layers/basic/BasicCompositor.cpp
index 97e8b485f080..cc934831b3b8 100644
--- a/gfx/layers/basic/BasicCompositor.cpp
+++ b/gfx/layers/basic/BasicCompositor.cpp
@@ -220,7 +220,6 @@ CreateBasicDeprecatedTextureHost(SurfaceDescriptorType aDescriptorType,
 
 BasicCompositor::BasicCompositor(nsIWidget *aWidget)
   : mWidget(aWidget)
-  , mWidgetSize(-1, -1)
 {
   MOZ_COUNT_CTOR(BasicCompositor);
   sBackend = LAYERS_BASIC;
@@ -540,7 +539,6 @@ BasicCompositor::BeginFrame(const nsIntRegion& aInvalidRegion,
   nsIntRect intRect;
   mWidget->GetClientBounds(intRect);
   Rect rect = Rect(0, 0, intRect.width, intRect.height);
-  mWidgetSize = intRect.Size();
 
   nsIntRect invalidRect = aInvalidRegion.GetBounds();
   mInvalidRect = IntRect(invalidRect.x, invalidRect.y, invalidRect.width, invalidRect.height);
diff --git a/gfx/layers/basic/BasicCompositor.h b/gfx/layers/basic/BasicCompositor.h
index ac969937cef0..e4500e6c0d49 100644
--- a/gfx/layers/basic/BasicCompositor.h
+++ b/gfx/layers/basic/BasicCompositor.h
@@ -119,10 +119,6 @@ public:
   virtual const char* Name() const { return "Basic"; }
 
   virtual nsIWidget* GetWidget() const MOZ_OVERRIDE { return mWidget; }
-  virtual const nsIntSize& GetWidgetSize() MOZ_OVERRIDE
-  {
-    return mWidgetSize;
-  }
 
   gfx::DrawTarget *GetDrawTarget() { return mDrawTarget; }
 
diff --git a/gfx/layers/composite/LayerManagerComposite.cpp b/gfx/layers/composite/LayerManagerComposite.cpp
index ea3a6b0e3c74..94d8d654bead 100644
--- a/gfx/layers/composite/LayerManagerComposite.cpp
+++ b/gfx/layers/composite/LayerManagerComposite.cpp
@@ -786,12 +786,6 @@ LayerComposite::Destroy()
   }
 }
 
-const nsIntSize&
-LayerManagerComposite::GetWidgetSize()
-{
-  return mCompositor->GetWidgetSize();
-}
-
 void
 LayerManagerComposite::SetCompositorID(uint32_t aID)
 {
diff --git a/gfx/layers/composite/LayerManagerComposite.h b/gfx/layers/composite/LayerManagerComposite.h
index d0097abaf4f6..1dde8755d8f6 100644
--- a/gfx/layers/composite/LayerManagerComposite.h
+++ b/gfx/layers/composite/LayerManagerComposite.h
@@ -198,8 +198,6 @@ public:
     CreateDrawTarget(const mozilla::gfx::IntSize &aSize,
                      mozilla::gfx::SurfaceFormat aFormat) MOZ_OVERRIDE;
 
-  const nsIntSize& GetWidgetSize();
-
   /**
    * Calculates the 'completeness' of the rendering that intersected with the
    * screen on the last render. This is only useful when progressive tile
diff --git a/gfx/layers/d3d11/CompositorD3D11.h b/gfx/layers/d3d11/CompositorD3D11.h
index acd767c5d598..664419788656 100644
--- a/gfx/layers/d3d11/CompositorD3D11.h
+++ b/gfx/layers/d3d11/CompositorD3D11.h
@@ -139,14 +139,6 @@ public:
   virtual void NotifyLayersTransaction() MOZ_OVERRIDE { }
 
   virtual nsIWidget* GetWidget() const MOZ_OVERRIDE { return mWidget; }
-  virtual const nsIntSize& GetWidgetSize() MOZ_OVERRIDE
-  {
-    NS_ASSERTION(false, "Getting the widget size on windows causes some kind of resizing of buffers. "
-                        "You should not do that outside of BeginFrame, so the best we can do is return "
-                        "the last size we got, that might not be up to date. So you probably shouldn't "
-                        "use this method.");
-    return mSize;
-  }
 
   ID3D11Device* GetDevice() { return mDevice; }
 
diff --git a/gfx/layers/d3d9/CompositorD3D9.h b/gfx/layers/d3d9/CompositorD3D9.h
index ca8d3542e47f..7b96fa561a3b 100644
--- a/gfx/layers/d3d9/CompositorD3D9.h
+++ b/gfx/layers/d3d9/CompositorD3D9.h
@@ -86,14 +86,6 @@ public:
   virtual void NotifyLayersTransaction() MOZ_OVERRIDE {}
 
   virtual nsIWidget* GetWidget() const MOZ_OVERRIDE { return mWidget; }
-  virtual const nsIntSize& GetWidgetSize() MOZ_OVERRIDE
-  {
-    NS_ASSERTION(false, "Getting the widget size on windows causes some kind of resizing of buffers. "
-                        "You should not do that outside of BeginFrame, so the best we can do is return "
-                        "the last size we got, that might not be up to date. So you probably shouldn't "
-                        "use this method.");
-    return mSize;
-  }
 
   IDirect3DDevice9* device() const
   {
diff --git a/gfx/layers/opengl/CompositorOGL.h b/gfx/layers/opengl/CompositorOGL.h
index 72a3db542fd8..e6427a593fba 100644
--- a/gfx/layers/opengl/CompositorOGL.h
+++ b/gfx/layers/opengl/CompositorOGL.h
@@ -158,9 +158,6 @@ public:
   virtual bool Resume() MOZ_OVERRIDE;
 
   virtual nsIWidget* GetWidget() const MOZ_OVERRIDE { return mWidget; }
-  virtual const nsIntSize& GetWidgetSize() MOZ_OVERRIDE {
-    return mWidgetSize;
-  }
 
   GLContext* gl() const { return mGLContext; }
   ShaderProgramType GetFBOLayerProgramType() const {