This problem does not occur if the computer is connected to the Internet and if the current time of the system clock is constantly adjusted by the synchronization to time servers on the Internet.
Moreover, this problem does not occur on an installed Mint (as opposed to LiveDisc), even if the computer is isolated from the Internet.
This is not an intermittent problem; I can always reproduce the problem by the following steps.
Steps that I took
Before booting, physically isolate the computer from LAN and the Internet. (I do not want to expose my computer to the Internet before the Firewall is enabled; the Firewall is disabled by default. Note that the effect of isolating the computer from network includes the incapability of automatic adjustment of the current time of system clock. By the way, if any data is to be absolutely protected from the Internet, the computer containing that data need be physically isolated from the Internet. For security, it is a good habit to isolate computers from the Internet when the Internet connection is unnecessary.)
Boot to the LiveDisc.
As soon as the booting of Mint 20.3 MATE LiveDisc completes, configure the Screensaver and the Power Management as follows.
Note that blank brackets [ ] indicates "unchecked"
and that v-holding brackets [v] indicates "checked".
Code: Select all
Control Center > Look and Feel >
Screensaver
Regard the computer as idle after: 2 hours
[ ] Activate screensaver when computer is idle
[ ] Lock screen when screensaver is active
Control Center > Hardware >
Power Management
On AC Power tab
Actions Put computer to sleep when inactive for: Never
Display Put display to sleep when inactive for: Never
General tab
Actions
When the power button is pressed: Suspend
When the suspend button is pressed: Suspend
Let the computer stay idle, and wait for 11 minutes.
Expected Result
The display never sleeps.
Actual Result
The display falls asleep after 10 minutes of idleness.
Consideration
This Mint 20.3 MATE LiveDisc is read-only and was made from "linuxmint-20.3-mate-64bit.iso" verbatim without any modification. The sha256 digest of the iso image is "27de0b1e6d743d0efc2c193ec88d56a49941ce3e7d58b03730a4bb1895c25be5".
I tested also on an installed Mint, and found that this problem does not occur on an installed Mint, but occurs only on LiveDisc. I guess that LiveDisc may be less thoroughly tested than an installed OS. While a problem on an installed OS may sometimes be difficult to reproduce because of too much customization of settings, a problem on LiveDisc must be easier to reproduce because of no customization.
This problem has been persistent through MATE Mint 20.1, 20.2 and 20.3. In contrast, Ubuntu MATE 20.04.3 LiveDisc is free from this problem. (The reason that I call this problem a "bug" is the discrepancy between the expected behavior and the actual behavior. The absence of this problem on Ubuntu is NOT the reason why I call this problem a "bug". I mention the absence on Ubuntu in order to help the developer spot the location of the bug in the source code, that is, where Mint is distinct from Ubuntu.)
On the "Power Management" control panel, the default value for "Put display to sleep when inactive for" is 30 minutes. On the "Screensaver" control panel, the default value for "Regard the computer as idle after" is "5 minutes". However, the display falls asleep upon "10 minutes" of idleness. I wonder where this "10 minutes" come from. Perhaps, "30 minutes" may be the default on the configuration file in "/etc" or "/lib", and "10 minutes" may be the hard-coded default. Anyway, I do NOT want the display to fall asleep.
To reproduce the problem, an AC-operated desktop/tower computer may be a better subject than an battery-operated notebook/laptop.
Another Experiment
Instead of setting "Put display to sleep when inactive for" to "Never" in the "Power Management" control panel, I also tested to see what will happen if the "Power Management" control panel is entirely left untouched and if only the "Screensaver" is changed as above. The expect result is that the display falls asleep upon "30 minutes" of idleness, because "30 minutes" is the default on the control panel. However, the actual result is that the display falls asleep upon "10 minutes" of idleness.
