https://docs.avaloniaui.net/docs/data-binding/binding-validation#default-behavior
The description is wrong.
What I see in action by default:
- It's a yellow border in dark mode
- There is no tooltip
- It's not a tooltip, it's a plain text displayed under the control, and I haven't find a way to remove the text and only show the border when error happens.
The Property code:
[ObservableProperty]
[NotifyDataErrorInfo]
[Required]
public partial CurrencyModel? RightCurrency { get; set; }
The Control code:
<ComboBox
ItemsSource="{Binding SomeValidSource}"
SelectedItem="{Binding RightCurrency}"
ItemTemplate="{StaticResource CurrencyComboBoxTemplate}" />

https://docs.avaloniaui.net/docs/data-binding/binding-validation#default-behavior
The description is wrong.
What I see in action by default:
The Property code:
The Control code: