Years ago, I remember watching a webcast of the introduction of the Aqua user interface when Mac OS X Public Beta was first demoed. The part I distinctly remember was realizing the brilliance of sheets. Like many great innovations, they were simple in retrospect and solved a problem you didn't realize you had: the modality problem — the fact that dialog boxes blocked interacting with the whole application even though only one window needed the information that you, as the user, had to provide. I watched in wonder as a save dialog blocked only the one window that needed saving, leaving all the other windows free. Finally, a solution to limit the modality. Because modality sucks. Back in 2000, sheets worked well because the smallest unit of user interaction with an application was a window. Soon after, though, things started to change. Web browsers in particular were among the first to start using tabs to put more than one document in a window.
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