Side airbag assembly

A vehicle comprising:

• a seatback frame;
• a tray supported by the seatback frame and being movable relative to the seatback frame in a vehicle-inboard direction from an undeployed position to a deployed position;
• an airbag supported by the tray and movable with the tray from the undeployed position to the deployed position;
• a rail fixed to the seatback frame and a sliding member fixed to the tray, the sliding member being slidably engaged with the rail in the vehicle-inboard direction;
• an actuator between the seatback frame and the tray to move the tray from the undeployed position to the deployed position;
• the tray including a first wall and a second wall, the first wall extending in the vehicle-inboard direction from the sliding member to the second wall, and the second wall extending from the first wall in a vehicle-forward direction in the undeployed position and in the deployed position;
• the airbag includes a base portion constrained between the seatback frame and the second wall of the tray in the deployed position;
• the airbag in the deployed position extending from the first wall in the seat-forward direction farther than the second wall extends from the first wall;
• and
• a computer having a processor and memory storing instructions executable by the processor to actuate the actuator in response to detection of certain vehicle impacts and detection of absence of an occupant vehicle inboard of the seatback frame.