Virtualization goes awry during scrolling

[tvass83]

Describe the bug
Scrolling through the items of a ListBox with virtualization on freezes the UI.

To Reproduce
Check the following simple repro app:
https://github.com/tvass83/AvaloniaIssues/tree/main/VirtualizationGoesAwry
OR

<ListBox HorizontalAlignment="Center"
         VerticalAlignment="Center"
         Height="60">
    <ListBoxItem Content="AAA" />
    <ListBoxItem Content="BBB" />
    <ListBoxItem Content="CCC" />
</ListBox>

1. Use the vertical scrollbar of the ListBox or use the keyboard to navigate through items

Expected behavior
The UI shall not freeze.

Desktop (please complete the following information):

• OS: Windows 10 version 20H2 (OS build 10.0.19042.804)
• Avalonia version: observed in 0.10.6.17, but still reproducible in 0.10.14

Additional context
It's not about the UI thread waiting for something to complete, but instead some kind of infinite loop where the hot path is the following:

Avalonia.Base.dll!Avalonia.Utilities.AvaloniaPropertyValueStore`1[System.__Canon]::TryFindEntry 0x0
Avalonia.Base.dll!Avalonia.Utilities.AvaloniaPropertyValueStore`1[System.__Canon]::TryGetValue 0x0
Avalonia.Base.dll!Avalonia.ValueStore::TryGetValue 0x0
Avalonia.Base.dll!Avalonia.ValueStore::TryGetValue 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::GetValueOrInheritedOrDefault 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::GetInheritedOrDefault 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::InheritanceParentChanged 0x0
Avalonia.Base.dll!Avalonia.StyledPropertyBase`1[System.__Canon]::RouteInheritanceParentChanged 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::set_InheritanceParent 0x0
Avalonia.Styling.dll!Avalonia.StyledElement::Avalonia.Controls.ISetInheritanceParent.SetParent 0x0
Avalonia.Controls.dll!Avalonia.Controls.Presenters.ContentPresenter::UpdateChild 0x0
Avalonia.Controls.dll!Avalonia.Controls.Presenters.ContentPresenter::ContentChanged 0x0
Avalonia.Controls.dll!Avalonia.Controls.Presenters.ContentPresenter+<>c::<.cctor>b__17_0 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObjectExtensions+<>c__DisplayClass23_0`1[System.__Canon]::<AddClassHandler>b__0 0x0
System.Reactive.dll!System.Reactive.Subjects.Subject`1[System.__Canon]::OnNext 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::RaisePropertyChanged 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::Avalonia.PropertyStore.IValueSink.ValueChanged 0x0
Avalonia.Base.dll!Avalonia.ValueStore::Avalonia.PropertyStore.IValueSink.ValueChanged 0x0
Avalonia.Base.dll!Avalonia.PropertyStore.BindingEntry`1[System.__Canon]::UpdateValue 0x0
Avalonia.Base.dll!Avalonia.PropertyStore.BindingEntry`1[System.__Canon]::OnNext 0x0
Avalonia.Base.dll!Avalonia.Reactive.BindingValueAdapter`1[System.__Canon]::OnNext 0x0
System.Private.CoreLib.dll!dynamicClass::IL_STUB_MulticastDelegate_Invoke 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::RaisePropertyChanged 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::Avalonia.PropertyStore.IValueSink.ValueChanged 0x0
Avalonia.Base.dll!Avalonia.ValueStore::NotifyValueChanged 0x0
Avalonia.Base.dll!Avalonia.ValueStore::SetExisting 0x0
Avalonia.Base.dll!Avalonia.ValueStore::SetValue 0x0
Avalonia.Base.dll!dynamicClass::IL_STUB_InstantiatingStub 0x0
Avalonia.Base.dll!Avalonia.StyledPropertyBase`1[System.__Canon]::RouteSetValue 0x0
Avalonia.Controls.dll!Avalonia.Controls.Generators.ItemContainerGenerator`1[System.__Canon]::TryRecycle 0x0
