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(Thumbnails can even display larger than 91 by 91 now, the previous display limit. If you have Apollo Maximus, check him out. His thumbnails is HUGE when you select it. )

Actually you have been able to make larger thumbnails for years. I think it was back from Poser 4 Pro but it may have been Poser 5 not sure. Anyway, I've made a lot of my thumbnails 182x91 for years (double height) because they are much nicer to look at and easier to find and see. They still look nice and big when clicked, and as small as anything else now when just displaying in the library.

I have the same problem with keyboard scrolling btw. When I use the scroll wheel on my mouse it skips soome bits, pushing them off screen. But with the keyboard I can only scroll 5 times before it stops responding. If I then click a thumbnail I can scroll 5 more times with the arrow keys ect.
So I'd say that is a genuine bug. It should either work all the time or not at all :)

Is there an official place to file bug reports?

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.