The belief* I had was that it is actually long periods of focus at a restricted set of ranges, eg some kind of atrophy. Though as in the footnote I realize this could be absorbed nonsense that followed the standard “screens == poor vision”. Happily when I was a child the threat to vision was reading books, which modern children are no longer at risk of ;)

* when I went to write this reply I realized that I don’t recall actually reading a paper on it, so it’s quite possible that it’s just another “everyone knows”-type fallacy