Avalonia.Controls.dll!Avalonia.Controls.Presenters.ItemVirtualizerSimple::RecycleContainersForMove 0x0
Avalonia.Controls.dll!Avalonia.Controls.Presenters.ItemVirtualizerSimple::set_OffsetValue 0x0
Avalonia.Controls.dll!Avalonia.Controls.Presenters.ItemVirtualizer::set_Offset 0x0
Avalonia.Controls.dll!Avalonia.Controls.Presenters.ItemsPresenter::Avalonia.Controls.Primitives.IScrollable.set_Offset 0x0
Avalonia.Controls.dll!Avalonia.Controls.Presenters.ScrollContentPresenter+<>c__DisplayClass50_0::<UpdateScrollableSubscription>b__2 0x0
System.Reactive.dll!System.Reactive.AnonymousSafeObserver`1[Avalonia.Vector]::OnNext 0x0
System.Reactive.dll!System.Reactive.Sink`1[Avalonia.Vector]::ForwardOnNext 0x0
System.Reactive.dll!System.Reactive.Linq.ObservableImpl.Skip`1+Count+_[Avalonia.Vector]::OnNext 0x0
Avalonia.Base.dll!Avalonia.Reactive.LightweightObservableBase`1[Avalonia.Vector]::PublishNext 0x0
Avalonia.Base.dll!Avalonia.Reactive.AvaloniaPropertyObservable`1[Avalonia.Vector]::PropertyChanged 0x0
System.Private.CoreLib.dll!dynamicClass::IL_STUB_MulticastDelegate_Invoke 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::RaisePropertyChanged 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::SetAndRaise 0x0
Avalonia.Controls.dll!Avalonia.Controls.Presenters.ScrollContentPresenter::set_Offset 0x0
Avalonia.Controls.dll!Avalonia.Controls.Presenters.ScrollContentPresenter+<>c::<.cctor>b__19_8 0x0
Avalonia.Base.dll!Avalonia.DirectProperty`2[System.__Canon,Avalonia.Vector]::InvokeSetter 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::SetDirectValueUnchecked 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject+DirectBindingSubscription`1[Avalonia.Vector]::OnNext 0x0
Avalonia.Base.dll!Avalonia.Reactive.SingleSubscriberObservableBase`1[Avalonia.Data.BindingValue`1[Avalonia.Vector]]::PublishNext 0x0
Avalonia.Base.dll!Avalonia.Reactive.TypedBindingAdapter`1[Avalonia.Vector]::OnNext 0x0
Avalonia.Base.dll!Avalonia.Reactive.BindingValueAdapter`1[System.__Canon]::OnNext 0x0
System.Private.CoreLib.dll!dynamicClass::IL_STUB_MulticastDelegate_Invoke 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::RaisePropertyChanged 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::SetAndRaise 0x0
Avalonia.Controls.dll!Avalonia.Controls.ScrollViewer::set_Offset 0x0
Avalonia.Controls.dll!Avalonia.Controls.ScrollViewer::set_VerticalScrollBarValue 0x0
Avalonia.Controls.dll!Avalonia.Controls.ScrollViewer+<>c::<.cctor>b__31_18 0x0
Avalonia.Base.dll!Avalonia.DirectProperty`2[System.__Canon,System.Double]::InvokeSetter 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::SetDirectValueUnchecked 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::SetValue 0x0
Avalonia.Base.dll!Avalonia.DirectPropertyBase`1[System.Double]::RouteSetValue 0x0
Avalonia.Markup.dll!Avalonia.Data.TemplateBinding::System.IObserver<System.Object>.OnNext 0x0
Avalonia.Base.dll!Avalonia.Reactive.LightweightObservableBase`1[System.__Canon]::PublishNext 0x0
System.Private.CoreLib.dll!dynamicClass::IL_STUB_MulticastDelegate_Invoke 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::RaisePropertyChanged 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::SetAndRaise 0x0
Avalonia.Controls.dll!Avalonia.Controls.Primitives.RangeBase::set_Value 0x0
Avalonia.Base.dll!Avalonia.DirectProperty`2[System.__Canon,System.Double]::InvokeSetter 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::SetDirectValueUnchecked 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::SetValue 0x0
