diff --git a/include/wx/window.h b/include/wx/window.h
index 8b960ba1f1..1b12bc2884 100644
--- a/include/wx/window.h
+++ b/include/wx/window.h
@@ -988,16 +988,21 @@ protected:
     // smarter SetSize / convenience function for expanding wxDefaultSize.
     // Note that it does not influence the value returned by GetBestSize
     // at all.
-    //
-    // FIXME: Can't we just make this the behaviour of SetSize when
-    //        wxSIZE_AUTO_* is in play and do away with this (undocumented)
-    //        method altogether?
-
     void SetBestSize(const wxSize& size)
     {
-        wxSize  best( DoGetBestSize() );
+        // the size only needs to be changed if the current size is incomplete,
+        // i.e. one of the components was specified as default -- so if both
+        // were given, simply don't do anything
+        if ( size.x == -1 || size.y == -1 )
+        {
+            wxSize sizeBest = DoGetBestSize();
+            if ( size.x != -1 )
+                sizeBest.x = size.x;
+            if ( size.y != -1 )
+                sizeBest.y = size.y;
 
-        SetSize( wxMax( size.x, best.x ), wxMax( size.y, best.y ) );
+            SetSize(sizeBest);
+        }
     }
 
     // more pure virtual functions