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When adding a new route using "Front", its presentation info order is set to 1, after the master-bus which has PI order 0. (see Session::ensure_route_presentation_info_gap). Mixer_UI::add_stripables, looks up the insert iterator: "after the route with order 0", but since the master bus is not in mixer's track-model, the insert-iterator is left "at end". A later call to `sync_presentation_info_from_treeview` then updated the new track's order key to be at the end.
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