A new feature in MacOS Sierra lets you put several files into one window and navigate between them using tabs.  This is a great way to work on multiple files at once, especially on a small screen. Apple calls this feature "Tabbed Windows".

When you create or open multiple files with Prism Mac, it looks like this. Each project file is in its own window with its own toolbar.

Until Apple released MacOS Sierra, this was the only way to work with multiple files in Prism Mac. But Sierra lets you put them all into one Window. Drop the Window menu on any of the Prism files, and choose "Merge All Windows".

Now there is only one Prism Window, with a set of tabs just below the toolbar that let you choose which project file you are working on:

Notes:

•To work on another window, click the + button to the right of the file selection tabs which will bring up the Welcome dialog.

•There are three ways to move a file into its own window and away from the tabbed window:  Control-click on the tab and choose Move Tab to New Window; grab the tab and drag it away from the joint window; or drop the Window menu and choose Move Tab to New Window.

•There is no way to have one window with one set of files, and another window with another set of files. Each file is either in its own window, or in the single tabbed window.

•Keyboard shortcut: Hold Control and click Tab to move to the next project file in the tabbed window. Hold shift and control and click tab to go to the prior project file.

•Putting multiple files in one tabbed window is simply a convenience to make it a bit easier to move between files. The files are still all distinct, saved in different names. Putting multiple files into one tabbed window does not merge the files.