I noticed that page previews is currently not working for ES5 users as ext.maths.popups is an ES6 module which causes the mw.loader call inside ext.popups/index.js to reject meaning the feature never gets activated. This was broken in Iefe98c1f0422dbf034e385b1a41a859d030a2cf4.

The problematic code is:

mw.loader.using( ['ext.math.popups', 'ext.popups.main' ] ) ).then( success, reject )

Because ext.math.popups is an es6 module it will be an unknown module on ES5 browsers leading to the reject handler to call and ext.popups.main to not be loaded.

I talked to @ovasileva about whether we should restore ES5 support for page previews, and given the recent trend away from IE11 (T178356) and inside Vector 2022 (most features are ES6 now), we decided it made sense to drop page previews support for ES5 users.