<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<Page.Resources>
<Style x:Key="widthOnFocus">
<Style.Triggers>
<Trigger Property="FrameworkElement.IsFocused" Value="True">
<Trigger.EnterActions>
<BeginStoryboard Name="changeWidth">
<Storyboard>
<DoubleAnimation Storyboard.TargetProperty="(FrameworkElement.Width)" To="300" Duration="0:0:5" />
</Storyboard>
</BeginStoryboard>
</Trigger.EnterActions>
<Trigger.ExitActions>
<RemoveStoryboard BeginStoryboardName="changeWidth" />
</Trigger.ExitActions>
</Trigger>
</Style.Triggers>
</Style>
</Page.Resources>
<StackPanel>
<TextBox Style="{StaticResource widthOnFocus}" Width="200" />
</StackPanel>
</Page>
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