What is the right behaviour now?

Your choice to stop behind the taxi is correct because when another vehicle signals and begins to change into your lane ahead of you, you must give them enough space to merge safely. According to German traffic rules, you should not attempt to pass the vehicle on the right or quickly switch lanes yourself in this situation, as this could lead to dangerous conflicts. By slowing down or stopping behind the taxi, you ensure a safe distance and allow the merging vehicle to enter your lane without pressure. This behavior helps prevent accidents and maintains smooth traffic flow. Always be attentive to signals from other drivers and adjust your speed accordingly to prioritize safety.