ScrollBar of ListBox is flickered when scrolling with various item sizes

[hamster620]

Describe the bug ScrollBar of ListBox is flickered when scrolling if items of ListBox have various sizes (height).

To Reproduce

1. Add a ListBox.
2. Set DataTemplate/ItemTemplate to generate items with various sizes
3. Scroll the ListBox.

Expected behavior ScrollBar should be smooth/stable when scrolling.

Screenshots

https://github.com/AvaloniaUI/Avalonia/assets/26860570/40abab8a-237b-42d6-9110-fd5edfeeddba

Desktop (please complete the following information):

• OS: macOS 13.4.1
• Version 11.0.0-rc2.1

[hez2010]

I think it is somwhat expected because we can only estimate the position of scrollbar. Otherwise we have to give up on virtualization and instead materialize all items in the list, which will lead to unacceptable performance.

[hamster620]

I noticed that starting from 11.0.0-rc1.1 the scrolling of ListBox is no longer aligned to top of first visible item which may causes the symptom I described. In our use case the item size varies according to number of lines of text of each item, so current scrolling behavior is unacceptable in this case.

In our app it is fine to elimate smooth scrolling of ListBox in order to keep thumb of scrollbar stable when scrolling. But I cannot find way to change behavior of VirtualizationStackPanel to disable smooth scrolling.

[robloo]

In our app it is fine to elimate smooth scrolling of ListBox in order to keep thumb of scrollbar stable when scrolling. But I cannot find way to change behavior of VirtualizationStackPanel to disable smooth scrolling.

You might have to switch the ItemsPanel from VirtualizationStackPanel to just StackPanel and disable virtualization entirely. This is only possible if you don't have so many items.