What is the right behaviour now?

In this situation, when another vehicle is overtaking you, the correct and safest action according to German traffic regulations is to brake if necessary to avoid a dangerous situation, especially if the overtaking vehicle does not have enough space to return to the lane safely. You should not accelerate or change your lane unexpectedly. Braking helps maintain a safe distance and gives the overtaking driver enough room to finish the maneuver without risk. This approach is designed to prevent accidents and promote predictable, safe behavior on the road. Continuing to drive steadily or swerving would not be correct, as sudden changes can create hazards during overtaking.