XAML - The property 'Content' is set more than once

Question

Very new to WPF and XAML. I can\'t get my head around why I can\'t place a WPF control where I would like in the following code. My issue is where the

Answer 1

Certain containers only allow 1 element, other containers allow >1 element. When you get the error message 'Content' is set more than once, it means you have tried to put more than one type of element in a container that only allows 1 element.

Maybe try this (not tested):

This is where the outline of the entire document will be placed.