I'm not into navboxes, but I managed to get it work i think :) I just had to add the "mv-collapsible" class to the table-tag in the Template:Navbox. This class is used to make any table collapsible. Since navboxes are nothing else than some advanced tables, this worked just fine, with 0 javascript :)

But one css rule is needed to color the "hide/show" button so you can see it:

/* ---- Collapsible Navbox ------------------------------ */
.va-navbox-title .mw-collapsible-toggle a:link {
    color: white !important;
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