WPF TabControl selected tab out-of-sync after validation

_This issue has been moved from [a ticket on Developer Community](https://developercommunity.visualstudio.com/t/WPF-TabControl-selected-tab-out-of-sync-/10950927)._

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[severity:It's more difficult to complete my work]
When using a WPF TabControl, in the event that the bound model reverts/does-not-accept the selection of a new tab, the selected header of the TabControl becomes out-of-sync. i.e. the model matches the `TabControl.SelectedItem`, but the `TabControl.SelectedIndex` is out-of-sync (left at the last clicked-on tab header).

This [minimum reproduceable sample](https://aka.ms/dc/file?name=B86d10911c03f42bf85b41c659a58d4f2638906911780433697_TabControlBug.zip&tid=86d10911c03f42bf85b41c659a58d4f2638906911780433697) demonstrates the issue we are seeing.
For context, below is a screenshot from the attached sample application (that targets net8.0-windows).
It binds the `TabControl.SelectedItem` property to the `MainWindowViewModel.SelectedTab` property using a TwoWay binding. When the 'Deny tab change' checkbox is checked (default), `MainWindowViewModel.SelectedTab` does not update its backing field in order to represent the model validating the selection and choosing to prevent it.

Steps to reproduce the TabControl's `SelectedItem` and `SelectedIndex` properties becoming out-of-sync:
1) starting the applicaiton, the first tab (tab 'A') is selected.
2) With the 'Deny tab change' checkbox checked, click on tab 'B'.
3) The `MainWindowViewModel.SelectedTab` does not change its value from 'A', but still raises a property changed event.
4) The `SelectedItem` and `SelectedIndex` properties of the TabControl become mis-matched, 'A' and 1 respectively. I.e. it shows the 'Content of A' in the content area of the control (which is expected), but the visibly selected tab header is 'B' (unexpected)

A similar out-of-sync issue is seen if TwoWay binding the view model to the `SelectedIndex` instead.

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### Original Comments

#### Feedback Bot on 14/8/2025, 09:16 AM:

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