<Grid x:Name="canvas">
    <Grid x:Name="main"/>
    <Grid x:Name="sidebar"/>
</Grid>

<Style TargetType="Grid"  x:Key="VisibleAnimation">
        <Setter Property="Visibility" Value="Collapsed"/>
        <Setter Property="RenderTransform">
            <Setter.Value>
                <TranslateTransform/>
            </Setter.Value>
        </Setter>
        <Style.Triggers>
            <Trigger Property="Visibility" Value="Visible">
                <Trigger.EnterActions>
                    <BeginStoryboard>
                        <Storyboard>
                            <DoubleAnimation Storyboard.TargetProperty="RenderTransform.(TranslateTransform.X)"
                         From="300" To="0" Duration="0:0:0.5"/>
                        </Storyboard>
                    </BeginStoryboard>
                </Trigger.EnterActions>
            </Trigger>
        </Style.Triggers>
    </Style>