But I am not against your proposal!
Good design, however, is a matter oh hierarchical decomposition (or "abstracton"). Though two levels are by far not enough for larger projects, they will help a lot. Of course the tab feature can be used for any number of levels, but you will not "see" the level, so the main use will be to separate the main module and one level of submodules, which should carry 80% of the code burden....
Puttiing everything in one file - folded or unfolded - is as awkward as putting every class into a separate file 