Hi,
I'm sure this was introduced in 1101 - I have not seen it before.
When you click on the thumbnail its size may change (sometimes it's changing, sometimes not). Surrounding thumbnails change the size as well.
Below you can see some pictures showing the same portion of window (same thumbs) , just clicked few times:

I hope it's not going to be difficult to fix since this one can really make you confused when clicking around :)

Thanks.