Hi
without staking too much credit knowing much about this, I believe closing the window and closing the app are different in that the former closes "the file" one is working on whilst allowing to save an instance of it, open another file, or start a new one; whereas the latter obviously closes the application altogether.

This behavior is or became common in most 'modern' OSs (post Windows 95, Microsoft called it Multiple Document Interface. M.D.I.). But, the case was, until one point (in the era of Single D.I.) one could only have one-file-open for each running instance of the application, from AutoCAD v 1 through AutoCAD 14, as is currently MoI or SketchUp in Windows (SDI), unlike SketchUp for Mac (MDI)...

The issue becomes, how many different files open simultaneously can a single application (MoI Mac or Windows) manage?

Cheers,

. Diego .