Maximized window hides automatically hiding taskbar

* .NET Core Version: 3.1
* Windows version: 2004 (Build 19041.450)
* Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

**Problem description:**

**Actual behavior:** Taskbar in automatic hide mode does not appear when moving the mouse pointer to the taskbar area in a maximized WPF app window.

**Expected behavior:** Taskbar should slide open regardless window normal/maximized state.

**Minimal repro:**

* Set the taskbar to auto hide mode.
* Create a new Visual Studio 2019 project of type WPF App for .NET Framework 4.8 or .NET Core 3.1.
* Run the app.
* Maximize the blank app window and hover the taskbar area. The taskbar will now not be shown.

**Findings:**

* Expected behavior in a Windows Forms app on .NET Framework 4.8 or .NET Core 3.1.
* Expected behavior in Windows Notepad. (to further exclude any Windows 10 bugs)
* Expected behavior in a dual screen setup when maximizing the window and hovering the taskbar area on the secondary screen.
* The same behavior is seen in Windows 10 Version 1909.
* The behavior does not seem to depend on DPI settings. Tested resolution on two systems in 2560x1440.