I3, LightDM and Slick-greeter: dual monitor issue at login screen

Latest i3wm edition, HP Elitebook 840 G2 laptop with additional 24" monitor. All working fine as a dual monitor system EXCEPT for the login screen.

This is a minor cosmetic issue but if I have the second monitor plugged in on boot the login screen background image size on the primary laptop screen gets the wrong size: there are black borders on the bottom and right edge of the login background image.

I understand “slick-greeter” does the login page and “lightdm” is the login manager. I have tried a number of things to no avail:

  1. created a lightdm.conf script to try to tell it about the monitors using xrandr
  2. tried to force slick-greeter to only have login screen on one monitor (this too failed).

I am not sure how to proceed any further. Suggestions?