[WinUI][dotnet8] - New window blinks when loading a content.

### Describe the bug

In .NET 7, windows wait to render the content before opening. In .NET 8 and later, an unintended flicker occurs while loading this content. This flicker comes with a default background. It also occurs using MAUI.

### Steps to reproduce the bug

1 - Create multiple windows with some content and an alternative background color.

### Expected behavior

It should open like in dotnet7, appearing all the window with a rendered content. (not a flickering before opening).

### Screenshots


### NuGet package version

None

### Windows version

_No response_

### Additional context

_No response_

The issue names no repository file or test. First reproduce the multiple-window case with content and an alternative background on .NET 8 or later, then compare it with .NET 7; done means the window opens with rendered content and no default-background flicker.

Written by the indexing model from the issue text.