Inverse jump boost effect/potion

To temporarily make players immobile the slowness effect is currently not sufficient, as it still allows players to move while jumping. By using the effect command players can be given a jump boost of over 127, which effectively disables the ability to jump. While this is most likely an unintended side effect, it is very useful for custom maps or gameplay.

However, this effect can not be carried by items such as potions or tipped arrows. To support this feature, a new effect could be added, most likely as a single-level effect that disables jumping.

From a larger perspective it might not be fun to freely allow players to brew a potion with this effect, as jumping is an essential part of regular gameplay.