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It seems like
g_app_info_launch_default_for_uri() first checks if the current app supports the MIME type and use that in that case, instead of asking directly the portal to open the default application. Indeed
> xdg-mime query default text/html org.gnome.Epiphany.Devel.desktop
But it's Builder that gets opened here when opening a remote link. But Builder doesn't seem to support well the http links through GVFS, as it crashes at https://gitlab.gnome.org/GNOME/gnome-builder/-/blob/master/src/plugins/beautifier/gb-beautifier-config.c#L658, likely because the workdir's path is NULL here (haven't checked yet with gdb).
At the end I might need to go through libportal, though GLib/GDK should work fine here…