gtkmm4 aboutdialog set_logo: does not respond to icon size requests.
In the gtkmm4 The call
aboutdialog->set_logo(Gdk::Texture::create_for_pixbuf(Gdk::Pixbuf::create_from_resource ("/org/gnome/plotter/plotter.png", 128, 128, true)));
No longer scales/resizes the icon, it always appears tiny, for example, replacing 128 with 256 doesn't change anything. The gtkmm4 book example code does this also.
In gtkmm3 the equivalent call was
aboutdialog.set_logo(Gdk::Pixbuf::create_from_resource("/org/gnome/plotter/plotter.png", 128, 128, true))
which still works fine. The C gtk4-demo aboutdialog seems to scale okay too.
gtkmm4 (about dialog looks kind of ridiculous)
gtkmm3 (about dialog looks fine)