There is no longer gtk_header_bar_set_[sub]title() after https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/1807 has been merged. The alternative is to use a vertical box with two labels, assigning GTK_STYLE_CLASS_[SUB]TITLE styles to them.
Created attachment 398438 [details] Patch
Committed r261144: <https://trac.webkit.org/changeset/261144> All reviewed patches have been landed. Closing bug and clearing flags on attachment 398438 [details].