[Improvement] `ListBox.draggable`-Style should not disable virtualization

Hi @wieslawsoltes

first of all this library is really really useful and I am happy with the state it is. 😄 However when I tested some performance in my App and I noticed that adding several hundred or thousand items to a ListBox.draggable makes the UI nearly unusable. After having a deeper look I noticed that virtualization is disabled in the Styles-Section:

  <Style Selector=":is(ListBox).draggable">
    <Setter Property="ItemsPanel">
      <Setter.Value>
        <ItemsPanelTemplate>
          <StackPanel Orientation="Vertical" />
        </ItemsPanelTemplate>
      </Setter.Value>
    </Setter>
  </Style>

If I replace the StackPanel with VirtualizingStackPanel it works, but dragging an item and scrolling is buggy (I think this is the reason for disabling virtualization?)

I know that I can add my own logic which I already do (I use adorner instead of transforms) but maybe others will face the same issue also, that is why I decided to open this ticket here. If I find a good solution I’ll open a PR for this.

Happy coding Tim

[wieslawsoltes]

@timunie Thanks for repro. I am working on fix and other improvements here https://github.com/wieslawsoltes/AvaloniaBehaviors/pull/85

[timunie]

I think it requires handling item container generator event (materialize, dematerialize etc.) and handle new containers.

Yes, maybe that is a good idea to do. I have to learn more about this

I think the scrolling needs to be handled also in some way as the RenderTransform needs to be adjusted accordingly.