There is a solid substance in a rigid cylinder with a piston placed on top such that the distance between the piston and the bottom of the cylinder is essentially zero, and then the substance decomposes and forms gas that expands and pushes the piston up against a constant external pressure to a maximum point. It then falls slowly as heat from the reaction is lost to the surroundings. Is it true that at the maximum point, the internal pressure is equal to the external pressure?
If the frictionless massless piston is not moving, then internal and external pressures must be equal. If the piston has mass, the internal pressure must be greater when it is stationary, to support the weight. If there is static friction, all bets are off.
Suppose the piston were a right cone rather than a right cylinder. Would the excess surface area change the outcome, all other things being equal?