Call SetProcessDPIAware() on Win32 backend so that the application is guarenteed to see the real dpi.

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -976,6 +976,9 @@ namespace Avalonia.Win32.Interop
         [DllImport("shcore.dll")]
         public static extern void GetScaleFactorForMonitor(IntPtr hMon, out uint pScale);
 
+        [DllImport("user32.dll", SetLastError = true)]
+        public static extern bool SetProcessDPIAware();
+
         [DllImport("user32.dll")]
         public static extern IntPtr MonitorFromPoint(POINT pt, MONITOR dwFlags);
 

src/Windows/Avalonia.Win32/Win32Platform.cs

@@ -77,6 +77,8 @@ namespace Avalonia.Win32
 
         public static void Initialize(bool deferredRendering = true)
         {
+            UnmanagedMethods.SetProcessDPIAware();
+
             AvaloniaLocator.CurrentMutable
                 .Bind<IClipboard>().ToSingleton<ClipboardImpl>()
                 .Bind<IStandardCursorFactory>().ToConstant(CursorFactory.Instance)