Once every app can be relied upon to work in a scalable (by screen resolution), full-height UI, there’s no reason they can’t begin to coexist on the same screen. The three-finger swipe currently used to navigate between full-screen apps could, in a pinch, resize them.
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If you liked this, you'll love my book. It's called Designed for Use: Create Usable Interfaces for Applications and the Web. In it, I cover the whole design process, from user research and sketching to usability tests and A/B testing. But I don't just explain techniques, I also talk about concepts like discoverability, when and how to use animations, what we can learn from video games, and much more.
