Explorer should check the Winlogon registry key to see if it is the shell.

In Windows, Explorer will check HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\Shell to see whether or not Explorer is set as the default shell. If it is not, then the process will only create a file explorer window and it will NOT launch the taskbar or desktop. It may also check the same key in the HKCU as well, but I will have to investigate more.

In ReactOS, Explorer does NOT check the Winlogon shell key. Regardless of what the default shell is, if there is no other shell window that has been marked with SetShellWindow, it will create the taskbar and desktop which is inappropriate behavior. If I were to set my default shell to Program Manager or Task Manager or CMD, then upon launching Explorer I would expect the Windows behavior and should only see the file explorer window appear.

• Expected Result
  When HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\Shell does not contain explorer.exe, launching explorer should only open a file explorer window. This is the behavior in Windows 2000 (possibly older) and later versions of Windows.
• Observed Result
  ReactOS 0.4.16-gd52031a launches the taskbar and desktop when opening explorer.exe. This is an issue for people using alternative shells.