user32: Fail creation of the desktop window earlier on when it already exists.

dlls/user32/desktop.c

@@ -117,6 +117,8 @@ LRESULT WINAPI DesktopWndProc( HWND hwnd, UINT message, WPARAM wParam, LPARAM lP
             ATOM atom;
             WCHAR buffer[37];
 
+            if (GetAncestor( hwnd, GA_PARENT )) return FALSE;  /* refuse to create non-desktop window */
+
             sprintfW( buffer, guid_formatW, guid->Data1, guid->Data2, guid->Data3,
                       guid->Data4[0], guid->Data4[1], guid->Data4[2], guid->Data4[3],
                       guid->Data4[4], guid->Data4[5], guid->Data4[6], guid->Data4[7] );

dlls/user32/win.c

@@ -2086,6 +2086,9 @@ HWND WINAPI GetDesktopWindow(void)
         SERVER_END_REQ;
     }
 
+    /* wait for the desktop process to be ready */
+    SendMessageW( thread_info->top_window, WM_NULL, 0, 0 );
+
     if (!thread_info->top_window || !USER_Driver->pCreateDesktopWindow( thread_info->top_window ))
         ERR( "failed to create desktop window\n" );
 

programs/explorer/desktop.c

@@ -799,7 +799,7 @@ void manage_desktop( WCHAR *arg )
     HDESK desktop = 0;
     GUID guid;
     MSG msg;
-    HWND hwnd, msg_hwnd;
+    HWND hwnd;
     HMODULE graphics_driver;
     unsigned int width, height;
     WCHAR *cmdline = NULL, *driver = NULL;
@@ -856,12 +856,12 @@ void manage_desktop( WCHAR *arg )
     hwnd = CreateWindowExW( 0, DESKTOP_CLASS_ATOM, NULL,
                             WS_POPUP | WS_CLIPSIBLINGS | WS_CLIPCHILDREN, 0, 0, 0, 0, 0, 0, 0, &guid );
 
+    if (hwnd)
+    {
         /* create the HWND_MESSAGE parent */
-    msg_hwnd = CreateWindowExW( 0, messageW, NULL, WS_POPUP | WS_CLIPSIBLINGS | WS_CLIPCHILDREN,
+        CreateWindowExW( 0, messageW, NULL, WS_POPUP | WS_CLIPSIBLINGS | WS_CLIPCHILDREN,
                          0, 0, 100, 100, 0, 0, 0, NULL );
 
-    if (hwnd == GetDesktopWindow())
-    {
         using_root = !desktop || !create_desktop( graphics_driver, name, width, height );
         SetWindowLongPtrW( hwnd, GWLP_WNDPROC, (LONG_PTR)desktop_wnd_proc );
         SendMessageW( hwnd, WM_SETICON, ICON_BIG, (LPARAM)LoadIconW( 0, MAKEINTRESOURCEW(OIC_WINLOGO)));
@@ -891,13 +891,6 @@ void manage_desktop( WCHAR *arg )
             }
         }
     }
-    else
-    {
-        DestroyWindow( hwnd );  /* someone beat us to it */
-        hwnd = 0;
-    }
-
-    if (GetAncestor( msg_hwnd, GA_PARENT )) DestroyWindow( msg_hwnd );  /* someone beat us to it */
 
     /* if we have a command line, execute it */
     if (cmdline)