Wrap demo.Render() in BeginChild()

[matt-edmondson]

Consider adding this to a BeginChild so the tabs are always visible, even when scrolled down the page so you can switch tab when you get to the end of the content?

Originally posted by @Damon3000s in #132 (comment)

[matt-edmondson]

@copilot is this still required?

[Damon3000s]

Tabbing in AdvancedWidgetsDemo.cs‎ was not correctly indented.