On longer surveys the set flight height though static, can see the UAV drift up or down over the ground surface, due to the UAV GPS drifting. The real-time slider height adjustment would help maintain a constant flying height above the ground surface.
DJI does not allow for real-time adjustments to waypoint missions. Currently the only way to change a mission is to pause it (drone hovers), change it (adjust altitude by relative delta of the mission) and reupload/resume the changed mission.
But you can use terrain elevation data to plan a terrain aware flight that keeps constant ground distance using our terrain planning functionality in DH web at app.droneharmony.com.