Rollable smartphone concepts have come and gone in the past few years, but we've yet to see an actual product using a rollable display (there are, however, rollable-screen laptops already).While the rollable smartphone concepts we've seen thus far have all rolled the screen out of the side, vivo has patented something else entirely - a rollable screen that becomes longer, not wider.The vivo rollable device pictured in the patent application becomes taller as you roll out the screen, which is definitely interesting to see, yet we can't help but wonder what the actual use cases for...