There are two possibilities:

1. Fake GPU software, which I haven't researched at all but probably exists

2. Forcing your graphics chipset to turn a specific head on and leave it on

Here's a Linux-specific example of how to do #2. Since I'm just talking to graphics hardware here, there should be equivalent operations for all of the following for other operating systems.

I don't connect an external display at any point.

First of all, how things look to begin with:

  $ xrandr   
  Screen 0: minimum 320 x 200, current 1024 x 768, maximum 4096 x 4096
  VGA-0 disconnected (normal left inverted right x axis y axis)
  DVI-0 disconnected (normal left inverted right x axis y axis)
  LVDS connected 1024x768+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
     1024x768      60.00*+
     800x600       59.86  
     848x480       59.66  
     720x480       59.71  
     640x480       59.38  

Now, `xrandr --output VGA-0 --auto` isn't going to do anything because --auto will tell the GPU to pick the best resolution. No screen is attached so there's no list of resolutions to pick from.

So, I'll generate a generic CVT modeline that sets up the VGA output for 1024x768@60Hz.

  $ cvt 1024 768 60
  # 1024x768 59.92 Hz (CVT 0.79M3) hsync: 47.82 kHz; pclk: 63.50 MHz
  Modeline "1024x768_60.00"   63.50  1024 1072 1176 1328  768 771 775 798 -hsync +

Cool. Now I'll tell X11 about the modeline (note the copy-paste):

  $ xrandr --newmode  "1024x768_60.00"   63.50  1024 1072 1176 1328  768 771 775 798 -hsync +

...And now do the XRandR dance to actually tell the output about the right modeline (this is Xorg-bureaucracy-specific):

  $ xrandr --addmode VGA-0 "1024x768_60.00"

Let's review the xrandr output:

  $ xrandr
  Screen 0: minimum 320 x 200, current 1024 x 768, maximum 4096 x 4096
  VGA-0 disconnected (normal left inverted right x axis y axis)
     1024x768_60.00  59.92  
  (...)

At this point I can go ahead and

  $ xrandr --output VGA-0 --mode "1024x768_60.00"

and my screen displays garbage for a fraction of a second and then figures itself out.

At this point I know the output is enabled (and the glitch didn't mean nothing) and in mirroring mode. I can prove this without a display attached by running xrandr again:

  $ xrandr
  Screen 0: minimum 320 x 200, current 1024 x 768, maximum 4096 x 4096
  VGA-0 disconnected 1024x768+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
     1024x768_60.00  59.92* 
  (...)

Two things: one, the mode has an asterisk next to it, meaning it's active (albeit without a screen attached), and two (the following will make sense in a minute if it doesn't straight away) Screen 0 has a "current" size of 1024x768. (Xorg "screens" represent a collection of video outputs you can move windows between.)

So, let's disable mirroring mode and make this new Schrödinger-pseudo-existent area its own independent surface.

  $ xrandr --output VGA-0 --left-of LVDS

At this point, i3 (my windowmanager) decides to move every single window onto the new offscreen surface area, and I go blind. Isn't minimalist software wonderful? (A new windowmanager is on my todo list; i3 drives me nuts. Anyway.)

Once I sort i3 out and get back to a terminal, xrandr shows something new and interesting:

  $ xrandr
  Screen 0: minimum 320 x 200, current 2048 x 768, maximum 4096 x 4096
  (...)

Oh hey, the Screen size is now 2048x768! Proof that I have two 1024x... outputs configured side-by-side. If I wanted to I could do --above or --below and have a 1024x1536 display.

Here's the technical moral of the story: since approximately the time of the dinosaurs monitors have used EDID to provide autodetection info to graphics chipsets, so forcing manual configuration is pretty deeply buried.

But that's what you need to do; "my screen is weird and needs a custom configuration" and "okay just set these parameters and leave the output head running" are analogous.

If you want to turn your screen off and have the GPU keep the head running, I recommend figuring out how to fish out the modeline your GPU is currently using, and then force the GPU to manually use that modeline. That should disable output autodetection and keep the output permanently active.

Despite the fact that I've demoed Linux output configuration here, you're (ironically) not too likely to have this problem on Linux as Xorg is fairly flexible, and wouldn't normally need these kinds of instructions provided to it under normal circumstances; for Linux it would be more "what's explicitly killing the output and how do I turn it off" as opposed to digging around in driver settings.

Also - I picked VGA for this example, honestly probably for nostalgic reasons. These instructions will work equivalently regardless of output type.

