How to make a StackPanel stretch vertically to fill the available space

The problem is moot. The StackPanel doesn't work like that. You need to use a DockPanel to make inner children of the Panel stretch vertically and make the Panel fill all available space. The elements inside should have DockPanel.Dock="Top" to fill only the space they require , then no Dock for the element(s) you want to stretch, then DockPanel.Dock="Bottom" for the rest of the elements. That's how you get that "Outlook feel", too.