Fully customized title bar covers window's black border on Windows 10

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### Describe the bug

I've created a top-level Win32 window with a XAML island filling its entire client area. When I set `ExtendsContentIntoTitleBar = true` on my `AppWindow` with a custom title bar, the black border drawn by DWM is covered across the top (but not the left or the right). I've tried compensating by manually drawing a border across the top of the `Grid` containing my title bar layout, but it still doesn't look right because the caption buttons overlap that border, instead of being inset inside of it.

Since this is on Windows 10, I cannot use DWM APIs to hide the rest of that border myself.

### Steps to reproduce the bug

1. Create a top-level Win32 window, add a XAML island within it;
2. Use `AppWindow.ExtendsContentIntoTitleBar = true` and set a custom title bar layout;
3. Write a simple app that just displays the window
4. Move the app's window in front of a light-colored background (I just move it in front of an empty notepad window)
5. Notice that there is a black border around three sides of the window, but it's missing across the top of the window where the custom title bar is drawn.

### Expected behavior

I expect that on Windows 10 a fully-customized title bar will not obscure the border across the top of the window.

### Screenshots


### NuGet package version

Windows App SDK 1.6.1: 1.6.240923002

### Packaging type

Unpackaged

### Windows version

Windows 10 version 22H2 (19045, 2022 Update)

### IDE

Visual Studio 2022

### Additional context

_No response_

Start by reproducing the Win32 window with a XAML island, AppWindow.ExtendsContentIntoTitleBar, and a custom title bar on Windows 10 version 22H2. Compare the top border with the left and right borders, including the caption buttons; done means the customized title bar no longer obscures the DWM-drawn top border.

Written by the indexing model from the issue text.