Bind to ViewModel property inside ItemContainerStyle for use in triggers

I have a TreeView whose DataContext is my ViewModel. The ViewModel has a property MyProp. Inside the TreeView, I have defined an ItemContainerStyle with a DataTrigger:

<TreeView.ItemContainerStyle>
    <Style TargetType="{x:Type TreeViewItem}">
        <Style.Triggers>
            <DataTrigger Binding={MyProp} Value="...">
            <!-- does not work because DataContext is the Item, not the ViewModel -->
                <Setter Property="Focusable" Value="False"/>
            </DataTrigger>
        </Style.Triggers>
    </Style>
</TreeView.ItemContainerStyle>

But since the ItemContainerStyle has an Item as its DataContext and not the ViewModel, the Binding in the DataTrigger does not work. So how do I tell the Binding that I want to bind to ViewModel.MyProp? It seems I cannot set DataContext in Styles and Triggers, so how else can I do it?

Did you tried to take your relative source:

     <DataTrigger Binding="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=DataContext.MyProp}" Value="  ">