I ran into this myself on my iPad just now. Contrary to @sebastian-king's experience, I added https://cdn.jsdelivr.net/npm/[email protected]/lib/jquery-taphold/taphold.js and it immediately worked, not sure if he was using a bad version or had an incompatible browser.
Anyway, I submitted #774. Not really expecting it to be merged, but hopefully it may help someone else.
Add taphold.js to enable web interface context menu on mobile
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The taphold feature of jquery-ui-contextmenu is meant to be backed by taphold.js, which is currently missing, as observed in #464. This PR adds it.
taphold.js from https://cdn.jsdelivr.net/npm/[email protected]/lib/jquery-taphold/taphold.js; taphold.min.js from https://cdn.jsdelivr.net/npm/[email protected]/lib/jquery-taphold/taphold.min.js (massaged comment to include author and license).
Fixes #464.