SUMMARY

The virtual desktops configuration module appears to be broken with regards to the setting of the arrangement of virtual desktops in multiple rows.

If you ask for the arrangement of virtual desktops in multiple rows, the requirement is not obeyed and the kwinrc file is not correctly updated.

For instance if you ask to have 4 virtual desktops and you ask for them to be arranged in two rows, using the system settings configuration module for virtual desktops, the requirement for two rows is ignored (or you do not get the apply button active at all after the change) and you get the 4 virtual desktops arranged in a single row. If at this point you go again to the configuration module, you will see that it is still set up for a single row.

This is a bug of the configuration module, not of kwin. If you edit kwinrc with a text editor and you put the correct info in there (by going to the [Desktops] section and by filling in the number you want at the Rows= entry, you get the behavior you want).

This may be the same issue as in bug 421992, but I am unsure because the description there is very brief.

The issue is seen in (k)ubuntu 20.04 (plasma 18.5), 20.10 (plasma 5.19.x) and in manjaro 20.10 (that now uses plasma 5.20.2, I believe).

STEPS TO REPRODUCE
1. Start with a new user, so that the plasma configuration is fresh and you have a single desktop (no virtual desktops)
2. Go to system settings -> Workspace behavior -> Virtual Desktops
3. Try to configure your system to use 4 virtual desktops arranged in 2 rows

OBSERVED RESULT

You get 4 virtual desktops in a single row and you cannot change that from the GUI. Only by manually editing kwinrc you can obtain the two rows.

EXPECTED RESULT

You should be able to setup your virtual desktops in your desired number of rows.

SOFTWARE/OS VERSIONS

Linux/KDE Plasma: (kubuntu) 20.04, 20.10, Manjaro 20.10
KDE Plasma Version: from 5.18.x to 5.20.x
KDE Frameworks Version: 5.68.0 - 5.74.0 - 5.75.0 
Qt Version: 5.12.1 - 5.15.1

ADDITIONAL INFORMATION