RibbonWindow has wrong size

* .NET Core Version: (e.g. 3.0 Preview1, or daily build number, use `dotnet --info`)
Version 5.0.2
* Windows version: (`winver`)
OS Version: 10.0.19042
* Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes
* Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? If yes, please file the issue via the instructions [here](https://docs.microsoft.com/visualstudio/ide/how-to-report-a-problem-with-visual-studio?view=vs-2019).
No
* Security issues and bugs should be reported privately, learn more via our [responsible disclosure guidelines](https://github.com/dotnet/wpf/blob/master/README.md#reporting-security-issues-and-security-bugs).

**Problem description:**
The visual size of a Wpf RibbonWindow is different to a Wpf Window with the same Height and Width in XAML. See attached projects. The rendering in the non client area of the RibbonWindow is not correct. See position of the Icon and the position of the Buttons.
[WpfApp1.zip](https://github.com/dotnet/wpf/files/5898461/WpfApp1.zip)
[WpfRibbonApp.zip](https://github.com/dotnet/wpf/files/5898462/WpfRibbonApp.zip)

**Actual behavior:**

**Expected behavior:**
The size of RibbonWindow and Window should be equal. Rendering of the Icon and the Buttons should be equal.

**Minimal repro:**

See attached projects