Based on user feedback and discussion of nb-usability team:

The state of Projects, Files and Favorites tab (expanded nodes) should be 
persisted between IDE sessions to find the "workspace" - IDE in the same state 
as when "leaving" - closing it.

This functionality has already been tested but was removed due to performance 
problems. But there are another performance improvements possible to make this 
feature really usable - for example late restoring of just visible tabs instead 
of restoring state of all the tabs during IDE startup.

Based on wider discussion, this feature should be optional - appropriate 
checkbox in IDE Options - as many users won't use it. On the other side many 
users are missing this behavior.

I'll provide more concrete data & proof of concept/reference implementation 
after retouche & related IDE changes come to stable and optimized state.