I'm not sure if I could have hit some key combo accidentally to do this, but I don't know one that would do this, although that isn't to say it doesn't exist. I had about 6 tabs open, 3 with open files in nano, then as I was editing one of them the entire terminal and all it's tabs vanished. Luckily when I started another terminal there was a filename.save and identical filename.save.1 of the file I was editing (although I only had that file open in one terminal window with nano) so nothing was lost. Trying Shift Ctrl Q or Alt F4 brings up a dialogue when a process is running or multiple tabs are open.
Not sure how artix related , if you think, that's a bug, please report in upstream (xfce bugzilla).
Unless it happens more often or others find it too then I'm not sure what it is, or if it was nano or the terminal or what. It could be a cosmic ray or random HW related bitflip. There was nothing in syslog or Xorg.log, I was looking at a maximised terminal window then it was an empty desktop. So I was only mentioning it in case anyone else encountered it, it's not easily reproducible as I often use the terminal in the same way including today, and it hasn't crashed before or since.
One other possibility is that I opened the terminal, then ran an update:
[2020-08-19T01:45:11+0100] [ALPM] upgraded iana-etc (20200720-1 -> 20200812-1)
[2020-08-19T01:45:12+0100] [ALPM] upgraded apache (2.4.43-2 -> 2.4.46-1)
[2020-08-19T01:45:12+0100] [ALPM] upgraded fuse-common (3.9.2-1 -> 3.9.3-1)
[2020-08-19T01:45:12+0100] [ALPM] upgraded fuse3 (3.9.2-1 -> 3.9.3-1)
[2020-08-19T01:45:13+0100] [ALPM] upgraded kbd (2.2.0-5 -> 2.3.0-2)
[2020-08-19T01:45:13+0100] [ALPM] upgraded mpg123 (1.26.3-1 -> 1.26.3-2)
[2020-08-19T01:45:13+0100] [ALPM] upgraded perl-libwww (6.46-1 -> 6.47-1)
[2020-08-19T01:45:13+0100] [ALPM] upgraded thin-provisioning-tools (0.8.5-3 -> 0.9.0-1)
[2020-08-19T01:45:13+0100] [ALPM] upgraded xfsprogs (5.6.0-2 -> 5.7.0-1)
And then I continued without rebooting or restarting the terminal - so perhaps something there replaced something that the original terminal / nano process was expecting to find.
Try change gtk2/3 theme, try use more "standard" theme like adwaita only. You can open xfce-terminal in another terminal so see what comes for output.
Thank you for the ideas, I was already using Adwaita and the window manage theme is set to default. Nothing similar has happened today so it's probably nothing to worry about, but I think it's useful to mention unusual things in case they are relevant.