<Trigger Property="Expander.IsExpanded" Value="True">
<Trigger.EnterActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation From="0" To="200" Duration="0:0:0.2" Storyboard.TargetProperty="Content.Height" />
</Storyboard>
</BeginStoryboard>
</Trigger.EnterActions>
</Trigger>
<Trigger Property="Expander.IsExpanded" Value="False">
<Trigger.ExitActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation From="200" To="0" Duration="0:0:0.2" Storyboard.TargetProperty="Content.Height" />
</Storyboard>
</BeginStoryboard>
</Trigger.ExitActions>
</Trigger>
<Trigger Property="Expander.IsExpanded" Value="True">
<Trigger.EnterActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation From="0" Duration="0:0:0.2" Storyboard.TargetProperty="Content.Height" />
</Storyboard>
</BeginStoryboard>
</Trigger.EnterActions>
</Trigger>
<Trigger Property="Expander.IsExpanded" Value="False">
<Trigger.EnterActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation To="0" Duration="0:0:0.2" Storyboard.TargetProperty="Content.Height" />
</Storyboard>
</BeginStoryboard>
</Trigger.EnterActions>
</Trigger>
<Trigger Property="Expander.IsExpanded" Value="True">
<Trigger.EnterActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation From="0" Duration="0:0:0.2" Storyboard.TargetProperty="Content.Height" />
</Storyboard>
</BeginStoryboard>
</Trigger.EnterActions>
</Trigger>
<Trigger Property="Expander.IsExpanded" Value="False">
<Trigger.ExitActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation To="0" Duration="0:0:0.2" Storyboard.TargetProperty="Content.Height" />
</Storyboard>
</BeginStoryboard>
</Trigger.ExitActions>
</Trigger>
<DoubleAnimation From="0" Duration="0:0:0.2" Storyboard.TargetProperty="Content.Height" />
<Trigger.ExitActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation To="0" Duration="0:0:0.2" Storyboard.TargetProperty="Content.Height" />
</Storyboard>
</BeginStoryboard>
</Trigger.ExitActions>
вариант «Reveal» Content