User blog comment:Harbumz/Introducing Vignette, Wikia's New Thumbnailer/@comment-876022-20150213005417/@comment-24880673-20150213053056

Animated GIFs are difficult to handle. We are now doing our best to thumbnail them, but some modes will work better than others (as you found out). The window cropping is probably the most likely to give you a thumbnail with artifacts. Scaling that image to width seems to work ok. You might have to experiment a little to get a decent thumbnail if you really want to window crop it.