The ViewBox control provides two properties to stretch the content. Those are Stretch and StretchDirection. When you don't specify the Stretch property to a ViewBox, it uses the default value for Stretch, which is Uniform.
When the Stretch property is set to Uniform, and the ViewBox does not match the aspect ratio of the content, it adds a white margin to it. It can be either at the top and bottom or at the left and right sides:
<Viewbox Stretch="Uniform">
<TextBlock Text="This is a text, inside a ViewBox"
Margin="10"/>
</Viewbox>
When it is set to Fill, it causes the content to completely fill the space without obeying the aspect ratio. Thus, you may see a distortion in the UI:
<Viewbox Stretch="Fill">
<TextBlock Text="This is a text, inside a ViewBox"
Margin="10"/>
</Viewbox>
When you set the Stretch property to UniformToFill, it maintains...