The behavior is unexpected as you are animating a DynamicResource brush that it is probably shared and animated in other places.

Anyway I tried this Style in Kaxaml and it complains about trying to animate an immutable object.

Besides inherited properties doesn't transfer its values beyond the ControlTemplate, so if you want to modify the Foreground property of a Control you should do it using a Setter that targets the control itself:

<ControlTemplate TargetType="TextBox">

...

<ControlTemplate.Triggers>

<Trigger Property="IsReadOnly" Value="True">

<Setter Property="Foreground" Value="{DynamicResource ControlReadOnlyBrush}"/>

</Trigger>

<ControlTemplate.Triggers>

</ControlTemplate>

Ah, of course. I forgot that it was a shared resource. I had the same problem with the border and background too until I instantiated a new SolidColorBrush from a color.

I'll try to change it, thanks! :)

Anyway, Sergio, shouldn't we be giving the same error as KaXaml/WPF?

Yes, this is something we have to fix.