Comment 32 for bug 1349838

You mean this?

   g_signal_connect_swapped (pad->priv->toolbar, "activate-undo", G_CALLBACK (xpad_pad_undo), pad);

It is not needed animore. This is though (otherwise there will be no undo element):

   MENU_ADD (_("_Undo"), "edit-undo", GDK_KEY_Z, GDK_CONTROL_MASK, xpad_pad_undo);

PS Looks like to bring back undo/redo of making bold or strikethrough one has to write GtkSourceUndoManager https://developer.gnome.org/gtksourceview/stable/GtkSourceUndoManager.html#gtk-source-undo-manager-can-undo-changed and this brings back to where it started. I'll try to find a way to do it without implementing Undo back again.