Galleries in slideshow mode reposition themselves when JS loads. This impacts all skins (shown in gif Minerva, but also impacts Vector).
Replication steps
Create a page with the following wikitext:
<gallery mode=slideshow heights=150px style="text-align:left"> File:Detroit Publishing Co. - A Yeoman of the Guard (N.B. actually a Yeoman Warder), full restoration.jpg|A Yeoman Warder, from a Victorian-era [[photochrom]] File:Official_program_-_Woman_suffrage_procession_March_3,_1913_-_crop.jpg|The Official program of the 1913 Woman Suffrage Procession held in Washington, D.C. File:Joseph Ferdinand Keppler - The Pirate Publisher - Puck Magazine - Restoration by Adam Cuerden.jpg|"The Pirate Publisher", a satire of copyright infringement from the magazine ''[[Puck (magazine)|Puck]]''. </gallery>
Expected behaviour
For users with JS, gallery should reserve the space it needs and not resize/rearrange itself when the code has loaded. It may possibly preload the icons, but definitely should reserve the space needed for that part of the interface.
With JS disabled, it should behave per normal - client-js and client-nojs classes will be needed here.
This blocks enabling slideshow on mobile (see parent task).