Expander is not using all available space after expanding inside Grid with ScrollViewer outside

[DrCQ]

Bug explanation

I have 3 Expander controls inside the Grid surrounded by the ScrollViewer. The intention is that the third Expander should - in the open state - always use all available space. To explain the problem following XAML is used:

<Grid>
  <ScrollViewer HorizontalScrollBarVisibility="Disabled" VerticalScrollBarVisibility="Auto">
    <Grid >
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Expander Grid.Row="0" Header="Fix hight 200" Margin="5">
            <Grid Height="200" Background="Yellow"/>
        </Expander>
        <Expander Grid.Row="1" Header="Fix hight 100" Margin="5">
            <Grid Height="100" Background="Green"/>
        </Expander>
        <Expander Grid.Row="2" Header="Maximal hight" Margin="5" Style="{x:Null}">
            <Grid Background="Blue" MinHeight="400"/>
        </Expander>
    </Grid>
  </ScrollViewer>
</Grid>

Setting the Expander Style to Null shows the proper behavior. Using the MaterialDesign default Styleshows, that the Expander is growing to the MinHeight value of the blue Grid inside only!

Window with Height set to 750 perfectly shows the expected behavior between ScrollViewer and Expanders.

Version

4.9.0

[nicolaihenriksen]

@DrCQ I believe this a simple matter of the VerticalContentAlignment of the Expander defaults to Top in MDIX, rather than Stretch which is the default value in the default style. If you explicitly set VerticalContentAlignment=Stretch on your Expander, I believe you get the expected behavior.

I have added a PR to change the default value because I think it is better to follow the default templates unless there is a specific reason not to; which I don't believe there is in this case.