No updateting in Windowmode! (X11)

I start writing a very simple test program setting up a windowmode using:
screen = SDL_SetVideoMode(640, 480, 0, 0);

when i’m then blitting an image to “screen” and performing an
SDL_updateRect(), nothing is to see just an black window!

but when i’m swtching to fullscreen or using from start fullscreen, it works.
when switching back to windowmode it works on.

very suspicious is that in an other programm from it works in windowmode
after i created an SDL-thread in wich the “SDL_WaitEvent(…)” function is
called. After that SDL_updateRect() allwais does what it schould.

I reduced both programms just to this to things and i do not see whats

PLEASE if anybody knows help…

thanks, stefan–