Interesting one, thanks for finding this! Going to https://en.wikipedia.org/w/index.php?title=User:קיפודנחש/gallery_bug&debug=true I get in the console:

mw.Api error:  badinteger 
error: Object { code: "badinteger", info: "Invalid value \"-36.4\" for integer parameter \"iiurlheight\"." }

@Aklapper

first of all, this is different than the error i see in the console when adding debug=1, which is

code: "urlparamnormal"
info: "Could not normalize image parameters for Bamfield_Tsunami_Hazard_Zone_sign.jpg."
docref: "See https://en.wikipedia.org/w/api.php for API usage. Subscribe to the mediawiki-api-announce mailing list at <https://lists.wikimedia.org/mailman/listinfo/mediawiki-api-announce> for notice of API deprecations and breaking changes."
__proto__: Object

2nd, i will recover original description, since regardless of the error message you see in console, the fact remains: you can't display a slideshow gallery inside a collapsed element.

please try to reproduce with any other gallery, or any other level of collapsabilty (e.g., have the gallery one of several things inside the collapser, or the collapsed being table and not div etc.

in hewiki, we use small script copied from frwiki years ago (not sure if they still use it) called "imgtoggle", to allow image expansion in-article, which meas the page is originally generated inside an element with hidden=none (basically, 2 copies of the gallery are created, using different widths= and heights= pairs, where only one of them is visible, and a button is added so the user can toggling which of them is visible. reliable users report this used to work until recently.

peace