ItemsControl / ListBox Issues Roll-up

[robloo]

Describe the bug

This is a Roll-up of Issues

Over the months the new ListBox and ItemsControl with many virtualization improvements has been put to the test. There are still some open issues with this group of controls after the rewrite that I want to summarize here. These issues are severe enough in some cases to prevent usage in production apps and also did not exist in the 0.10 version which performed almost flawlessly in general use (without virtualization).

• Virtualization is broken and items are not positioning correctly. It is believed some of these issues visible in the ItemsControl come from the VirtualizingStackPanel itself.
  • VirtualizingStackPanel render only first item #12432 removed as it relates more to render transform usage
  • ListBox/VirtualizingStackPanel places several items at the same "line" #12744
  • Rendering artifacts in ListView #13607 (comment)
  • Closed in PR Fix overlapping items in VirtualizingStackPanel #13765 👍
• n-th child selector does not update correctly when changing bound collections or switching items source or even scrolling in the worse-case
  • :nth-child(even) stops working in ListBox after scrolling #12381
  • Closed in PR Fix nth-child styles on virtualized lists #13770
• ObservableCollection changes are not picked up and processed correctly. It seems to corrupt the state in some cases if the bound collection is cleared/reset OR items are added/remove.
  • Unable to repro this in a simple test case. It also may be more related to switching ItemsSource or virtualization problems which are both listed separately.
  • May be related to ListBox's selection bug after scrolling (VirtualizingStackPanel) #11666 as well which has a fix
• Changes in an item view model that raise property changed events will not always update the UI (within an ObservableCollection bound to a ListBox for example)
  • If a child item type implements INotifyPropertyChanged and is within an ObservableCollection, any changes to the child are NOT applied to the ItemsControl. This is a problem if the app is recycling it's own content and re-using existing item instances within an existing ObservableCollection and just applying new property values.
• Switching ItemsSource does not always reset state without setting to null first
  • ListBox/VirtualizingStackPanel reverses order of containers when ItemsSource binding changes, breaking Tab navigation #11846
• Selection / Focus has some broken/missing edge cases
  • ListBox's selection bug after scrolling (VirtualizingStackPanel) #11666
  • Tabbing into a ListBox should always focus the selected item #11878
  • SelectionMode.AlwaysSelected isn't properly enforced when DataContext changes and SelectedItem isn't bound. #12733
  • ListBox.SelectAll sometimes fails to select items when SelectedItem is bound #13676

Other

• ListView doesn't update the ItemTemplate values using a IDataTemplate #11531
• ListBox ArgumentOutOfRangeException #12185

Expected behavior

Everything should work as well as WPF and 0.10.

Additional context

This is a rough issue for now. There are many different issues and this is a first attempt to start bringing them all together and make sense of it. Expect this issue to change quite a bit as things are narrowed down and better understood. As I find better bug reports those will be linked in as well.

See individual issues and discussions for more details.

[grokys]

Thanks for putting this together @robloo ! I hope to start looking into this issues next week.

[robloo]

@grokys I'm glad I can help a little. Great work with these controls!

[grokys]

ObservableCollection changes are not picked up and processed correctly. It seems to corrupt the state in some cases if the bound collection is cleared/reset OR items are added/remove.

Do we have a simple repro for this anywhere?

[robloo]

ObservableCollection changes are not picked up and processed correctly. It seems to corrupt the state in some cases if the bound collection is cleared/reset OR items are added/remove.

Do we have a simple repro for this anywhere?

Not that I've seen in the bug reports. This is just a general impression I've had and I've seen it with internal apps as well. I'll spend some time to try and find an exact repro.

[robloo]

I'm attempting to isolate some repro for these narrowed cases:

1. ItemsSource changing references in the view model (i.e. set a new ObservableCollection to the property bound to ItemsSource)

   There have been a few different reports of issues here and I've seen some cases myself. However, I can't repro it in a simple test case. There is still an open bug report for somethin related though: ListBox/VirtualizingStackPanel reverses order of containers when ItemsSource binding changes, breaking Tab navigation #11846

2. Adding/Removing items from an existing ObservableCollection isn't displaying correctly in the UI.

   This one I'm not 100% sure is a stand-alone issue. It could be related to older issues and the VirtualizaingStackPanel fixes already made. Because I'm not sure this is a real problem I'm going to ignore it without further information. Also note I cannot repro it in a simple test case.

3. Changing existing view models within an existing observable collection (recycling).

   I've seen issues with this in a real app and there are some other bug reports and comments I've seen to indicate it's a problem. For example it might be related to this somehow: Combobox selectedValue in datagrid is changed when scrolling datagrid #13751. However, I also cannot repro this in a simple case. I will attempt to isolate it more in the actual app.

   Also note that UWP/WinUI marshalled INotifyPropertyChanged automatically to the UI thread in the case of bindings and things. I don't think Avalonia does that. This MAY be a contributor as code I'm working with was ported so this type of thing can be hard to find after the fact. I'm attempting to confirm all UI updating code paths are getting run on the UI thread.

   A takeaway here is if Avalonia does not automaticaly marshel INotifyPropertyChanged to the UI thread for binding perhaps it should and that could be done with the binding refactoring.

Bottom line, don't waste your time on the points I can't confirm. Those were general categories of issues I thought there were problems but it needs to be understood more.

[timunie]

To add: #13751

[timunie]

Yet probably another one: #13875

[maxkatz6]

#14389 #14365

[MrJul]

Another one: #14718

[timunie]

adding #14814

[timunie]

yet another: #15194

@robloo we may want to update the check-list the other day to reflect current status better.