SetParent needs to clear 'managed' flag for former top level window.

dlls/x11drv/window.c

diff --git a/dlls/x11drv/window.c b/dlls/x11drv/window.c
index 78bc35b..c896c91 100644
--- a/dlls/x11drv/window.c
+++ b/dlls/x11drv/window.c
@@ -1139,6 +1139,11 @@
                 /* destroy the old X windows */
                 destroy_whole_window( display, data );
                 destroy_icon_window( display, data );
+                if (data->managed)
+                {
+                    data->managed = FALSE;
+                    RemovePropA( data->hwnd, managed_atom );
+                }
             }
         }
         else  /* new top level window */