Actually, now that I'm back on Arch and Cinnamon, I realise that the flicker was much less pronounced when on the Manjaro KDE live image.

I had a very similar issue for a few months with an nVidia card. It drove me mad trying to figure out what the problem was. It was actually the card: it eventually died.

What I find rather curious though, is that the flickering doesn't manifest itself at all when I have two active displays, as I mentioned in the opening post.

This leads me to think that the drivers use timings from the secondary display (just visible to the left in the later parts of the video, and which has pixel 0,0 so games often go fullscreen on that instead of the primary, making trouble when they aren't built for portrait mode, which is most games outside of cell phones) even on the primary display, and that for some reason those timings work better than what the primary display itself says to use.

Also, the screen-flickers-in-solo-mode issue have really existed ever since I got the screen about two years ago, just that I haven't bothered trying to solve it as it worked fine with two screens connected.

I can't find a setting for refresh rate in the display settings, so that is a tad more difficult to check, but as for resolutions go, no.

When a game have acted up and wanted to put itself on my secondary (LG) screen (because it's to the left and has pixel coordinate 0,0 so applications frequently treat it as if it is the primary), I've disabled the secondary screen and set the primary (Dell) to 1920x1080 (16:9, same ratio the screen has) in order to get rid of the flicker and be able to play. 1920x1200 (16:10) and 1920x1440 (4:3) also work fine, as do 1280x720.

Mm.. is it possible that maybe your Radeon has some problem supporting double 16:9 1440 resolutions?
I've found other people reporting flickering problem with that GPU, but they're different cases usually about only 1 monitor.

The flicker only happens in solo-mode though, I decrease the resolution to get rid of that. The reason I disable the secondary monitor is that many games start on that one instead (and they're not usually optimised for portrait-mode screens, so I'd prefer if they did not use that one).

Ah, ok then.

Btw, maybe check this dude's solution anyway: https://community.amd.com/thread/190349

He's using Windows though, which means his drivers are different from mine. I'm not using the Crimson/Catalyst/fglrx driver, but the open Radeon driver as it's less likely to break with kernel and X server updates.

I don't think the legacy Catalyst driver is updated for Xorg X server 1.19 (or even 1.18), and I'm not sure AMDGPU (with or without the closed -PRO addition) does not yet support 270 and 270X out of the box (I think support is there, but it's experimental and not yet enabled by default) as they're 1st gen GCN chips, unlike 260 and 290 which are 2nd gen and 285 which is 3rd gen.

I'll see if I can get AMDGPU to work, and if it fixes the issue for me.

Are you near any sources of interference? I don't know if this is one of those monitors, but some of the higher resolution monitors are more susceptible to interference from radio signals.

You need a dual-link cable to properly rule out DVI. I do wonder if you are running a lower refresh rate with both monitors enabled. I would want to try a different DisplayPort cable as well.

Update. I just got a dual-link DVI cable, and tested the auto-detected resolutions and framerates:

DVI-D DL:
2560x1440 59.95 flicker
1920x1200 59.95 flicker
1600x1200 60.00
1680x1050 59.88 flicker
1280x1024 75.02
1280x1024 60.02
1280x800 59.91 flicker
1152x864 75.00
1024x768 75.03
1024x768 60.00
800x600 75.00
800x600 60.32
640x480 75.00
640x480 59.94
720x400 70.08

DisplayPort
2560x1440 59.95 flicker
1920x1200 59.88
1920x1080 60.00
1920x1080 50.00
1920x1080 59.94
1920x1080 24.00
1920x1080 23.98
1920x1080i 60.00
1920x1080i 50.00
1920x1080i 59.94
1600x1200 60.00
1680x1050 59.95
1280x1024 75.02
1280x1024 60.02
1280x800 59.81
1152x864 75.00
1280x720 60.00
1280x720 50.00
1280x720 59.94
1024x768 75.03
1024x768 60.00
800x600 75.00
800x600 60.32
720x576 50.00
720x576i 50.00
720x480 60.00
720x480 59.94
720x480i 60.00
720x480i 59.94
640x480 75.00
640x480 60.00
640x480 59.94
720x400 70.08

On a sidenote, the GOG forums are really shitty at 720x400 and 640x480. Please fix ASAP, I would very much like to visit the forums from my Performa 475 where none of the available browsers can handle anything remotely modern with regards to web standards or security. Please rebuild it in pure HTML4, with minimal JavaScript and CSS.

Weird.. oh well, at least you can still use 1920x1080 or 1920x1200 60Hz