Recently I had been battling with adorners in WPF. I have a watermark service adapted from the code in this SO answer, which works great, adding watermarks to my text boxes.
However sometimes user controls that are shown above a textbox with a watermark on it would show the watermark above everything else, which became pretty annoying.
After some research with google and snoop, I discovered how adorners work, and the fact that when an adorner is added to an element, it is in fact added to the first adorner decorator up the tree, and because by default the adorner decorator is right at the top of the tree the adorners appear over everything.
The simple fix is to add an adorner decorator around the specific element with the watermark, that way the watermark stays with the element.
<AdornerDecorator Grid.Row="0" Grid.Column="1">
<TextBox x:Name="UsernameTextbox" Text="">