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Been using XOOPS since the beginning times. And have used it on all my sites......The one thing all users seem to ask me nowadays. Can I use my phone to check posts, reply to forums, download etc.....I unfortunately always have to say no.... There was a pseudo module quite a time ago......and then haw haw.....but no real interaction with XOOPS was available.....So for XOOPS to be MOBILE compatible would be a GIANT leap for Xoops.
So my suggestion - A CORE update/fix/rewrite or module that makes XOOPS mobile.
It is after all the way of the future - Oops the future is already here.....