How do you style only the top level items in TreeView?

Question

For the 2nd day I'm scouring the web and have not found a solution. Take an element like this:

<TreeView ItemsSource="{Binding Types}" Width="300">
   <TreeView.Resources>
      <HierarchicalDataTemplate DataType="{x:Type models:Type}"
                                ItemsSource="{Binding SubTypes}">
         <TextBlock Text="{Binding Name}"/>
         <HierarchicalDataTemplate.ItemTemplate>
            <DataTemplate DataType="{x:Type SubType}">
               <TextBlock Text="{Binding Name}"/>
            </DataTemplate>
         </HierarchicalDataTemplate.ItemTemplate>
      </HierarchicalDataTemplate>
   </TreeView.Resources>
</TreeView>

I use the Material NuGet library for base styles. Now however I need to disable the hover, select, etc. on the first level items and only allow the selection/hover for the subitems.

But everything I seem to find is about styling the contents of each item or styling everything globally.

A <- remove selection/hover (pref single click too but that's another topic)
  A1 <- maintain original style, hover and select
  A2 <- maintain original style, hover and select
  A3 <- maintain original style, hover and select
B <- remove selection/hover (pref single click too but that's another topic)
  B1 <- maintain original style, hover and select
  B2 <- maintain original style, hover and select
  B3 <- maintain original style, hover and select

Answer 1

Sounds like you don't really want each top-level item to act like a normal TreeViewItem. In that case, why not move the top-level items outside of the TreeView?

Basically, you'd have an ItemsControl of the top-level items, where the item template acts a bit like an Expander containing a TreeView of the items underneath it. You could style the top-level items to look however you like.

The downside would be that the trees under the top-level items would be virtualized individually, not as a whole. That is, they would not share containers. Unless you have a ton of top-level items, that probably won't be a big deal.

Example:

<ItemsControl xmlns:s="clr-namespace:System;assembly=mscorlib"
              ItemsSource="{Binding Types}">
  <ItemsControl.Resources>
    <ControlTemplate x:Key="ExpanderButtonTemplate" TargetType="ToggleButton">
      <Border Background="Transparent" Padding="3,2">
        <ContentPresenter />
      </Border>
    </ControlTemplate>
    <Style TargetType="Expander">
      <Setter Property="Template">
        <Setter.Value>
          <ControlTemplate TargetType="Expander">
            <DockPanel LastChildFill="True">
              <ToggleButton DockPanel.Dock="Top"
                            IsChecked="{Binding IsExpanded, RelativeSource={RelativeSource TemplatedParent}}"
                            Template="{StaticResource ExpanderButtonTemplate}">
                <ContentPresenter ContentSource="Header" />
              </ToggleButton>
              <Border>
                <ContentPresenter x:Name="contentSite" Visibility="Collapsed" />
              </Border>
            </DockPanel>
            <ControlTemplate.Triggers>
              <Trigger Property="IsExpanded" Value="True">
                <Setter TargetName="contentSite" Property="Visibility" Value="Visible" />
              </Trigger>
            </ControlTemplate.Triggers>
          </ControlTemplate>
        </Setter.Value>
      </Setter>
    </Style>
  </ItemsControl.Resources>
  <ItemsControl.ItemTemplate>
    <DataTemplate>
      <Expander Header="{Binding Name}">
        <TreeView ItemsSource="{Binding SubTypes}" BorderThickness="0" Padding="0">
          <TreeView.ItemTemplate>
            <HierarchicalDataTemplate DataType="{x:Type models:Type}"
                                      ItemsSource="{Binding SubTypes}">
              <TextBlock Text="{Binding Name}"/>
              <HierarchicalDataTemplate.ItemTemplate>
                <DataTemplate DataType="{x:Type models:SubType}">
                  <TextBlock Text="{Binding Name}"/>
                </DataTemplate>
              </HierarchicalDataTemplate.ItemTemplate>
            </HierarchicalDataTemplate>
          </TreeView.ItemTemplate>
        </TreeView>
      </Expander>
    </DataTemplate>
  </ItemsControl.ItemTemplate>
</ItemsControl>

Click on one of the top-level items to expand the tree beneath it.