app_do_GetFeaturesAsTopoJSON unpredictable (and mostly wrong)
As found with pgtopo_update_gui#13 (closed) the app_do_GetFeaturesAsTopoJSON function is generating the "arcs" array of the TopoJSON object with unpredictable arc indices, thus producing unreliable output.
I'm surprised we didn't catch this before, but maybe given arcs are inserted in the arcs map in the order they are spotted we may get the result we need, except if PostgreSQL decides to use some form of optimized scan strategy.