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Monitor Layout Controls...

  xmonitor     xrandr wrapper to layout monitors more easilly (personal)
      https://antofthy.gitlab.io/software/#xmonitor

  arandr       GUI program generating xrandr commands to use
  lxrandr      GUI interface for xrandr for setup and confuration of a
               second X windows, such as projectors

  lxqt-config-randr
               GUI for specific xrandr controls in with display of results
               Lets you try it, with a timeout fall back,
               But does not output the xrandr line it uses!

  xrandr       The primary display control (run time)
               But has so many options it is hard to use and understand

  GDM uses a "monitors.xml" file to define a xrandr to run
    cp ~/.config/monitors.xml ~gdm/.config/
    You can also put a xrandr command in  /etc/gdm/Init/Default
    BUT ONLY if you set "WaylandEnable=false" in /etc/gdm/custom.conf

  autorandr
    https://github.com/phillipberndt/autorandr

Alternative check for monitor being online (for script polling)
  head /sys/class/drm/*/status
This updates on poll after a few minutes passed on last check.

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Panning (scrolling display) mode

  Let display pan as the pointer moves around it
    xrandr --output DP1-1 --panning 3840x2160

  More options let you specify how the mouse controls the panning

  Back to normal
    xrandr --output DP1-1 --panning 0x0

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Change resolutions of a monitor (add new modes)
  https://ubuntuforums.org/showthread.php?p=8595940

  xrandr
     # reports no mode for 1024 768
  cvt 1024 768
     # outputs a mode long for that resolution

  xrandr --newmode <modeline>

  # for example
  xrandr --newmode "1024x768_60.00" 63.50  1024 1072 1176 1328  768 771 775 798 -hsync +vsync

  # Now add it
  xrandr --addmode VGA1 1024x768_60.00

  # and switch to it
  xrandr --output VGA1 --mode 1024x768

NOTE this is all at runtime and temporary
They need to be added to "/etc/gdm/Init/Default"  to setup before login.

More detail on xrandr at
  https://wiki.ubuntu.com/X/Config/Resolution

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Transform the display
Note mouse movements are also transformed

Rotate 180

  xrandr --output DP1-1 --reflect xy
  xrandr --output DP1-1 --reflect normal

OR

  xrandr --output DP1-1 --rotate inverted
  xrandr --output DP1-1 --rotate normal

Flip (text is hard to read)

  xrandr --output DP1-1 --reflect x
  xrandr --output DP1-1 --reflect y
  xrandr --output DP1-1 --reflect normal

Scale

  xrandr --output DP1-1 --scale 2x2
  xrandr --output DP1-1 --scale 1x1

Affine Transformation

  # rotate 30 (seems larger)
  xrandr --output DP1-1 --transform .8660,-.5,0,.5,.8660,0,0,0,1

  # shear
  xrandr --output DP1-1 --transform 1,-.2,0,0,1,0,0,0,1

  # back to normal
  xrandr --output DP1-1 --transform 1,0,0,0,1,0,0,0,1
  xrandr --output DP1-1 --transform none

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Monitor goes blank with "no signal", but xrandr shows it connected...

Posted answer to...
  https://askubuntu.com/questions/951901/#1254962
  https://askubuntu.com/questions/405645/
  https://askubuntu.com/questions/148725/

If I tell xrandr make any sort of change (it must be a change)
then the display is reset and it all comes back.

For example, have xrandr turn it off and on (pause between actions)...

    xrandr --output DP-1-2 --off
    xrandr --output DP-1-2 --auto

However it losses its position information and just becomes a clone, until
I again reset the position.  Any windows on that display get moved to make them
at least partially visible in the other display, though I have found it shift
a LOT of windows which should not have been effected too.  Basically it is
a horrible fix.

This fixes the problem some of the time...

    xset dpms force suspend    # or use standby or off (same result)
    xset dpms force on

This is the best solution I found so far, but depends on your setup...

Move the display just a little (one pixel), then moving it back!
It is enough of a change to get xrandr to resurect the monitor and does not
cause my application windows to suddenly change position.

    xrandr --output DP-1-2 --pos 1921x0;    # shift one pixel
    xrandr --output DP-1-2 --pos 1920x0;    # shift it back

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