Millennial couples, and many Gen-X pairs, know this smartphone-related spat all too well. One partner innocently ignores the other because the person’s head is utterly consumed by the 5- or 6-inch screen in front of his or her face. For some, the only thing that keeps this I-can’t-believe-you’re-glued-to-your-phone-right-now battle from turning into a war is that both parties are often guilty as charged at various points during the day.