Why are tab headers displayed in the content area of tabs in a XAML TabControl?

Answer 1

The TabControl will accept your controls as its controls only if they can be cast to TabItem, not UserControl, or SmartFormAreaView, etc.

So you either fill regular TabItems with your visual tree, or you subclass TabItems, or you subclass the TabControl to override its IsItemItsOwnContainerOverride method, to accept your type as the container.

The method should look as follows:

protected override bool IsItemItsOwnContainerOverride(object item)
{
    return item is YourControlTypeHere || item is TabItem;
}

Answer 2

For any ItemsControl, if the items added to its Items collection (either directly or via ItemsSource) are not instance of that control's item container, then each item is wrapped in an instance of the item container. The item container is a class such as TabItem or ListBoxItem. The item container is normally a ContentControl or HeaderedContentControl, and your actual item is assigned to its Content property, so you can use templates etc to control how the content is presented. You can also style the item container itself using the ItemControl's ItemContainerStyle property.

In this particular case, you should bind ItemsSource to a list of SmartFormAreaPresenters. Then use something like this for the tab control:

<TabControl ItemsSource="{Binding SmartFormAreaPresenters}">
  <TabControl.ItemContainerStyle>
    <Style TargetType="{x:Type TabItem}">
      <Setter Property="Header" Value="{Binding HeaderText}" />
    </Style>
  </TabControl.ItemContainerStyle>

  <TabControl.ContentTemplate>
    <DataTemplate DataType="{x:Type local:SmartFormAreaPresenter}">
      <local:SmartFormAreaView />
    </DataTemplate>
  </TabControl.ContentTemplate>
</TabControl>

where HeaderText is a suitable property on your SmartFormAreaPresenter. You should also remove the TabItem from your SmartFormAreaView definition. The DataContext of each View will automatically be set to the appropriate Presenter.

See Dr. WPF's blog for an excellent discussion of various ItemsControl related topics.