If you have a lot of virtual desktops, ie. more than 15 on a 1920x1080 monitor, the strip of virtual desktops of the overview effect doesn't show any desktop beyond the 15th. It *is* possible to see the other desktops by left/right-clicking on the strip and dragging it to the left, but it seems uninintuitive to me. I would have expected to be able to move the strip by scrolling over it with the mousewheel and, perhaps, even have a scrollbar.
It is scrollable, but you're right that it doesn't visually show a scrollbar. We should fix that.
At least on 5.24.0, it's not scrollable; likely a patch that didn't land at this release.
(In reply to Nate Graham from comment #1) > It is scrollable, but you're right that it doesn't visually show a > scrollbar. We should fix that. It is not scrollable with a standard mouse that doesn't include a horizontal scroll wheel, at least not in Plasma 5.26.
You can scroll horizontally using a one-axis wheel mouse by scrolling up and down while holding down the Alt key (or maybe it's shift, I can never remember).
*** Bug 492373 has been marked as a duplicate of this bug. ***