Describe the bug

When scrolled to the bottom of a list with a ScrollViewer that is configured to anchor at the extent (VerticalAnchorRatio = 1.0) then the viewport should automatically shift to track the very end, but it doesn't always.

Steps to reproduce the bug

Steps to reproduce the behavior:

1. Clone the https://github.com/micahl/bottomsuplist repo
2. Checkout the oldscrollviewer branch which uses the Windows.UI.Xaml.Control.ScrollViewer to host an ItemsRepeater
3. Build and run the solution
4. Ensure you're scrolled all the way to the bottom.
5. Enter a few text messages in quick succession.

Expected behavior

The newly added messages at the bottom of the list should always appear in view.

Actual behavior
The viewport may not automatically shift so that the message doesn't appear in view and at that point it tracks what is in view so that all subsequent messages continue to appear below the fold.

Version Info

NuGet package version:
Microsoft.UI.Xaml 2.1.190405004]