diff --git a/gfx/layers/ipc/AsyncPanZoomController.cpp b/gfx/layers/ipc/AsyncPanZoomController.cpp
index d17150c9e26..5bacdbb4f8f 100644
--- a/gfx/layers/ipc/AsyncPanZoomController.cpp
+++ b/gfx/layers/ipc/AsyncPanZoomController.cpp
@@ -1346,8 +1346,8 @@ void AsyncPanZoomController::UpdateCompositionBounds(const ScreenIntRect& aCompo
   // has gone out of view, the buffer will be cleared elsewhere anyways.
   if (aCompositionBounds.width && aCompositionBounds.height &&
       oldCompositionBounds.width && oldCompositionBounds.height) {
-    ScreenToScreenScale adjustmentFactor(float(aCompositionBounds.width) / float(oldCompositionBounds.width));
-    SetZoomAndResolution(mFrameMetrics.mZoom * adjustmentFactor);
+    float adjustmentFactor = float(aCompositionBounds.width) / float(oldCompositionBounds.width);
+    SetZoomAndResolution(CSSToScreenScale(mFrameMetrics.mZoom.scale * adjustmentFactor));
 
     // Repaint on a rotation so that our new resolution gets properly updated.
     RequestContentRepaint();
diff --git a/layout/base/Units.h b/layout/base/Units.h
index 7131aadc29d..6c3832e50ee 100644
--- a/layout/base/Units.h
+++ b/layout/base/Units.h
@@ -69,7 +69,6 @@ typedef gfx::ScaleFactor<LayoutDevicePixel, ScreenPixel> LayoutDeviceToScreenSca
 typedef gfx::ScaleFactor<ScreenPixel, LayoutDevicePixel> ScreenToLayoutDeviceScale;
 typedef gfx::ScaleFactor<LayerPixel, ScreenPixel> LayerToScreenScale;
 typedef gfx::ScaleFactor<ScreenPixel, LayerPixel> ScreenToLayerScale;
-typedef gfx::ScaleFactor<ScreenPixel, ScreenPixel> ScreenToScreenScale;
 
 /*
  * The pixels that content authors use to specify sizes in.