fltk-1.3.0_rc3-r1 is set to the same slot as all ebuilds <= fltk-1.1.9-r2 This is not correct, the APIs are different. Yoshimi for instance, will not link against fltk-1.3.0_rc3-r1 but it will link against fltk-1.1.9-r2. Please create separate slots for these. Reproducible: Always
SLOTting has nothing to do with APIs, and everything with concurrent installation.
true, although major changes to APIs are generally concurrent to changes in slots Is it possible to have to both versions of libraries installed, while having the library softlinks point to the newest of the two? (or perhaps toggle between the two using eselect)
slotting is fine, and 1.1.* is deprecated and will be replaced by 1.3.* in the same slot. the packages simply need to be fixed.