After returning from standby (sleep), 2 monitors are perceived as one on the login screen and background image are stretches across two monitors.

Linux LMDE 7 Cinnamon

Standard Screensaver "Time"

My comp with 2 monitors:
Left (additional) - HDMI 1920x1080 (it first in system (by driver priority))
Primary - DP 2560x1440 (it second in system (by driver priority))

GPU: NVIDIA GT 1030 with proprietary driver

in /etc/lightdm/lightdm.conf:

[Seat:*]
...
display-setup-script=xrandr --output DP-0 --primary --mode 2560x1440 --rate 185 --output HDMI-0 --mode 1920x1080 --rate 60 --left-of DP-0
...

With this configuration, the additional monitor (on the left) has an offset of +0+0, and the primary monitor has an offset of +1920+0.
I'm describing this so you can understand the system configuration.

Let's say the computer is idle for a while, and the following happens:

1. The screensaver turns on.
2. After a while, the signal and the monitors turn off (this is how the settings are configured), meaning the computer hasn't yet entered standby mode, but the monitors have already go dark (no signal).
3. After a while, the monitors also turn off—the lights of power on monitors go out.
4. Turn both monitors back on.
5. Any mouse or keyboard action.
6. The computer returns to work—it displays the login screen (the welcome screen).

And here's the worst part!

On the login (greeting) screen the background image stretches to span the width of both monitors, and the login dialog box also shifts to the left (although the dialog box itself appears undisturbed). It's as if the system is treating the two monitors as one.

After entering the login information, we log in, and everything displays normally—each display (monitor) in its proper place. The two monitors are combined into a single workspace, but the background image is duplicated on each—everything as it should be.

The main problem is that the login window, after returning from the screen saver (specifically, after turning off the monitor), displays something strange and incomprehensible.

Either this is a screensaver bug, or a system bug, or... can you help me set it up correctly?

Start with the /etc/lightdm/lightdm.conf display-setup-script and reproduce the listed screensaver, monitor-power-off, sleep, and wake sequence on the two-monitor setup. Compare the greeter layout before and after wake; done means the login background and dialog treat the monitors separately and match their configured positions.