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The problem is that the keyboard changes the visible area but the scroll view has no idea that just happened. The scroll view still thinks it has the whole screen to display its content, and from its perspective, that’s plenty of room. We need to tell the scroll view that the visible area is smaller now that the keyboard is there.
iOS tells you about the keyboard, but doesn’t tinker with your views.
Just because iOS knows that the keyboard is there, it doesn’t know how your app wants to handle it. That’s up to you!