There was a small gap in the corners of the thumbnailArea highlight rectangle that was noticable on dark thumbnails.
- Remove radius from thumbnailArea highlight
Having the same radius as the thumbnailArea allowed it to fit perfectly when the thumbnails weren't filling the corners of the thumbnailArea, but it still left a small gap on thumbnails that completely filled the thumbnailArea. Removing the radius makes the highlight slightly overlap with the inner corners of the thumbnail outline, but it's not really noticeable.
Another alternative would be to turn the thumbnail outline into a real outline instead of a rounded rectangle behind the thumbnailArea. Then it could go on top of the thumbnailArea and the thumbnail highlight could fit perfectly inside of it. This would give previews of rectangular images rounded corners and is significantly more complicated than the current change, but I feel like it should be possible.
If you and anyone else reviewing this patch thinks it's OK, I'll just remove the radius for simplicity and consistency, but you seemed like you didn't want small bits of whiteness in the corners:
BTW, that last screenshot you quoted has 3x scaling