Describe the bug

When you resize an app with a ListDetailView in a WinUI 3 NavigationView the BackButton disappears and re-appears intermittently.

• Is this bug a regression in the toolkit? If so, what toolkit version did you last see it work:

Steps to Reproduce

• Can this be reproduced in the Sample App? (Either in a sample as-is or with new XAML pasted in the editor.) If so, please provide custom XAML or steps to reproduce. If not, let us know why it can't be reproduced (e.g. more complex setup, environment, dependencies, etc...)

Steps to reproduce the behavior:

1. In the attached app (App2.sln)
2. Go to ListDetail Page
3. Resize the app to narrow
4. Resize the app to wide

=> Back button disappears

Repeating step 3 and 4 makes the back button disappear and re-appear intermittently.
The problem is that there are states where you cannot go back from the detail to the list or navigate backward between pages without resizing the app.

Expected behavior

The back button should not disappear

Screenshots

Environment

NuGet Package(s): 
CommunityToolkit.WinUI.UI.Controls

Package Version(s): 
7.0.1

Windows 10 Build Number:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [x] May 2020 Update (19041)
- [ ] Insider Build (build number: )

App min and target version:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [ ] May 2020 Update (19041)
- [ ] Insider Build (xxxxx)

Device form factor:
- [x] Desktop
- [ ] Xbox
- [ ] Surface Hub
- [ ] IoT

Visual Studio 
- [ ] 2017 (version: )
- [ ] 2019 (version: ) 
- [x] 2019 Preview (version: Version 16.10.0 Preview 1.0)

Additional context

Add any other context about the problem here.
App2.zip