Modern FLTK has a new "native" file chooser, that on certain platforms gives you a more familiar file chooser. This currently works on Windows and Mac OS X. On X11 you still get the old boring one.
*** Bug 4543 has been marked as a duplicate of this bug. ***
We're getting more feedback that this isn't just a cosmetic issue, but a functional one. The FLTK file chooser follows a very ancient design which current users aren't familiar with as other file choosers have advanced over the years. This could be more than an annoyance and could actively prevent users from using some functionality (primarily local drives).