An assembly for a vehicle includes a front pillar and a rear pillar spaced along a vehicle-longitudinal axis. The front pillar and the rear pillar 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 lower bar and an upper bar movable upwardly relative to the lower bar along the first track and the second track. The upper bar is movable between a lowered position and a raised position. The assembly includes a crossmember pivotally coupled to the lower bar and the upper bar.