ComboBox in a ScrollViewer in Xaml Islands can be scrolled off the window

Steps to reproduce the bug In a XAML Islands application, create a ComboBox in a ScrollViewer:

<Page>
    <ScrollViewer>
        <StackPanel>
            <ComboBox>
                <ComboBoxItem Content="Foo" />
                <ComboBoxItem Content="Bar" />
                <ComboBoxItem Content="Baz" />
            </ComboBox>
        </StackPanel>
    <ScrollViewer>
</Page>

(The exact one is over at https://github.com/microsoft/terminal/blob/main/src/cascadia/TerminalSettingsEditor/Launch.xaml#L38-L67)

Run the app, open the comboBox, then mouse over the rest of the scroll viewer and start scrolling.

Steps to reproduce the behavior:

1. Go to ‘…’
2. Click on ‘…’

Expected behavior In a non-islands app, you can’t scroll the scrollviewer while the combobox is open. You have to dismiss the box first.

Actual behavior

The combo box’s entries float off the top of the window into space

Version Info

NuGet package version:

[Microsoft.UI.Xaml.2.5.0-prerelease.201202003]

[DHowett]

This is amazing.

[DHowett]

There being no activity on an issue means the issue continues existing, not that it disappears!