Expander Animation isn't smooth when bottom aligned

[michael-hawker]

Describe the bug

The Expander doesn't animate nicely in certain layout scenarios.

Steps to Reproduce

<Page
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:controls="using:Microsoft.Toolkit.Uwp.UI.Controls"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

  <Page.Resources>
    <SolidColorBrush Color="{ThemeResource SystemChromeLowColor}" x:Key="SystemControlForegroundChromeLowBrush"/>
  </Page.Resources>

  <Grid>
        <controls:Expander x:Name="Expander" 
                           VerticalAlignment="Bottom"
                           Header="This is the header - expander 2"
                           HorizontalContentAlignment="Stretch"
                           IsExpanded="True">
          <Grid Height="256"
                Background="{ThemeResource SystemControlBackgroundBaseHighBrush}">
            <TextBlock HorizontalAlignment="Center"
                       TextWrapping="Wrap"
                       Text="This is the expanded content without a content overlay"
                       VerticalAlignment="Center"
                       Foreground="{ThemeResource SystemControlForegroundChromeLowBrush}" />
          </Grid>
        </controls:Expander>
  </Grid>
</Page>

1. Use XAML above in sample app (Expander)
2. Click on the Expander to collapse
3. Note how that it 'jumps' to the bottom after the animation has played.

Expected behavior

Expander's height should animate along with content collapsing so it smoothly collapses?

Screenshots

If applicable, add screenshots to help explain your problem.

Environment

Store App

Windows 10 Build Number:

• Fall Creators Update (16299)
• April 2018 Update (17134)
• October 2018 Update (17763)
• May 2019 Update (18362)
• May 2020 Update (19041)
• Insider Build (build number: )

Hello michael-hawker, thank you for opening an issue with us!

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[michael-hawker]

If you set ExpandDirection to Up and have the Expander VerticalAlignment to Top you see the same problem.

[michael-hawker]

We should test this same scenario against the WinUI Expander to see behavior, if it exhibits the same issue, we should file an issue on WinUI and close this one since we'll eventually deprecate ours.