diff --git a/widget/nsBaseWidget.cpp b/widget/nsBaseWidget.cpp
index db6a6bd670db..e8ae12c2441a 100644
--- a/widget/nsBaseWidget.cpp
+++ b/widget/nsBaseWidget.cpp
@@ -792,37 +792,19 @@ NS_IMETHODIMP nsBaseWidget::MakeFullScreen(bool aFullScreen, nsIScreen* aScreen)
   if (aFullScreen) {
     if (!mOriginalBounds)
       mOriginalBounds = new nsIntRect();
-    GetScreenBounds(*mOriginalBounds);
-    // convert dev pix to display pix for window manipulation
-    CSSToLayoutDeviceScale scale = GetDefaultScale();
-    mOriginalBounds->x = NSToIntRound(mOriginalBounds->x / scale.scale);
-    mOriginalBounds->y = NSToIntRound(mOriginalBounds->y / scale.scale);
-    mOriginalBounds->width = NSToIntRound(mOriginalBounds->width / scale.scale);
-    mOriginalBounds->height = NSToIntRound(mOriginalBounds->height / scale.scale);
+    *mOriginalBounds = GetScaledScreenBounds();
 
     // Move to top-left corner of screen and size to the screen dimensions
-    nsCOMPtr<nsIScreenManager> screenManager;
-    screenManager = do_GetService("@mozilla.org/gfx/screenmanager;1");
-    NS_ASSERTION(screenManager, "Unable to grab screenManager.");
-    if (screenManager) {
-      nsCOMPtr<nsIScreen> screen = aScreen;
-      if (!screen) {
-        // no screen was passed in, use the one that the window is on
-        screenManager->ScreenForRect(mOriginalBounds->x,
-                                     mOriginalBounds->y,
-                                     mOriginalBounds->width,
-                                     mOriginalBounds->height,
-                                     getter_AddRefs(screen));
-      }
-
-      if (screen) {
-        int32_t left, top, width, height;
-        if (NS_SUCCEEDED(screen->GetRectDisplayPix(&left, &top, &width, &height))) {
-          Resize(left, top, width, height, true);
-        }
+    nsCOMPtr<nsIScreen> screen = aScreen;
+    if (!screen) {
+      screen = GetWidgetScreen();
+    }
+    if (screen) {
+      int32_t left, top, width, height;
+      if (NS_SUCCEEDED(screen->GetRectDisplayPix(&left, &top, &width, &height))) {
+        Resize(left, top, width, height, true);
       }
     }
-
   } else if (mOriginalBounds) {
     Resize(mOriginalBounds->x, mOriginalBounds->y, mOriginalBounds->width,
            mOriginalBounds->height, true);
@@ -1758,6 +1740,36 @@ nsBaseWidget::GetRootAccessible()
 
 #endif // ACCESSIBILITY
 
+nsIntRect
+nsBaseWidget::GetScaledScreenBounds()
+{
+  nsIntRect bounds;
+  GetScreenBounds(bounds);
+  CSSToLayoutDeviceScale scale = GetDefaultScale();
+  bounds.x = NSToIntRound(bounds.x / scale.scale);
+  bounds.y = NSToIntRound(bounds.y / scale.scale);
+  bounds.width = NSToIntRound(bounds.width / scale.scale);
+  bounds.height = NSToIntRound(bounds.height / scale.scale);
+  return bounds;
+}
+
+already_AddRefed<nsIScreen>
+nsBaseWidget::GetWidgetScreen()
+{
+  nsCOMPtr<nsIScreenManager> screenManager;
+  screenManager = do_GetService("@mozilla.org/gfx/screenmanager;1");
+  if (!screenManager) {
+    return nullptr;
+  }
+
+  nsIntRect bounds = GetScaledScreenBounds();
+  nsCOMPtr<nsIScreen> screen;
+  screenManager->ScreenForRect(bounds.x, bounds.y,
+                               bounds.width, bounds.height,
+                               getter_AddRefs(screen));
+  return screen.forget();
+}
+
 nsresult
 nsIWidget::SynthesizeNativeTouchTap(nsIntPoint aPointerScreenPoint, bool aLongTap,
                                     nsIObserver* aObserver)
diff --git a/widget/nsBaseWidget.h b/widget/nsBaseWidget.h
index 20b833f55ebd..aa53880aaff3 100644
--- a/widget/nsBaseWidget.h
+++ b/widget/nsBaseWidget.h
@@ -284,6 +284,13 @@ public:
     return aClientSize;
   }
 
+  // return the widget's outside dimensions
+  // in global coordinates in display pixel.
+  nsIntRect GetScaledScreenBounds();
+
+  // return the screen the widget is in.
+  already_AddRefed<nsIScreen> GetWidgetScreen();
+
   // return true if this is a popup widget with a native titlebar
   bool IsPopupWithTitleBar() const
   {