diff --git a/widget/cocoa/nsChildView.h b/widget/cocoa/nsChildView.h
index 8f07c70ddae..4b175f6f98a 100644
--- a/widget/cocoa/nsChildView.h
+++ b/widget/cocoa/nsChildView.h
@@ -390,6 +390,10 @@ public:
   // pixels" and the Cocoa "points" coordinate system.
   CGFloat                 BackingScaleFactor() const;
 
+  mozilla::DesktopToLayoutDeviceScale GetDesktopToDeviceScale() final {
+    return mozilla::DesktopToLayoutDeviceScale(BackingScaleFactor());
+  }
+
   // Call if the window's backing scale factor changes - i.e., it is moved
   // between HiDPI and non-HiDPI screens
   void                    BackingScaleFactorChanged();
diff --git a/widget/cocoa/nsCocoaWindow.h b/widget/cocoa/nsCocoaWindow.h
index f1c9227a572..9a4608d1b63 100644
--- a/widget/cocoa/nsCocoaWindow.h
+++ b/widget/cocoa/nsCocoaWindow.h
@@ -324,6 +324,10 @@ public:
     virtual double          GetDefaultScaleInternal() override;
     virtual int32_t         RoundsWidgetCoordinatesTo() override;
 
+    mozilla::DesktopToLayoutDeviceScale GetDesktopToDeviceScale() final {
+      return mozilla::DesktopToLayoutDeviceScale(BackingScaleFactor());
+    }
+
     NS_IMETHOD              SetTitle(const nsAString& aTitle) override;
 
     NS_IMETHOD Invalidate(const LayoutDeviceIntRect& aRect) override;
diff --git a/widget/cocoa/nsCocoaWindow.mm b/widget/cocoa/nsCocoaWindow.mm
index 87dfe38df66..1fce279b21e 100644
--- a/widget/cocoa/nsCocoaWindow.mm
+++ b/widget/cocoa/nsCocoaWindow.mm
@@ -291,8 +291,9 @@ nsresult nsCocoaWindow::Create(nsIWidget* aParent,
     }
     // now we can convert newBounds to device pixels for the window we created,
     // as the child view expects a rect expressed in the dev pix of its parent
-    DesktopToLayoutDeviceScale scale(BackingScaleFactor());
-    return CreatePopupContentView(RoundedToInt(newBounds * scale));
+    LayoutDeviceIntRect devRect =
+      RoundedToInt(newBounds * GetDesktopToDeviceScale());
+    return CreatePopupContentView(devRect);
   }
 
   mIsAnimationSuppressed = aInitData->mIsAnimationSuppressed;
@@ -307,8 +308,8 @@ nsresult nsCocoaWindow::Create(nsIWidget* aParent,
                                const LayoutDeviceIntRect& aRect,
                                nsWidgetInitData* aInitData)
 {
-  DesktopToLayoutDeviceScale scale(GetDefaultScaleInternal());
-  DesktopIntRect desktopRect = RoundedToInt(aRect / scale);
+  DesktopIntRect desktopRect =
+    RoundedToInt(aRect / GetDesktopToDeviceScale());
   return Create(aParent, aNativeParent, desktopRect, aInitData);
 }
 
diff --git a/widget/nsBaseWidget.cpp b/widget/nsBaseWidget.cpp
index 8d03648e40a..b5963803c39 100644
--- a/widget/nsBaseWidget.cpp
+++ b/widget/nsBaseWidget.cpp
@@ -831,9 +831,9 @@ NS_IMETHODIMP nsBaseWidget::MakeFullScreen(bool aFullScreen, nsIScreen* aScreen)
 
   if (aFullScreen) {
     if (!mOriginalBounds) {
-      mOriginalBounds = new CSSIntRect();
+      mOriginalBounds = new LayoutDeviceIntRect();
     }
-    *mOriginalBounds = GetScaledScreenBounds();
+    GetScreenBounds(*mOriginalBounds);
 
