Accessibility MenuItem PositionInSet is not getting correctly set in certain situations

[anramanath]

• .NET Core Version: 3.1
• Does the bug reproduce also in WPF for .NET Framework 4.8?: Unknown
• Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? No
• Security issues and bugs should be reported privately, learn more via our responsible disclosure guidelines.

Problem description:

When populating the subMenuItems in the code-behind, narrator announces the positionInSet / SizeOfSet. This doesn't happen when using the ItemsSource and (optionally) modifying the ContainerStyle in .xaml code.

If you have a menuItem with an itemSource (optionally with a container style), the positionIsSet automation property does not get set correctly and narrator subsequently does not read positionInSet / sizeOfSet. positionInSet does seem to work when each menuItem is added using an event handler for subMenuOpened.

Actual behavior:

Windows narrator does not announce "positionInSet of sizeOfSet" when menuItems are set through .xaml file ItemsSource. Only works when set in codebehind.

Expected behavior:

positionInSet should always be set no matter how the menuItem is instantiated. Windows narrator should always announce "positionInSet of sizeOfSet".

Minimal repro:

This does not announce positionInSet / sizeOfSet

<MenuItem Header="Open"
                    AutomationProperties.AutomationId="AID_Open"
                    ItemsSource="{Binding SomeSourceCollection, Mode=OneWay}">
                            <MenuItem.ItemContainerStyle>
                                <Style TargetType="{x:Type MenuItem}" BasedOn="{StaticResource {x:Type MenuItem}}">
                                    <Setter Property="MenuItem.Header" Value="{Binding File, Mode=OneTime}" />
                                </Style>
                            </MenuItem.ItemContainerStyle>
                        </MenuItem>

vs.

When populating in the code-behind, the positionInSet is properly set and announced.

.xaml file

<MenuItem x:Name="openMenuItem" 
                                  Header="Open"
                                  AutomationProperties.AutomationId="AID_Open"/>

.xaml.cs file (codebehind)

this.openMenuItem.ItemsSource = SomeSourceCollection
                .Select((menuItemHeader, index) =>
                {
                    var subMenuItem = new MenuItem()
                    {
                        Header = menuItemHeader,
                    };

                    return subMenuItem ;
                });

[mslukebo]

I did some digging in WPF's code to figure out why this is happening. Here's what I found:

Inside MenuItemAutomationPeer.GetPositionInSetCore, when the base call returns -1 the method goes on to call ItemAutomationPeer.GetPositionInSetFromItemsControl(parent, owner). The value returned by the original method will be less than or equal to the value this call returns.

However, ItemAutomationPeer.GetPositionInSetFromItemsControl(parent, owner) will return -1 when Owner.ItemsSource is anything except a collection of MenuItems. The GetPositionInSetFromItemsControl(parent, owner)call looks for owner inside parent.Items. Items will just be the collection ItemsSource is bound to, not the collection of MenuItems that the items get placed inside of in the UI. For this to work correctly, the peer would need to be searching for owner.DataContext in the items.

A potential workaround to this problem is ensuring that you only bind a collection of MenuItems to MenuItem.ItemSource. This is odd, since you don't need to do this for, say, a ListView or ListBox.