Page "Multiple document interface" Paragraph 35
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Wikipedia
Mac OS and its GUI are document-centric instead of window-centric or application-centric.
Every document window is an object with which the user can work.
The menu bar changes to reflect whatever application the front window belongs to.
Application windows can be hidden and manipulated as a group, and the user may switch between applications ( i. e., groups of windows ) or between individual windows, automatically hiding palettes, and most programs will stay running even with no open windows.
Indeed, prior to Mac OS X, it was purposely impossible to interleave windows from multiple applications.
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