     // Move to top-left corner of screen and size to the screen dimensions
     nsCOMPtr<nsIScreen> screen = aScreen;
@@ -847,8 +847,13 @@ NS_IMETHODIMP nsBaseWidget::MakeFullScreen(bool aFullScreen, nsIScreen* aScreen)
       }
     }
   } else if (mOriginalBounds) {
-    Resize(mOriginalBounds->x, mOriginalBounds->y, mOriginalBounds->width,
-           mOriginalBounds->height, true);
+    if (BoundsUseDesktopPixels()) {
+      DesktopRect deskRect = *mOriginalBounds / GetDesktopToDeviceScale();
+      Resize(deskRect.x, deskRect.y, deskRect.width, deskRect.height, true);
+    } else {
+      Resize(mOriginalBounds->x, mOriginalBounds->y, mOriginalBounds->width,
+             mOriginalBounds->height, true);
+    }
   }
 
   return NS_OK;
diff --git a/widget/nsBaseWidget.h b/widget/nsBaseWidget.h
index 5fc502befa8..5461dc62cdc 100644
--- a/widget/nsBaseWidget.h
+++ b/widget/nsBaseWidget.h
@@ -186,6 +186,11 @@ public:
   bool                    BoundsUseDesktopPixels() const {
     return mWindowType <= eWindowType_popup;
   }
+  // Default implementation, to be overridden by platforms where desktop coords
+  // are virtualized and may not correspond to device pixels on the screen.
+  mozilla::DesktopToLayoutDeviceScale GetDesktopToDeviceScale() override {
+    return mozilla::DesktopToLayoutDeviceScale(1.0);
+  }
   NS_IMETHOD              MoveClient(double aX, double aY) override;
   NS_IMETHOD              ResizeClient(double aWidth, double aHeight, bool aRepaint) override;
   NS_IMETHOD              ResizeClient(double aX, double aY, double aWidth, double aHeight, bool aRepaint) override;
@@ -514,7 +519,7 @@ protected:
   nsCursor          mCursor;
   nsBorderStyle     mBorderStyle;
   nsIntRect         mBounds;
-  CSSIntRect*       mOriginalBounds;
+  LayoutDeviceIntRect* mOriginalBounds;
   // When this pointer is null, the widget is not clipped
   mozilla::UniquePtr<LayoutDeviceIntRect[]> mClipRects;
   uint32_t          mClipRectCount;
diff --git a/widget/nsIWidget.h b/widget/nsIWidget.h
index 6377fb5d2e5..b2da2ade51b 100644
--- a/widget/nsIWidget.h
+++ b/widget/nsIWidget.h
@@ -429,10 +429,8 @@ class nsIWidget : public nsISupports {
                       const DesktopIntRect& aRect,
                       nsWidgetInitData* aInitData = nullptr)
     {
-        // GetDefaultScaleInternal() here is a placeholder, to be replaced by
-        // GetDesktopToDeviceScale in a later patch
-        mozilla::DesktopToLayoutDeviceScale scale(GetDefaultScaleInternal());
-        LayoutDeviceIntRect devPixRect = RoundedToInt(aRect * scale);
+        LayoutDeviceIntRect devPixRect =
+          RoundedToInt(aRect * GetDesktopToDeviceScale());
         return Create(aParent, aNativeParent, devPixRect, aInitData);
     }
 
@@ -546,6 +544,13 @@ class nsIWidget : public nsISupports {
      */
     virtual float GetDPI() = 0;
 
+    /**
+     * Return the scaling factor between device pixels and the platform-
+     * dependent "desktop pixels" used to manage window positions on a
+     * potentially multi-screen, mixed-resolution desktop.
+     */
+    virtual mozilla::DesktopToLayoutDeviceScale GetDesktopToDeviceScale() = 0;
+
     /**
      * Returns the CompositorVsyncDispatcher associated with this widget
      */
@@ -858,7 +863,7 @@ class nsIWidget : public nsISupports {
     NS_IMETHOD GetBounds(LayoutDeviceIntRect& aRect) = 0;
 
     /**
-     * Get this widget's outside dimensions in global coordinates. This
+     * Get this widget's outside dimensions in device coordinates. This
      * includes any title bar on the window.
      *
      * @param aRect   On return it holds the  x, y, width and height of
@@ -878,7 +883,7 @@ class nsIWidget : public nsISupports {
      * @param aRect   On return it holds the  x, y, width and height of
      *                this widget.
      */
-    NS_IMETHOD GetRestoredBounds(mozilla::LayoutDeviceIntRect& aRect) = 0;
+    NS_IMETHOD GetRestoredBounds(LayoutDeviceIntRect& aRect) = 0;
 
     /**
      * Get this widget's client area bounds, if the window has a 3D border
@@ -889,7 +894,7 @@ class nsIWidget : public nsISupports {
      * @param aRect   On return it holds the  x. y, width and height of
      *                the client area of this widget.
      */
-    NS_IMETHOD GetClientBounds(mozilla::LayoutDeviceIntRect& aRect) = 0;
+    NS_IMETHOD GetClientBounds(LayoutDeviceIntRect& aRect) = 0;
 
     /**
      * Get the non-client area dimensions of the window.