Aegisub, on both available versions of the package fail to build with an error like this:

/var/tmp/portage/media-video/aegisub-9999/work/aegisub-9999/src/colour_button.cpp:21:10: fatal error: boost/gil/gil_all.hpp: No such file or directory

this file (boost/gil/gil_all.hpp) has been removed from boost-gil in a refactor commit 32fec9f05ba8d558c7af7b1bd537c80f31c2b826 (falling at some point between boost-1.68 and boost-1.69)
As 1.4 uses boost-1.70, which has that file removed, Aegisub fails to build.

The ebuild for aegisub should require and use boost from an earlier version slot.