Я решил эту проблему, отредактировав копию ControlTemplate для элемента управления IntegerUpDown и добавив свойство isReadOnly в WaterMarkTextBox.
<ControlTemplate x:Key="ControlControlTemplate1" TargetType="{x:Type Control}">
<xctk:ButtonSpinner x:Name="PART_Spinner" AllowSpin="{Binding AllowSpin, RelativeSource={RelativeSource TemplatedParent}}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" ButtonSpinnerLocation="{Binding ButtonSpinnerLocation, RelativeSource={RelativeSource TemplatedParent}}" Background="{TemplateBinding Background}" HorizontalContentAlignment="Stretch" IsTabStop="False" ShowButtonSpinner="{Binding ShowButtonSpinner, RelativeSource={RelativeSource TemplatedParent}}" VerticalContentAlignment="Stretch">
<xctk:WatermarkTextBox IsReadOnly="True" x:Name="PART_TextBox" AutoMoveFocus="{Binding AutoMoveFocus, RelativeSource={RelativeSource TemplatedParent}}" AutoSelectBehavior="{Binding AutoSelectBehavior, RelativeSource={RelativeSource TemplatedParent}}" AcceptsReturn="False" BorderThickness="0" Background="Transparent" ContextMenu="{TemplateBinding ContextMenu}" Foreground="{TemplateBinding Foreground}" FontWeight="{TemplateBinding FontWeight}" FontStyle="{TemplateBinding FontStyle}" FontStretch="{TemplateBinding FontStretch}" FontSize="{TemplateBinding FontSize}" FontFamily="{TemplateBinding FontFamily}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" IsTabStop="True" IsUndoEnabled="True" MinWidth="20" MaxLength="{Binding MaxLength, RelativeSource={RelativeSource TemplatedParent}}" Padding="{TemplateBinding Padding}" TextAlignment="{Binding TextAlignment, RelativeSource={RelativeSource TemplatedParent}}" TextWrapping="NoWrap" TabIndex="{TemplateBinding TabIndex}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}" WatermarkTemplate="{Binding WatermarkTemplate, RelativeSource={RelativeSource TemplatedParent}}" Watermark="{Binding Watermark, RelativeSource={RelativeSource TemplatedParent}}"/>
</xctk:ButtonSpinner>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="BorderBrush" Value="{DynamicResource {ComponentResourceKey ResourceId=ControlMouseOverBorderKey, TypeInTargetAssembly={x:Type Themes:ResourceKeys}}}"/>
</Trigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding IsReadOnly, RelativeSource={RelativeSource Self}}" Value="False"/>
<Condition Binding="{Binding AllowTextInput, RelativeSource={RelativeSource Self}}" Value="False"/>
</MultiDataTrigger.Conditions>
<Setter Property="IsReadOnly" TargetName="PART_TextBox" Value="True"/>
</MultiDataTrigger>
<DataTrigger Binding="{Binding IsReadOnly, RelativeSource={RelativeSource Self}}" Value="True">
<Setter Property="IsReadOnly" TargetName="PART_TextBox" Value="True"/>
</DataTrigger>
<Trigger Property="IsKeyboardFocusWithin" Value="True">
<Setter Property="BorderBrush" Value="{DynamicResource {ComponentResourceKey ResourceId=ControlSelectedBorderKey, TypeInTargetAssembly={x:Type Themes:ResourceKeys}}}"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
</Trigger>
<Trigger Property="IsFocused" Value="True">
<Setter Property="FocusManager.FocusedElement" TargetName="PART_TextBox" Value="{Binding ElementName=PART_TextBox}"/>
<Setter TargetName="PART_TextBox" Property="Background" Value="{StaticResource LightGreyBrush}" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
Чтобы отредактировать копию шаблона, щелкните правой кнопкой мыши элемент управления, затем отредактируйте шаблон, отредактируйте копию или текущую