Modal dialog on top of non-modal window sends main window to back

* .NET Core Version: .NET 4.7
* Windows version: win10
* 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...)? No

**Problem description:**
Modal dialog on top of non-modal window sends main window to back. Ie, when we close a child window its Owner gets send back (we lost focus)

**Actual behavior:**
The parent window lost focus after closing a child window after closing a modal.

**Expected behavior:**
No matter if a modal dialog has been shown before, if a user closes a child window, the parent window should stay visible.

**Minimal repro:**
Here is the problem and some source code to reproduce: https://stackoverflow.com/questions/10757625/why-does-closing-the-last-child-window-minimize-its-parent-window