as in description, dropbox should depend only on ncurses:5/5, which is stab package, providing library.
|| ( sys-libs/ncurses:5/5
should be just:
sys-libs/ncurses:5/5 as this is corrrect dep. Under certain conditions portage unable to understand correct order of deps, i.e, it can get unmerged from box with --depclean. And then after world update it cannot choose correct one. Precisely it's maybe a some kind of portage dep resolution bug. Easy workaround is to use just one correct dep.
!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:
(sys-libs/ncurses-6.0-r1:0/6::gentoo, installed) pulled in by
sys-libs/ncurses:0/6= required by (app-editors/nano-2.5.3:0/0::gentoo, installed)
(and 27 more with the same problem)
(sys-libs/ncurses-5.9-r5:0/5::gentoo, ebuild scheduled for merge) pulled in by
sys-libs/ncurses:0/5 required by (net-misc/dropbox-3.18.1:0/0::gentoo, installed)
above (in Actual results) is the output of emerge -uNDa @world.
You cant, the dep in the ebuild is correct. It is due to a failed SLOTmove for ncurses a while ago.
for stable users the correct dep is ncurses:0/5
for ~arch uses the correct dep is ncurses:5/5
5.9-r99:5/5 is a blank ebuild only for bridging the broken slotmove and will eventually be removed.
~arch will have 6.0-r1:0/6 and ~5.9-r101:5/5 installed.
stable will have 5.9-r5:0/5 installed