diff --git a/components/compositing/compositor.rs b/components/compositing/compositor.rs
index dc3313fb595..a4061351939 100644
--- a/components/compositing/compositor.rs
+++ b/components/compositing/compositor.rs
@@ -9,6 +9,7 @@ use compositor_thread::{InitialCompositorState, Msg, RenderListener};
 use delayed_composition::DelayedCompositionTimerProxy;
 use euclid::Point2D;
 use euclid::point::TypedPoint2D;
+use euclid::rect::TypedRect;
 use euclid::scale_factor::ScaleFactor;
 use euclid::size::TypedSize2D;
 use gfx_traits::{Epoch, FragmentType, ScrollRootId};
@@ -140,8 +141,11 @@ pub struct IOCompositor<Window: WindowMethods> {
     /// The scene scale, to allow for zooming and high-resolution painting.
     scale: ScaleFactor<f32, LayerPixel, DevicePixel>,
 
-    /// The application window size.
-    window_size: TypedSize2D<u32, DevicePixel>,
+    /// The size of the rendering area.
+    frame_size: TypedSize2D<u32, DevicePixel>,
+
+    /// The position and size of the window within the rendering area.
+    window_rect: TypedRect<u32, DevicePixel>,
 
     /// The overridden viewport.
     viewport: Option<(TypedPoint2D<u32, DevicePixel>, TypedSize2D<u32, DevicePixel>)>,
@@ -376,7 +380,8 @@ impl webrender_traits::RenderDispatcher for CompositorThreadDispatcher {
 impl<Window: WindowMethods> IOCompositor<Window> {
     fn new(window: Rc<Window>, state: InitialCompositorState)
            -> IOCompositor<Window> {
-        let window_size = window.framebuffer_size();
+        let frame_size = window.framebuffer_size();
+        let window_rect = window.window_rect();
         let scale_factor = window.hidpi_factor();
         let composite_target = match opts::get().output_file {
             Some(_) => CompositeTarget::PngFile,
@@ -388,7 +393,8 @@ impl<Window: WindowMethods> IOCompositor<Window> {
             port: state.receiver,
             root_pipeline: None,
             pipeline_details: HashMap::new(),
-            window_size: window_size,
+            frame_size: frame_size,
+            window_rect: window_rect,
             scale: ScaleFactor::new(1.0),
             viewport: None,
             scale_factor: scale_factor,
@@ -756,7 +762,18 @@ impl<Window: WindowMethods> IOCompositor<Window> {
 
     fn send_window_size(&self, size_type: WindowSizeType) {
         let dppx = self.page_zoom * self.hidpi_factor();
-        let initial_viewport = self.window_size.to_f32() / dppx;
+
+        let window_rect = {
+            let offset = webrender_traits::DeviceUintPoint::new(self.window_rect.origin.x, self.window_rect.origin.y);
+            let size = webrender_traits::DeviceUintSize::new(self.window_rect.size.width, self.window_rect.size.height);
+            webrender_traits::DeviceUintRect::new(offset, size)
+        };
+
+        let frame_size = webrender_traits::DeviceUintSize::new(self.frame_size.width, self.frame_size.height);
+        self.webrender_api.set_window_parameters(frame_size, window_rect);
+
+        let initial_viewport = self.window_rect.size.to_f32() / dppx;
+
         let msg = ConstellationMsg::WindowSize(WindowSizeData {
             device_pixel_ratio: dppx,
             initial_viewport: initial_viewport,
@@ -892,11 +909,16 @@ impl<Window: WindowMethods> IOCompositor<Window> {
             self.update_zoom_transform();
         }
 
-        if self.window_size == new_size {
+        let new_window_rect = self.window.window_rect();
+        let new_frame_size = self.window.framebuffer_size();
+
+        if self.window_rect == new_window_rect &&
+           self.frame_size == new_frame_size {
             return;
         }
 
-        self.window_size = new_size;
+        self.frame_size = new_size;
+        self.window_rect = new_window_rect;
 
         self.send_window_size(WindowSizeType::Resize);
     }
@@ -1485,7 +1507,7 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                                  target: CompositeTarget)
                                  -> Result<Option<Image>, UnableToComposite> {
         let (width, height) =
-            (self.window_size.width as usize, self.window_size.height as usize);
+            (self.frame_size.width as usize, self.frame_size.height as usize);
         if !self.window.prepare_for_composite(width, height) {
             return Err(UnableToComposite::WindowUnprepared)
         }
@@ -1523,7 +1545,7 @@ impl<Window: WindowMethods> IOCompositor<Window> {
             debug!("compositor: compositing");
 
