SUMMARY
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Currently on Plasma 5.27 the Overview displays all the Virtual Desktop Thumnails regardless of whether the Virtual Desktops are configured for display as a Grid or not. In Plasma 6.x this is not what happens..  

2. In the case of Plasma 6.x, if the Virtual Desktops are configured to display as a Grid, the Overview functions as if it were 'Display all Windows on Current Desktop'. Conversely, if the Virtual Desktops are configured so that the Overview correctly display all Virtual Desktops the Grid effect does not display as a Grid, but as a row of Virtual Desktops.



STEPS TO REPRODUCE
On Plasma 6.x
1.   in Workspace Behaviour -> Virtual Desktops create 4 Virtual Desktops in 2 rows of 2
2.  Trigger both Overview and Desktop Grid alternately
3.  Note that when Overview is triggered it displays with no Virtual Desktops along the top bar, which is missing. Note that when Desktop Grid displays a a Grid
4.  in Workspace Behaviour -> Virtual Desktops create 4 Virtual Desktops in a single row of 4
5.  Trigger both Overview and Desktop Grid alternately
6. Note that overview display correctly with the Overview Bar at the top display thumbnails of the 4 Desktops, and the Virtual Desktops are selectable

Note that when Desktop Grid is triggered, it displays the Virtual Desktops as a single row, not as a grid.

OBSERVED RESULT
When the Virtual desktops are created in Multiple rows the Desktop Grid display correctly as a Grid, but Overview appears as if it is the 'Display all Windows on the Current Desktop' effect

When the Virtual Desktops are created in a single row, the Overview displays correctly, but the Desktop Grid does not.

EXPECTED RESULT
That Overview on Plasma 6 act as it does on Plasma 5, and ignore the rows when the Virtual Desktops are created in multiple rows, for the Grid effect, and therefore display correctly as an Overview, as it does of Plasma 5.27.


SOFTWARE/OS VERSIONS
 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION

I have been testing KDE Plasma on KDE Neon Unstable and now Stable, Manjaro KDE Unstable, Fedora 39 KDE  All running KDE 6.01 and 6.02, KUbuntu 22.04 and Manjaro KDE Stable, both running KDE 5.27.

With the exception that on KDE 5.27 the Overview has limited options... no ability to disable Add/Remove Virtual Desktops, both the Overview and the Grid functions well.

In comparison there are issues with the Over/Grid combination on Plasma 6 that make the Overview and the Grid an either or choice, but where neither functions well.