The current single-project paradigm was decided upon early on when Simply Fortran was first being designed.  While multiple projects would be useful, only displaying a single project maintains simplicity.  The original concept was, if a user wants multiple projects, another instance of Simply Fortran could be started.

That said, I'm only explaining the thinking in the above paragraph being the single-project design.  The design decision certainly isn't set in stone.  Multiple projects in a single session would be useful, but some careful planning will be necessary to ensure the overall simplicity of the IDE isn't compromised significantly.  I'll add it to the list of items to consider.

When switching from one project tab to another, there'd be no need for Simply Fortran to swap any of the files tabs.
The project tabs would just be great way to switch to a different project.

Does one really ever need to display more than one project?  Very yes!  A project under development often calls library routines.  When a library, or several libraries, is/are also (earlier or concurrent) software development projects, it often happens when developing an application project that one needs to make small changes to one of the library routines.  Example, changing some of teh passed parameters.
---
John