Hello, Klaus!
I noticed a strange halo that arises when using a transparent gradient png-mask.
This effect appears on iPhone 7, iOS 10, Chrome / Safari.
Did you know something about this problem?
http://n.frylark.ru/@temp/_bugs/mask_png/ - example pano
http://n.frylark.ru/@temp/_bugs/mask_png/data/14_01_snap.jpg - halo
http://n.frylark.ru/@temp/_bugs/mask_png/data/14_01_mask.png - mask