Bug 492373

SUMMARY
Since introduction in 5.24, the Overview screen's Desktop Bar will overflow off-screen with extreme desktop counts. While this does not cause a crash or error, it causes controls to draw off-screen. 

With default DPI/no scaling, the horizontal (1 x n) Desktop Bar will overflow at:
2560x1440 - 16 desktops
1920x1080 - 13 desktops
Similar overflows would occur on a left-edge Desktop Bar (when desktops are n x 1 layout), and in a non-linear state (2x2, 3x3, 4x4 grid). A 4x4 grid user with 16 desktops would be the most likely to encounter this issue in practice. 

Screenshots from my Plasma desktop illustrate the issue at 4-9-16 desktop count. 
Similar screenshots from my Macbook demonstrate the same count. 

MacOS's design mitigates overflow in order of appearance of the effects (vice versa as desktops are removed):
-At 10 desktops: shrink previews + zoom/accordion on hover
-At 12 desktops: hide labels unless hovered
-At 16 desktops: prevent creating more desktops

Obviously KDE Plasma is not MacOS, we can find an elegant way to handle this that doesn't just copy Apple. 

I think that reading in the screen dimensions+DPI, then defining a ratio of "desktop count : screen length" could be sufficient to trigger an overflow state, where a solution like a scrollbar container could kick in, or scale the previews down and up in that state. 

I'll help where I can, but it's not a high priority for now.