Avalonia/samples/ControlCatalog/Pages/AutoCompleteBoxPage.xaml

<l:AutoCompleteBoxPage xmlns="https://github.com/avaloniaui"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:l="clr-namespace:ControlCatalog.Pages;assembly=ControlCatalog"
  x:Class="ControlCatalog.Pages.AutoCompleteBoxPage">
  <l:AutoCompleteBoxPage.Styles>
    <Style Selector="l|AutoCompleteBoxPage[SideBarEnabled=True] Grid#sidebar">
      <Style.Animations>
        <Animation Duration="0:0:0.5" FillMode="Both" Easing="QuadraticEaseOut">
          <KeyFrame Cue="100%">
            <Setter Property="Width" Value="250" />
          </KeyFrame>
        </Animation>
      </Style.Animations>
    </Style>
    <Style Selector="l|AutoCompleteBoxPage[SideBarEnabled=False] Grid#sidebar">
      <Style.Animations>
        <Animation Duration="0:0:0.5" FillMode="Both" Easing="QuadraticEaseIn">
  
          <KeyFrame Cue="100%">
            <Setter Property="Width" Value="0" />
          </KeyFrame>
        </Animation>
      </Style.Animations>
    </Style>
  </l:AutoCompleteBoxPage.Styles>
  <Grid RowDefinitions="80,*">
    <Button Name="sidebar_button" Grid.RowSpan="1" />
    <Grid Grid.Row="1" Grid.ColumnDefinitions="Auto,*">
      <Grid Name="sidebar" Background="Red" Width="0" Grid.ColumnSpan="1" />
    </Grid>
  </Grid>
</l:AutoCompleteBoxPage>