Let us say that we have 3 main containers. The first container is 300 (doc host), the second is 500, and the third is 100. When the window is expanded, I would like the second container to grow to fit the screen keeping the other two as fixed height. Currently I'm using DockingPanel.InitialSize for the Containers, I'm also using ProportionalStackPanel.RelativeSize for another part, but that won't help me get the other two as fixed height. I'm probably overlooking something obvious.