I have an issue discussed previously on Ask Different but have not found a solution in my use case.
I have macOS High Sierra installed on a MBP 6,2 (i.e. a vintage 15" i5, mid-2010, NVIDIA GeForce GT 330M with 256MB VRAM).
With a new external monitor (Viewsonic VP3268-4K) set at 2560x1440 and connected with mini-DisplayPort to my MBP, the external monitor loses it's video signal when I try to deactivate the internal LCD display by clamshell-ing it (I use a refrigerator magnet to make the MBP believe the lid is closed).
However, this works fine when another external display (1920x1200) is connected via mini-DP --> DVI cable.
-- What Works --
Deactivating the unused MBP internal display worked fine with my older external monitor (1920x1200) connected to the MBP via mini-DP --> DVI adapter cable. When clamshell-ed the internal LCD display goes dark and also disappears from the System Preferences --> Displays --> Arrangements tab page. My mouse cursor remains only within the external display. Everything works as expected.
The new (2560x1440) external display connected via mini-DP works with no issues as long as the MBP internal display is also active. I can turn down the internal display's backlighting to remove the visual distraction, but this still leaves it's video framebuffer active: it is listed in System Preferences... and the mouse pointer is not restricted to the external monitor video space and can go off into the internal display area.
-- Fixes Tried so Far --
Lots of reading online, here (lots of good info) and other sites led me to try editing boot NVRAM parameters to force the internal display to be inactive, terminal commands suggested in other postings were tried (see below), different DP adapters, changing the new external monitor's settings: force DP v1.1. but nothing tried had the desired effect: to clamshell the internal display while using the external monitor (2560x1440) via DP. Unfortunately I cannot use DVI with the new external monitor because single channel DVI only supports (1920x1200) and I do not have a dual channel DVI cable or Apple cable adapter.
The terminal commands I've tried (after disabling SIP) are:
sudo nvram boot-args="niog=1" sudo nvram -d boot-args sudo nvram boot-args="iog=0x0" sudo nvram boot-args=niog=1
I've also tried resetting the external monitor settings by removing the Folder
Can anyone offer any other suggestions?