Galaxy S6 :: Touchscreen Lagging / Unresponsive / Vibrating Randomly

I factory reset my phone and during the process of signing in, agreeing to T&C's etc, a strange problem started occurring - the touchscreen became super unresponsive (had to press something three of five times for it to respond), however, at the same time, the phone would quickly vibrate on touch multiple times, but again, not actually do anything. Moreover, a weird blue text box started appearing around the options I touched and I can't swipe down to access the normal settings menu.

And to top it all off, if I long hold the power button to force restart/power off, it just won't do it. (tried the old "turn it off and on" trick to see it'll do the trick)

Everything was all normal until this point, had my phone for a couple of months, haven't dropped it or anything.