my WISH is for the indication of the part of the text, you are viewing in the main window, in the yellow-bordered rectangle in the thumbnail-view to be reversed..
now the part of the text, you are viewing in the main window, in the yellow-bordered rectangle is grayed and the rest unchanged i.e. bright in the thumbnail-view.
for me it is clearer/more logical if the part of the thumbnail(s), you are viewing in the main window, in the yellow-bordered rectangle is bright and the rest is grayed in the thumbnail-view.

Reproducible: Always

Steps to Reproduce:
1.start okular with thumbnails
2.
3.

Actual Results:  
the part of the text, you are viewing in the main window, is grayed and the rest unchanged i.e. bright in the thumbnail-view.

Expected Results:  
the part of the thumbnail(s), you are viewing in the main window, is bright and the rest is grayed in the thumbnail-view.

not sure if this is a bug as such, but for me it was unexpected behavior.
thus I submit this as a wish