Passing fundamentals to functions no longer works.
This was discussed briefly when the argcache work was being done but slipped by.
Essentially, the argcache was written prior to GJS supporting fundamental types (argcache originates in 2013, we started supporting fundamentals in 2014) and the regression in GJS isn't especially noticeable currently because in the Shell fundamentals are really only used in off-by-default features (e.g. mouse zoom, looking glass highlighting) because they access graphics libraries which deal in fundamental types.
From gnome-shell#3237 (closed), the below error occurs when a fundamental is passed into a function - I think because GJS is attempting to use the GObject argcache handlers instead of handling the fundamental case.
Oct 06 14:37:42 localhost.localdomain gnome-shell[4659]: JS ERROR: TypeError: Object 0x299711dc9040 is not a subclass of GObject_Object, it's a GFundamental_Object
vfunc_get_preferred_size@resource:///org/gnome/shell/ui/magnifier.js:55:37
To prevent regressions with fundamentals in the future we need to build out Regress' fundamental support and make testFundamental.js
more comprehensive (there is currently one very minimal test)