xosd compiles with xinerama enabled by default. However, using either --disable-xinerama or --enable-xinerama disables xinerama, which means the "$(use_enable xinerama)" line disables xinerama support regardless of your use flag. Reproducible: Always
can you attach your build.log too please.
Created attachment 401496 [details] xosd build log, xinerama use flag disabled
Created attachment 401498 [details] xosd build log, USE="xinerama"
Created attachment 401500 [details] xosd build log, USE="-xinerama"
Finally managed to get around to getting the build logs. I uploaded three build logs, USE="xinerama", USE="-xinerama", and a local overlay where I had the "\ $(use_enable xinerama)" part of the ebuild commented out. You should be able to see in the build logs that both --disable-xinerama or --enable-xinerama are disabling xinerama, and I first noticed this when trying to compile xosd locally outside of portage.
can you retry with x11-libs/xosd-2.2.14_p2_p1 version please?