GridSplitter does not work when IsItemsHost is true

• .NET Core Version: 5.0
• Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

GridSplitter does not work, when it is added programmatically to ItemsControl.Items that uses a Grid with IsItemsHost="true"

Reason: GridSplitter.InitializeData checks if Parent is a Grid. In this case the LogicalParent is the ItemsControl. Shouldn’t it check the VisualParent? repro.zip

[miloush]

@IAmTheCShark yes sorry I meant the WPF is doing extra work. The purpose of Panel.IsItemsHost is to prevent the panel from having logical children. Similar with ContentControl.ContentIsNotLogical which is used with items. Your PR would make some of the items sometimes see the panel and that worries me. Maybe an opt-in (or associating GridSplitter with a custom Grid) would do. Maybe my counterexample is artificial and I am overly cautious, while everyone would expect the behavior you are expecting and GridSplitter in fact should have checked visual parent. You can also create your own GridSplitter that behaves as suggested, although sadly this aspect is not easily overridable.

I have used Grid as items panel many times, however, never with a GridSplitter. That’s why I was enquiring about the scenario.

[miloush]

The inconsistency is most visible when the items control overrides IsItemsIsOwnContainer. For example, while GridSplitter in pure ItemsControl would work, it wouldn’t work in ListBox as its visual parent would be ContentPresenter. I am getting more and more convinced myself this is not a good idea.