This code:
<StackPanel Orientation="Horizontal">
<RichTextBox />
<Button Content="Dialog" />
</StackPanel>
shows the button somewhere on the left side of the StackPanel ONTO the RichTextBox, WHY?
edit: Oh I just saw its a width problem. The RTB has nearly no width and the button is righthand of it.
edit: seems I run into this bug: http://stackoverflow.com/questions/350863/wpf-richtextbox-with-no-width-set
solution does not work for me!