All methods I tried with software-only methods didn't work or caused weird artifacts on my displays. Instead of using purely software-based opensourced solutions by hacking the system, I resorted to buying a small dummy video output/virtual display plug for US$3.00. I purchased it on taobao.com, however, they're also available on Amazon.com, Newegg.com, and AliExpress.com.
The plug tricks the system to believe I have another display attached. I then shared the extra display's desktop to my Android device using a software developed by a nice person, called Deskreen. Alternatively, I could use another opensource method to share the desktop called, Weylus and have touch input also. VNC Server should be able to get touch input too.
Mini DisplayPort (mDP) dummy output plug:
Extended Desktop (3840x1291):
For now, this is now the most elegant solution on GNU/Linux. Comparatively, superDisplay on Windows 10 is still much simpler. As superDisplay only need installation of a Windows driver applet and counterpart app on Android, then minor settings and connection could be by Wi-Fi/Ethernet or USB.