Cannot hold jump+sneak to stay still when going up/down scaffolding

Under the precedent set by existing climbable blocks (e.g. vines, ladders) as well as flight in creative and spectator modes, the established control scheme is: jump = go up, sneak = go down (or go down faster), jump+sneak = don't go up or down.

Scaffolding as implemented in 19w04b does not have this behavior. Instead, the jump key overrides the sneak key so that holding both makes the player climb up. To be consistent with the above, jumping and sneaking should cancel each other out when both are held at once.