ScrollBars for ListBox within Listbox

Hello,

I have an problem that I can not solve for quite some time.
For simplicity, I have a sample code to demonstrate the idea...

<Grid>
<ListBox>
<ListBoxItem>
<Expander Header="OuteItem">
<ListBox>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
</ListBox>
</Expander>
</ListBoxItem>
<ListBoxItem>
<Expander Header="OuteItem">
<ListBox>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
</ListBox>
</Expander>
</ListBoxItem>
<ListBoxItem>
<Expander Header="OuteItem">
<ListBox>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
</ListBox>
</Expander>
</ListBoxItem>
<ListBoxItem>
<Expander Header="OuteItem">
<ListBox>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
<TextBlock>InnerItem</TextBlock>
</ListBox>
</Expander>
</ListBoxItem>
</ListBox>
</Grid>


The OuterItem listbox is scrollable and is just fine.
Anyone has any idea how to make InnerItem ListBox scrollable???
Thank you so much!

I think perhaps the inner items do not have a scrollbar because the inner ListBox is not limited in height and just continues to expand to accommodate each new item. If you were to restrict the height of each inner ListBox then a scrollbar would appear whenever the contents became too large to fit in the defined height.