How does ItemsControl remember its focused element? - wpf

Question

Consider the following layout:
<Button Grid.Row="0">Before</Button>
<ItemsControl x:Name="MainItemsControl"
Grid.Row="1"
KeyboardNavigation.TabNavigation="Once"
KeyboardNavigation.DirectionalNavigation="Contained"
IsTabStop="False">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel IsItemsHost="True"
Orientation="Horizontal" />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ListBoxItem>Test1</ListBoxItem>
<ListBoxItem>Test2</ListBoxItem>
<ListBoxItem>Test3</ListBoxItem>
</ItemsControl>
<Button Grid.Row="2">After</Button>
With regards to keyboard navigation, this works as you would expect:
With three <Tab>s, you can navigate from the first button to the itemscontrol and finally to the second button.
When tabbing into the ItemsControl, you can use cursor keys to navigate the contained items
When tabbing out of the ItemsControl and back into it, the last focused item is restored
Now, let's replace the ListBoxItems with another level of ItemsControls:
<ItemsControl x:Name="MainItemsControl"
[...]
<ItemsControl KeyboardNavigation.TabNavigation="Once"
KeyboardNavigation.DirectionalNavigation="Continue"
IsTabStop="False">
<ListBoxItem>One1</ListBoxItem>
<ListBoxItem>One2</ListBoxItem>
<ListBoxItem>One3</ListBoxItem>
</ItemsControl>
<ItemsControl KeyboardNavigation.TabNavigation="Once"
KeyboardNavigation.DirectionalNavigation="Continue"
IsTabStop="False">
<ListBoxItem>Two1</ListBoxItem>
<ListBoxItem>Two2</ListBoxItem>
<ListBoxItem>Two3</ListBoxItem>
</ItemsControl>
</ItemsControl>
By and large, this still works as desired. You now can use left+right arrow keys to navigate between "One" and "Two" items.
However, when I focus a "Two" item, then tab out of the ItemsControl and back into it, the focus of the last selected "One" item (i.e. the last focus of the leftmost inner ItemsControl) is restored.
What kind of mechanism causes the the ItemsControl to remember its inner focus and why does it break when I add another layer of ItemsControls inside?
(Note: No manual FocusScopes or the like are defined so far. By tinkering with FocusManager, you can make WPF restore the correct focus. I am wondering about the vanilla default behavior.)