Q: When are people supposed to sit after communion? When the tabernacle closes? When the priest sits?
A: GIRM 86 says that the song continues “for as long as the Sacrament is being administered to the faithful.” GIRM 43 says that “if appropriate, they may sit or kneel during the period of sacred silence after Communion.”
I read this to mean that the posture may change when communion to the faithful comes to a close. Remember that the rubrics of the Mass are more interested in the communion of the faithful than in the reservation of the Blessed Sacrament. When the communion song is over, when the last person has received, it would be appropriate for people to sit (or kneel) while the remaining hosts are reposed, the optional hymn after communion is sung, the vessels are purified, and the period of silent thanksgiving ensues. The rubrics say nothing about changing posture when the tabernacle door clangs nor when the priest sits. They are concerned about the communion of the faithful.