I have switched my panel from being on the bottom to being on the side. The desktop is configured to have desktops in one row (by default) but doing so and having the pager in a vertical panel means that the desktop are only a few pixels large, and rather invisible. So I had to open system settings and change to have them in 2 rows. I understand that we want to keep the same shape in the plasmoid as when the desktops are presented or other operations happen, but perhaps it could rotate itself of 90°? It's more of a design problem really, but at first I was not even sure of what was that small grey box that I was seeing on the panel. Reproducible: Always
There is a 'Layout:' dropdown in Activity Pager settings - it also can be added/used in Pager settings - but it's 'Default' option should be changed into 'Automatic' to get optimal UX after changing of panel orientation
*** This bug has been marked as a duplicate of bug 348792 ***