Raspberry Pi 400 with 3440×1140 screen resolution
New Raspberry Pi 4 and 400 support HD which is switched off by default. Even turning it on will not show Ultra Wide HD resolution 3440×1140 on the list of available options, so how to add custom resolution? Simply edit a configuration file. The instruction works on recent versions of Raspberry Pi OS and Manjaro.
To do so, open file
sudo nano /boot/config.txt
You can leave all hash-tags and simply paste the code below on the end of file:
hdmi_cvt=3440 1440 60 7 0 0 1
Ctrl+x, y and Enter will save the file.
Now restart you machine.
This solution works on Raspberry Pi 400 (it shouldn't make any difference on RPi 4) with iiyama Red Eagle G-MASTER monitor.