General Support: How keep navigation thumbnail order constant for all large images (in Softer)?

I am using Plogger3 with Softer theme, with thumbnail navigation enabled.
I have at most 9 images in an album. The size of my large images and thumbnails allow me to show all the navigation thumbnails under the large image.
The images are sorted by filename ascending. The image names will be similar to 1.jpg, 2.jpg, etc.

As I click from large image to large image, the order of the thumbnails changes, keeping the thumbnail for the current large image centered under the large image.

Instead of this change in thumbnail order, as I click from large image to large image I want to keep the thumbnail order constant, with the first thumbnail corresponding to large image 1.jpg, the second thumbnail corresponding to large image 2.jpg, etc.

I've looked at plog-functions.php, especially the code starting at line 1973 [ function plogger_get_thumbnail_nav() ] and ending at line 2059 [ /*** End of Picture loop functions ***/ ], but have not found a way to alter the code to keep the thumbnail order constant.

Advice on which file and which code I should alter and how to alter it would be greatly appreciated.