             // Paint the scene.
-            let size = webrender_traits::DeviceUintSize::from_untyped(&self.window_size.to_untyped());
+            let size = webrender_traits::DeviceUintSize::from_untyped(&self.frame_size.to_untyped());
             self.webrender.render(size);
         });
 
diff --git a/components/compositing/windowing.rs b/components/compositing/windowing.rs
index 85d73af2f9f..84a1365af0d 100644
--- a/components/compositing/windowing.rs
+++ b/components/compositing/windowing.rs
@@ -7,6 +7,7 @@
 use compositor_thread::{CompositorProxy, CompositorReceiver};
 use euclid::{Point2D, Size2D};
 use euclid::point::TypedPoint2D;
+use euclid::rect::TypedRect;
 use euclid::scale_factor::ScaleFactor;
 use euclid::size::TypedSize2D;
 use msg::constellation_msg::{Key, KeyModifiers, KeyState};
@@ -106,8 +107,10 @@ impl Debug for WindowEvent {
 }
 
 pub trait WindowMethods {
-    /// Returns the size of the window in hardware pixels.
+    /// Returns the rendering area size in hardware pixels.
     fn framebuffer_size(&self) -> TypedSize2D<u32, DevicePixel>;
+    /// Returns the position and size of the window within the rendering area.
+    fn window_rect(&self) -> TypedRect<u32, DevicePixel>;
     /// Returns the size of the window in density-independent "px" units.
     fn size(&self) -> TypedSize2D<f32, DeviceIndependentPixel>;
     /// Presents the window to the screen (perhaps by page flipping).
diff --git a/ports/cef/window.rs b/ports/cef/window.rs
index 4c31a890774..ecf8a1f2984 100644
--- a/ports/cef/window.rs
+++ b/ports/cef/window.rs
@@ -19,7 +19,8 @@ use wrappers::CefWrap;
 
 use compositing::compositor_thread::{self, CompositorProxy, CompositorReceiver};
 use compositing::windowing::{WindowEvent, WindowMethods};
-use euclid::point::Point2D;
+use euclid::point::{Point2D, TypedPoint2D};
+use euclid::rect::TypedRect;
 use euclid::scale_factor::ScaleFactor;
 use euclid::size::{Size2D, TypedSize2D};
 use gleam::gl;
@@ -206,6 +207,12 @@ impl WindowMethods for Window {
         }
     }
 
+    fn window_rect(&self) -> TypedRect<u32, DevicePixel> {
+        let size = self.framebuffer_size();
+        let origin = TypedPoint2D::zero();
+        TypedRect::new(origin, size)
+    }
+
     fn size(&self) -> TypedSize2D<f32, DeviceIndependentPixel> {
         let browser = self.cef_browser.borrow();
         match *browser {
diff --git a/ports/glutin/window.rs b/ports/glutin/window.rs
index 79582972de7..04cc8fbe2b0 100644
--- a/ports/glutin/window.rs
+++ b/ports/glutin/window.rs
@@ -9,6 +9,7 @@ use compositing::compositor_thread::{self, CompositorProxy, CompositorReceiver};
 use compositing::windowing::{MouseWindowEvent, WindowNavigateMsg};
 use compositing::windowing::{WindowEvent, WindowMethods};
 use euclid::{Point2D, Size2D, TypedPoint2D};
+use euclid::rect::TypedRect;
 use euclid::scale_factor::ScaleFactor;
 use euclid::size::TypedSize2D;
 #[cfg(target_os = "windows")]
@@ -797,6 +798,12 @@ impl WindowMethods for Window {
         }
     }
 
+    fn window_rect(&self) -> TypedRect<u32, DevicePixel> {
+        let size = self.framebuffer_size();
+        let origin = TypedPoint2D::zero();
+        TypedRect::new(origin, size)
+    }
+
     fn size(&self) -> TypedSize2D<f32, DeviceIndependentPixel> {
         match self.kind {
             WindowKind::Window(ref window) => {