<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>