Desktop background/wallpaper resizing undesirably on login

Hi, I've encountered an annoying problem that has stumped me as to how to resolve it...

I use Zorin 12 on a laptop with an external monitor, for two displays. The external monitor is oriented vertically. I use a custom image as my desktop background / wallpaper image (attached for reference).

Every time I log into system after a reboot with the external monitor connected, the background image initially displays the way I want it to, showing a centered partial image on the laptop screen and the full image scaled to fit the external monitor.

Invariably, roughly 10 to 20 seconds after logging in, the background image on the second monitor changes to be an ultra-zoomed-in view, showing the very center of the image only. The section of the image that is displayed corresponds to the area of the image on the laptop screen that you would get by taking the largest segment of the image displayed on the laptop screen that would fit in the aspect ratio of the external display. I believe that there is some "helpful" widget or setting that is snipping the largest image it can from the laptop screen (which is the Primary screen) and applying the snipped image as the wallpaper for the external display.

I've tested this by resizing the image so that the image displayed on the laptop screen remains the same but the image for the external monitor is smaller in the vertical dimension, and therefore distorted when scaled to fit the monitor. The post-login wallpaper change produces an image that is not distorted.

When I toggle the Activities-picker (with the "start" button), the background image underneath the activities overlay is still in the correct scale.

I am able to correct the resolution issue on the external display by changing the background image to something else, and then back to my custom image. This is, however, rather tedious to have to do after every reboot.

With the timing and the testing I've done, and the fact that Zorin 12 is the only distro I've used based on Ubuntu 16.04 that does this, I have to think that whatever is happening is a Zorin-provided package/tool/widget/helper/thing. I'd really like to know how to disable it.

I maybe totally off on this one but wondered if a solution for a slightly different issue in 18.04 might resolve the issue?:

https://askubuntu.com/posts/1104530/revisions

I maybe totally off on this one but wondered if a solution for a slightly different issue in 18.04 might resolve the issue?:

https://askubuntu.com/posts/1104530/revisions

Thanks for the thought, but no, I've tried using most, if not all of the various solutions in that thread (as applied to wallpaper scale/position/display, rather than text-size), and none of them address the issue properly.

Not that they don't necessarily work, mind you, just that they get either suppressed or overridden by whatever is causing the problem to begin with. (I have used most of these solutions in other Ubuntu-based distros to good effect at one time or another.)

In the case of the login script, I hard-coded the script to apply the specific background image, and changed my wallpaper to a different image. On re-boot, the wallpaper changed to the correct image and scaling, and then re-scales to the correct image and the wrong scaling.

I have verified that the login script, at least, does work as expected (sets the correct image and scaling, which then persists properly) on a basic install of Ubuntu 16.04, as well as in Linux Mint. As such, I'm reasonably certain that the fly in the ointment is something either included or turned on by the Zorin-specific implementation.

I've attached a clearer image to illustrate what's happening. The green border is the desired scaling and display for the external monitor. (Again, this scaling *is* shown when I push the start button for the Activities-picker.) The blue border is the desired, and achieved, scaling and display for the laptop screen. The red border denotes the approximate scaling and display that I get for the desktop wallpaper/background on the external monitor whenever I reboot.

Lovin' the XKCD comic. I am assuming your Zorin 12.4 is completely up to date. Could you specify which desktop environment you are using?

This line:

and the fact that Zorin 12 is the only distro I've used based on Ubuntu 16.04 that does this, I have to think that whatever is happening is a Zorin-provided package/tool/widget/helper/thing

makes me wonder if sending a PM to Azorin on here may be your best bet. It may take him some time to respond, but this is a non-emergency question.