"witholding" means the opposite of what you think it does, it means "preventing" or "holding back" in this context :)

Also I suspect a lot of people, certainly myself, would find this behaviour quite unsociable, and hard to bear during every conversation. The reason I think, is because it is so easy for someone doing this to slip into focussing on social media, or some news article, privately. It's also hard to tell whether you were temporarily ignoring me or just refreshing your memory on something we are talking about.