Screen view
Why does my screen sometimes move down like it wants to do a split screen but not let me do anything until I tap it a few times to make my screen go back to normal?
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that would be a feature called 'Reachability'. Extremely helpful when you are doing stuff on your phone one-handed and can't quite reach a portion of the screen. this happens when you do a slight downward swipe at the bottom of the screen
Bring screen items within reach on iPhone - Apple Support
you don't have to tap randomly on the screen to cancel it..
if it's pulled down, notice that subtle upward arrow right where the screen splits? just tap on that, or tap and drag up.. or the feature times out in ~10 seconds and the screen returns to normal