nvidia-prime can't log in if boot in discrete mode on Thinkpad with hardware video mux

1) to save power, sudo prime-select intel
and boot in hybrid graphics

This is the only way I know to get the nvidia card off which is the only reason I boot like this
This still works, although it is really slow.

2) to go back to multi-monitors, reboot, at bios turn on discrete graphics

and it does not reach password entry. The greeter crashes/restarts/crashes/restarts....

In the 17.10 build of the driver, even 396.18 which is newer than the 18.04 driver, step 2 simply boots you with nvidia graphics.

The current situation is really horrible. Overall, optimus experience is extremely degraded in 18.04. Since I have the very latest beta in 17.10, why has there been such a huge change which is even in the best case really slow, and the worse case result in boot failure? Why can't we keep the approach used since 16.04 or may be earlier?

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: nvidia-prime 0.8.7
ProcVersionSignature: Ubuntu 4.15.0-15.16-generic 4.15.15
Uname: Linux 4.15.0-15-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu5
Architecture: amd64
Date: Thu Apr 19 16:20:45 2018
Dependencies:

EcryptfsInUse: Yes
InstallationDate: Installed on 2017-12-14 (125 days ago)
InstallationMedia: Ubuntu 17.10 "Artful Aardvark" - Release amd64 (20171018)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_AU:en
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_AU.UTF-8
 SHELL=/bin/bash
SourcePackage: nvidia-prime
UpgradeStatus: Upgraded to bionic on 2018-03-09 (41 days ago)