the Arrange Page was in Logic 9. Now Logic Pro X has an Arrange Page on steroids called the "Main Window", that single window with all the window pane functionality.

In my opinion, "Main Window" is a really bad choice for the terminology, because it is used int he OSX Human Interface Guidelines for the Active Window that has key focus.
the command "Close Window" (cmd+W) only closes the currently Active Window. Technically you can have the Main Window and the Score Window open. Closing the Main Window has still the Project running. TO bring back the Main Window, just used the command in the Window Menu (cmd+1)
However, if up close the last window, then it will close the Project.
Hope that helps
Edgar Rothermich - LogicProGEM.com
(Author of the "Graphically Enhanced Manuals")
http://DingDingMusic.com/Manuals/
'I may receive some form of compensation, financial or otherwise, from my recommendation or link.'