gtksourceview3 was superseded in 2018 by gtksourceview4 (a new major
upstream release, but still using GTK 3). There's a porting guide here,
which is mostly "stop using deprecated APIs" plus a few function renames:
https://gnome.pages.gitlab.gnome.org/gtksourceview/gtksourceview5/porting-guide-3-to-4.html

gtksourceview3 is unmaintained and hasn't had an upstream release for more
than 5 years, so it would be great if the remaining gtksourceview3
applications can move to gtksourceview4 before trixie.

gtksourceview4 has itself been superseded by gtksourceview5, which is
for GTK 4 applications. Ideally GTK 3 applications should move to GTK 4,
but that's a larger and more intrusive change. Porting guide:
https://docs.gtk.org/gtk4/migrating-3to4.html,
https://gnome.pages.gitlab.gnome.org/gtksourceview/gtksourceview5/porting-guide-4-to-5.html

Thanks,
    smcv

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