WPF: How to make controls stretch in a StackPanel?
You want DockPanel instead:
<DockPanel DockPanel.Dock="Bottom" LastChildFill="True"> <TextBlock DockPanel.Dock="Right" ... /> <Slider ... /></DockPanel>DockPanel gets you the "Anchor" functionality you want. StackPanel simply stacks things next to each other.
I notice from your code block that your StackPanel is already docked inside a dockpanel, so I've included the same docking attribute in my code sample above.