This comment deserves a github gist, and a separate top-level post.

>At this point, i3 (my windowmanager) decides to move every single window onto the new offscreen surface area, and I go blind.

Xmonad can't do stuff like this because it treats each monitor as a separate virtual screen.

If I wanted a bigger monitor I would of just bought a bigger monitor, dammit …

A permanent option sometimes required for headless operation: a hardware emulator/terminator such as https://amzn.com/B00FLZXGJ6 ($8.50).

You must have some dodgy hardware. This is definitely not normal, at least with HDMI.

I just tried it with the monitor right in front of me, even unplugging it from the wall didn't cause the laptop to drop the monitor like it would if I had disconnected the HDMI cable.

What I had learnt from investigating this issue is:

* It's been a problem since at least 2012

* HDMI cables have an active-low connection pin, therefore it's easier to fool the video card into thinking the monitor is still connected.

* DisplayPort is an active-high signal, therefore if the monitor powers off, it's the same thing as the cable being physically disconnected.

Ideally I would be able to switch the monitor off in such a way that it still maintains a connection, just that the display is off.

Edit: formatting

Is it necessary to you that the display actually be disconnected from power? I use display port on most of my displays and don't have the issue with simply turning off the monitor. But, that's because the monitor doesn't REALLY turn off. It just goes into an ultra low power mode. If that satisfies you, perhaps you need to get a different monitor is all.

Perhaps. I should take a look at the monitor configuration and see if there's a setting I can change. Something like "monitor power button puts monitor to sleep" rather than "monitor power button turns power off to monitor".

I don't turn the power off at the source, i.e. unplug the monitors.

Sounds perfectly normal to me. I’ve only ever seen the behavior daemin describes.

See my reply to the subthread commentor.

I've never really understood the desire to use the button on the display to turn off monitors - does allowing the monitors to be switched off by the OS not solve basically the same problem ?

I leave my PC running 24/7 pretty much, so at night I want the monitors turned off so it doesn't illuminate the whole living room (which I can see from the bedroom - it is a small apartment).

I used to turn them off with the button on the monitor (when all my monitors were DVI), so if the mouse was moved by accident during the night it wouldn't wake the monitors up. Right now I have to let it go to sleep normally, and then hope there isn't some strange vibration that is big enough to trigger a wake.

Edit: Better language and more info.

Every monitor I've owned since the early 2000s will switch itself off (or at least go into a standby state where the screen is entirely off and there's a tiny blinky LED indicating that it's still powered) after a period of inactivity. I know some monitors never give up waiting for a signal but at this point maybe it'd be simpler to treat yourself to a new monitor?

Edit: Oh, you're talking about things waking your computer up? Maybe turn off 'wake on USB'? Although that's less convenient otherwise. I know when my desktop computer is fully hibernated, mouse motions don't wake it but button presses do. And you can get into this state (assuming Windows 8+) by right clicking on the Windows icon / 'start' button and selecting 'sleep'.

Disabling wake from USB is not a good idea given I have a USB keyboard and mouse.

Essentially this is a convenience thing, it used to work perfectly fine, then they added HDCP to make things less convenient.

Like I said, I'd be fine if the software (well Windows in my case) has a switch to disable HDCP and restore the old functionality of always assuming there's a monitor there.

In my experience, Windows 10 seemed to have issues with the "Wake on USB" setting about 6-8 months ago. No matter if I let my computer hibernate or forcibly sleep my computer, it would turn on within 20-30 minutes of me being away. No matter what settings I set, this would happen constantly.

I came up with a trick for this. Wrote a little script for windows (I'm sure you could do the same thing for linux) that just sends the "sleep" command to the monitors at will.

I can dig it up for you if you want it when I get home.

EDIT: Upon re-read this doesn't really solve your accidental wake up problem. Oops.

You can disable mouse-wakeup, so that the only way to wake from sleep is a keyboard press.

Just leave your mouse upside down.

> I've never really understood the desire to use the button on the display to turn off monitors - does allowing the monitors to be switched off by the OS not solve basically the same problem ?

Right now, my monitor is in my bedroom. If I let the OS shut off the monitor, it still shows a little white pulsing light to show it's in sleep mode. That's really annoying when I'm trying to sleep.

And my monitor has an extremely bright pulsing white LED. I have a piece of orange electrical tape over it, and it is still on the bright side. Dell e2414h if anyone's curious.