the following C++ code works fine, except that I do not know how to select multiple folders: each time I click on a folder the previous one is unselected…
When using the option Fl_Native_File_Chooser::BROWSE_MULTI_FILE for files, I am able to select various files

Fl_Native_File_Chooser fnfc;
fnfc.title("Select folders");
fnfc.type(Fl_Native_File_Chooser::BROWSE_MULTI_DIRECTORY);

fnfc.directory(DefaultFolder);           // default directory to use
vector<string> VS;
// Show native chooser
switch (fnfc.show()) {
case -1: Console << format("ERROR: {}n", fnfc.errmsg()) << endl;    break;  // ERROR
case  1: Console << format(("CANCELn"));                      break;  // CANCEL
default:
    for (int i = 0; i < fnfc.count(); i++)
        VS.push_back(fnfc.filename(i));
}
return VS;

I would expect to select multiple folders, as it works for files