Property

Name
Header
MainContent
SideContent

Example

 <dev:SamplePanel Header="Header">
    <dev:SamplePanel.MainContent>
        <TextBlock Text="Main Content" />
    </dev:SamplePanel.MainContent>
    <dev:SamplePanel.SideContent>
        <TextBlock Text="Side Content" />
    </dev:SamplePanel.SideContent>
</dev:SamplePanel>

Demo

you can run demo and see this feature.