A design file shows a product on a good day. The account has data in it, the network is fast and the name fits on one line.
Before I build a screen I write down every state it can be in and what the person should do next in each one. The list is usually three times longer than the number of frames I was given.
Taking that list back to the designer early is the most useful thing I do on a project. It is also where most of the real design decisions turn out to be hiding.
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