WPF问题：需要在另一个上面绘制控件

I've a shell window constructed of an Header, Main Content and Footer. The main content is a tab control. According to the design, one of the tab items content (a user control) needs to expand a little bit over the footer. At first I thought about implmenting this with a negative Margin, but the footer no matter what I did always get drawn on top of my content.I've a shell window constructed of an Header, M