Apparently the problem is software-related, nothing to do with the in-cell touch display, as it mostly appears in certain situation where Apple now separates the keyboard pop-up window from the app it is running in, like during the above-mentioned Apple ID input. It has, however, also been appearing in app icons and individual letters.
…Apple is aware of the issue & that they’re working on it. It will be patched in an upcoming iOS update. It’s confirmed to be a software issue. Why it only happens on some devices, she didn’t have details. The bottom line is that they’re working on it now & we will have a solution.”There has allegedly already been a customer complaint reply from Apple's service, saying that the issue will be fixed with the next update: “
Have you experienced the "static lines" on-screen keyboard software glitch with your iPhone 5 unit?