Since kde 4.10.1 at least, I can no longer see preview of the window icon on the pager's virtual desktop button. Reproducible: Always
Yes, it just started for me also on Fedora 18, upon updateing to 4.10.1. Very disconcerting.
Can't reproduce this with KDE 4.10.1, Kubuntu 13.04 beta 1 Might be a good idea to check your configuration, e.g. test with a new user.
(In reply to comment #2) > Can't reproduce this with KDE 4.10.1, Kubuntu 13.04 beta 1 > Might be a good idea to check your configuration, e.g. test with a new user. OK, after the next update cycle, the preview thumbnails are back. Relieved.
Did you use air or Oxygen standart theme? Try to change theme to check. I do remember some theme engin change may cause old theme broken.
I tried this a new user, and the bug appears to persist more. The pager widget takes space, but you cant see it. It's completely blank there. Right clicking on that area shows pager options, but the pager itself is invisible. Blindly clicking on it does nothing (no desktop switch).
*** This bug has been marked as a duplicate of bug 316577 ***