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YouTube is now hiding the skip button on mobile too
(www.androidpolice.com)
This is a most excellent place for technology news and articles.
It is an ad-jumper!
You're right. it's uBlock or some other addition that removes the pre-roll ads. But googling for Sponsorblock is a good start to find such apps. I even got some patched youtube app for my TV... So it's no ads for me and not even sponsored segments. Not on the computer, not on my phone and neither on the TV.