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Using Expose within Spaces
To drag a file into an application that's running in another one of your Spaces, use Expose while in Spaces overview mode. First, press F8 to go into Spaces overview mode, then press F9 to Expose all application windows. Hover over the application window you want and wait a moment or press the Space bar, and both that space and that application come to the foreground, ready for you to drop the file.
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