ignore frame extents if window has no decorations (#14441)

src/Avalonia.X11/X11Window.cs

@@ -593,6 +593,9 @@ namespace Avalonia.X11
 
         private Thickness? GetFrameExtents()
         {
+            if (_systemDecorations != SystemDecorations.Full)
+                return new Thickness(0);
+
             XGetWindowProperty(_x11.Display, _handle, _x11.Atoms._NET_FRAME_EXTENTS, IntPtr.Zero,
                 new IntPtr(4), false, (IntPtr)Atom.AnyPropertyType, out var _,
                 out var _, out var nitems, out var _, out var prop);