How? Sensor fusion with a gyroscope will help transient problems, like walking past some iron rebar, but would have to drift back to the magnetometer's reading

How? Sensor fusion with a gyroscope will help transient problems, like walking past some iron rebar, but would have to drift back to the magnetometer's reading if in a long-term situation. Using the GPS as a compass is pretty sketchy, too, AFAIK. Is there some smarter trick?