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See if it still happens in r179. It could happen when navigating back from an image right after changing images (the old image didn't get removed because it was still waiting for the new image to be displayed). It's hard to reproduce, let me know if you still see it happening.
I waited a bit to see if it would still happen, which it has not.
So it seems to be fixed on my end.
Thank you for the quick fix and reply.
Seemingly random image will eventually replace the background (standard black) for all other images.
The size of the new background image is the same as the original image it chooses to display. So it is not stretching the entire background and is sometimes less noticeable, because most images will overlap.
I can not quite tell why and when this happens, but this is especially noticeable when browsing tags or between similar artists and images.
Script is up to date as is Firefox.