Content of a Button Style appears only in one Button instance

Question

I have a Viewbox:

Answer 1

Setting the x:Shared attribute of the ViewBox to false as suggested by @ASh will indeed work but why don't you just include the ViewBox in the ControlTemplate like this?

<Style x:Key="SampleStyle" TargetType="{x:Type Button}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type Button}">
                <Border Background="Transparent" >
                    <Viewbox>
                        <Grid>
                            <Ellipse Stroke="#e2e2e0" StrokeThickness="6" Fill="#d5273e" Width="128" Height="128"/>
                        </Grid>
                    </Viewbox>
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

A template is a template and a control instance is an instance.

Answer 2

I think a good approach is to use DataTemplate. So you will have

<DataTemplate x:Key="SampleViewbox">
    <Viewbox>
        <Grid>
            <Ellipse
                    Width="128"
                    Height="128"
                    Fill="#d5273e"
                    Stroke="#e2e2e0"
                    StrokeThickness="6" />
        </Grid>
    </Viewbox>
</DataTemplate>

And for style

<Style x:Key="SampleStyle" TargetType="{x:Type Button}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type Button}">
                <Border Background="Transparent">
                    <ContentPresenter ContentTemplate="{StaticResource SampleViewbox}" />
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

Answer 3

Viewbox is FrameworkElement which cannot belong to multiple parents. Every time buttons request a resource {StaticResource SampleViewbox} they get the same instance.

to change that behavior add x:Shared="False" attribute

<Viewbox x:Key="SampleViewbox" x:Shared="False">