Bug: Asset pane thumbs often have black bars above and below thumb

[dealio]

I've noticed that often the thumbnail images of assets in the asset list pane (top pane on the right) will have black bars added above and below the thumbnail image, though the image that I supplied to the application does not contain those black bars. These black bars never overlay the thumbnail, but they obscure the image so much that sometimes its difficult to tell what the image is. Note that I have noticed black bars on the left and right sides of thumbnail images as well, though the original images had no such bars. The combination together makes for a screen of thumbnails that are hard to discern. Also, the thumbnails are very tiny on my 4K monitor, but that's another matter (feature request: allow specifying the size of the thumbnails in Preferences, or auto-size them using monitor DPI).