diff --git a/widget/windows/nsWindow.cpp b/widget/windows/nsWindow.cpp
index df0511de6d6..ddcc0517a4b 100644
--- a/widget/windows/nsWindow.cpp
+++ b/widget/windows/nsWindow.cpp
@@ -2868,7 +2868,9 @@ nsWindow::MakeFullScreen(bool aFullScreen, nsIScreen* aTargetScreen)
     SetSizeMode(mOldSizeMode);
   }
 
-  UpdateNonClientMargins();
+  // If we are going fullscreen, the window size continues to change
+  // and the window will be reflow again then.
+  UpdateNonClientMargins(mSizeMode, /* Reflow */ !aFullScreen);
 
   bool visible = mIsVisible;
   if (mOldSizeMode == nsSizeMode_Normal)