Locking mechanism for downhole positioning of sleeves

A well tool can include a chamber having a volume that changes when a member displaces, and a valve device that selectively opens and permits displacement of the member, and that selectively closes and prevents displacement of the member. A method of operating a well tool can include displacing a member, thereby changing a chamber volume, and then closing a valve device, thereby preventing displacement of the member. A well system can include a well tool connected in a tubular string, a flow passage of the tubular string extending longitudinally through the well tool, the well tool including a chamber having a volume that changes when a member displaces, and a shifting tool, a valve device opening and member displacement being permitted when the shifting tool engages the well tool, and the valve device closing and member displacement being prevented when the shifting tool disengages from the well tool.