Avalonia.Base.dll!Avalonia.DirectPropertyBase`1[System.Double]::RouteSetValue 0x0
Avalonia.Markup.dll!Avalonia.Data.TemplateBinding::System.IObserver<System.Object>.OnNext 0x0
Avalonia.Base.dll!Avalonia.Reactive.LightweightObservableBase`1[System.__Canon]::PublishNext 0x0
System.Private.CoreLib.dll!dynamicClass::IL_STUB_MulticastDelegate_Invoke 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::RaisePropertyChanged 0x0
Avalonia.Base.dll!Avalonia.AvaloniaObject::SetAndRaise 0x0
Avalonia.Controls.dll!Avalonia.Controls.Primitives.Track::set_Value 0x0
Avalonia.Controls.dll!Avalonia.Controls.Primitives.Track::ThumbDragged 0x0
Avalonia.Interactivity.dll!Avalonia.Interactivity.Interactive::<AddHandler>g__InvokeAdapter|4_0 0x0
Avalonia.Interactivity.dll!Avalonia.Interactivity.Interactive+<>c__4`1[System.__Canon]::<AddHandler>b__4_1 0x0
Avalonia.Interactivity.dll!Avalonia.Interactivity.EventRoute::RaiseEventImpl 0x0
Avalonia.Interactivity.dll!Avalonia.Interactivity.EventRoute::RaiseEvent 0x0
Avalonia.Interactivity.dll!Avalonia.Interactivity.Interactive::RaiseEvent 0x0
Avalonia.Controls.dll!Avalonia.Controls.Primitives.Thumb::OnPointerMoved 0x0
System.Reactive.dll!System.Reactive.Subjects.Subject`1[System.ValueTuple`2[System.__Canon,System.__Canon]]::OnNext 0x0
Avalonia.Interactivity.dll!Avalonia.Interactivity.EventRoute::RaiseEventImpl 0x0
Avalonia.Interactivity.dll!Avalonia.Interactivity.EventRoute::RaiseEvent 0x0
Avalonia.Interactivity.dll!Avalonia.Interactivity.Interactive::RaiseEvent 0x0
Avalonia.Input.dll!Avalonia.Input.MouseDevice::MouseMove 0x0
Avalonia.Input.dll!Avalonia.Input.MouseDevice::ProcessRawEvent 0x0
Avalonia.Input.dll!Avalonia.Input.InputManager::ProcessInput 0x0
Avalonia.Win32.dll!Avalonia.Win32.WindowImpl::AppWndProc 0x0
Avalonia.Win32.dll!Avalonia.Win32.WindowImpl::WndProc 0x0
Avalonia.Win32.dll!dynamicClass::IL_STUB_ReversePInvoke 0x0
user32.dll!UserCallWinProcCheckWow
user32.dll!DispatchMessageWorker
Avalonia.Win32.dll!dynamicClass::IL_STUB_PInvoke 0x0
Avalonia.Win32.dll!Avalonia.Win32.Win32Platform::RunLoop 0x0
Avalonia.Base.dll!Avalonia.Threading.Dispatcher::MainLoop 0x0
Avalonia.Controls.dll!Avalonia.Controls.ApplicationLifetimes.ClassicDesktopStyleApplicationLifetime::Start 0x0
Avalonia.Controls.dll!Avalonia.ClassicDesktopStyleApplicationLifetimeExtensions::StartWithClassicDesktopLifetime 0x0
VirtualizationGoesAwry.dll!VirtualizationGoesAwry.Program::Main 0x0

[maxkatz6]

Hi! I haven't yet checked the source nor tried to reproduce it. Will try to do it later this week.

But I want to say, that there is no "virtualization" when you use controls source. I.e. when all your items are controls and not models. There is nothing to recycle this way, all items are materialized already, so virtualization is impossible.

[tvass83]

@maxkatz6 That sounds pretty weird for two reasons:

1. If I set VirtualizationMode=None on the ListBox then I can't repro anymore
2. I see the usage of ItemVirtualizerSimple in the callstack

In any case, please give it a try and thanks for picking this up, I really appreciate it!

[maxkatz6]

Yeah, it seems like a duplicate of this old ticket - #2936

[maxkatz6]

In general, recommendation to use data bindings so recycling can work properly. Or if there is small number of items and it's just easier to define all items in XAML, then you need to disable virtualization, which is broken, when it's not possible to recycle items.