From #wikimedia-multimedia:

2017-03-01 20:29:56 <matanya> marktraceur: hi, T140596: Have a constant height between images would be appreciated in rotating reader-interactive "slider" gallery mode was merged for a few minutes on he.wiki
2017-03-01 20:30:45 <matanya> it seems like it caused the left-right buttons to go between items of the slideshow to jump between images