Keep the Windows 10 Action Center Always Open Until Closed contains the registry files to force the Action Center to remain on top until closed.
Once the Action Center is open, you will need to click the Taskbar icon (see screenshot below) to close it. This modification is handy if you frequently close the Action Center before reading or removing notifications.
Download 'Windows 10 Action Center Always Open Until Closed' and extract the two files.
Double-click 'Action Center Always Open - ON.reg' or 'Action Center Always Open - OFF.reg' (Default).
Click Yes when prompted by User Account Control. Click Yes when prompted by Registry Editor.