@davidfor:
Okay, I'll try to get you a set of those three cases, need to swap stuff around between OS X/Linux, might take a while

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I'll take a look at what happens with the "Show book covers full screen" option disabled on my end, I'd completely forgotten that this even existed ^^.
I updated #255 w/ the gif.
EDIT: Oookay, indeed, w/ "Show book covers full screen", the letterboxing is not that great (it's not eye-gougingly awful either, but, still). Am I right in assuming that nickel simply takes the full-screen image and resizes it? Of that's the case, I'm not terribly hopeful for a solution that handles both cases properly :/.