ListBox does not update its layout when put inside a container with RenderTransform transition.

[LilyStilson]

I have a custom expander style that has animation on expansion of its content. The animation is done using <TransformOperationsTransition /> on the parent container of <ContentPresenter />. Height is not being changed at any point, I've used Avalonia.Simple theme's Expander as a base for this one.

I've recorded a video that demonstrates this behavior. I want to make sure whether it is indented or not and is there a way to fix it?
Without that expansion transition, there's no issue. The video also shows that all other elements work perfectly fine with the transition.

Screen.Recording.2024-12-03.at.0.00.58.mp4

The code for the expander style is here
And the Main Window's ListBox is here

[StefanKoell]

I've seen a similar issue. If I'm not mistaken the ListBox is using a virtualized stack panel by default. I was able to workaround the issue by switching to a normal stack panel. Would be interesting if this is also the case for you.

[LilyStilson]

I don't think you can bind to <StackPanel />, you can to <ItemsRepeater /> though. I've tried the latter option - same issue.

Screen.Recording.2024-12-03.at.12.36.34.mp4

[StefanKoell]

This is what I did in my ListBox:

            <ListBox.ItemsPanel>
                <ItemsPanelTemplate>
                    <StackPanel />
                </ItemsPanelTemplate>
            </ListBox.ItemsPanel>

Also, what I noticed myself: ItemsRepeater is a weird beast which does weird stuff. When I don't use a ListBox, I usually use an ItemsControl instead.

[LilyStilson]

Sorry, I misunderstood what you meant here, I didn't know you could change the underlying panel in ListBox x)
After changing it to <StackPanel /> the issue went away, thanks :D

[StefanKoell]

So it seems that the virtualized panel which is used by default has some issues. Maybe it's worth opening an issue with a small repro project if you want.