An assembly for a vehicle includes a front pillar and a rear pillar spaced along a vehicle-longitudinal axis. The pillars define a door opening. The assembly includes a first track supported by the front pillar and a second track supported by the rear pillar. The assembly includes a bar movably engaged with the first track and the second track. The bar is movable between a lowered position and a raised position. The bar is elongated completely across the door opening in the raised position.