WinUI: NavigationPage transitions show white window/background behind content

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### Description

When using `PushAsync` and `PopAsync` to transition with NavigationPage on Windows 10, a white background appears at the edges of the window where the transition is occurring. This area is always white even when the content page has a different background color or a background image.


https://user-images.githubusercontent.com/44978869/205174626-1b78a566-0b66-4431-9876-1892136c2070.mp4

### Steps to Reproduce

1. Create a new MAUI app
2. Replace `MainPage = new AppShell()` with `MainPage = new NavigationPage(new MainPage())` in App.xaml.cs
3. Add a second content page and give both pages a background color other than white
4. Write event handlers in the code behind to allow navigating between the two pages with `App.Current.MainPage.Navigation.PushAsync/PopAsync`

Expected behavior: The transition animation is seamless.

Actual behavior: A white background is visible during the transition.

NOTE: The linked repro project is passing `animated: false` to `PushAsync/PopAsync` because of another bug report I'm filing. You can change it to `animated: true`, but the white background is visible either way (due to the other bug).

### Link to public reproduction project repository

https://github.com/ArrowCase/TestMauiTransition

### Version with bug

7.0 (current)

### Last version that worked well

Unknown/Other

### Affected platforms

Windows

### Affected platform versions

Windows SDK 10.0.19041.0

### Did you find any workaround?

None.

### Relevant log output

_No response_

Start by running the linked TestMauiTransition reproduction on Windows and inspect the NavigationPage behavior around PushAsync and PopAsync, with animated transitions enabled. Compare the transition against pages using non-white background colors or images; done means no white area is visible at the window edges during navigation.

Written by the indexing model from the issue text.