I'm talking about that effect when I'm reading a text (any kind of text, textbook, blog, forum text) and suddenly I discover that two minutes passed and I advanced six lines in the text, but I just have no idea of what I read. It's like a time blackhole, and now I have to re-read it.

I do this all the time. I have seen it referred to in literature (a character reading a page three times before realising he can't take it in, as a way to show that he's extremely distracted), but that's not quite the same as just zoning out.