WPF: How to make HeaderedContentControl.Content fit height?

Problem here is HeaderedContentControl uses StackPanel internally to layout header and content.

To fix that, use a Grid instead or re-template HeaderedContentControl to use Grid.

Example:

<ControlTemplate TargetType="HeaderedContentControl">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>
        <ContentControl Content="{TemplateBinding Header}" Grid.Row="0" />
        <ContentControl Content="{TemplateBinding Content}" Grid.Row="1" />
    </Grid>
</ControlTemplate>