Comment 18 for bug 1746146

From the JPEG FAQ:
> In particular it is possible to do 90-degree rotations and
> flips losslessly, if the image dimensions are a multiple of the file's
> block size (typically 16x16, 16x8, or 8x8 pixels for color JPEGs).

In contrast to eog, gthumb does detect this situation and displays a dialog with the following text:

This transformation may introduce small image distortions along one or more edges, because the image dimensions are not multiples of 8.

The distortion is reversible, however. If the resulting image is unacceptable, simply apply the reverse transformation to return to the original image.

You can also choose to discard (or trim) any untransformable edge pixels. For practical use, this mode gives the best looking results, but the transformation is not strictly lossless anymore.

It then gives the options: "Trim", "Cancel" and "Accept distortions".