The Bug
Fishes and Tadpoles aren't affected by jump boost when flopping around despite them jumping
Reproduce
/summon cod ~ ~1 ~ {PersistenceRequired:1,ActiveEffects:[{Id:8,Amplifier:1,Duration:999999}]}
Observed Result
Fishes and Tadpoles weren't jumping higher with jump boost
Expected Result
Fishes and Tadpoles would be affected by jump boost
Code Analysis
Code analysis provided by @unknown
The reason this occurs is because the flop is set to a static 4.0 on the y axis in the abstract fish class and isn't adding the jump boost modifier
Current Code
net/minecraft/world/entity/animal/AbstractFish.java
public void aiStep() {
if (!this.isInWater() && this.onGround && this.verticalCollision) {
this.setDeltaMovement(this.getDeltaMovement().add((double)((this.random.nextFloat() * 2.0F - 1.0F) * 0.05F), (double)0.4F, (double)((this.random.nextFloat() * 2.0F - 1.0F) * 0.05F)));
this.onGround = false;
this.hasImpulse = true;
this.playSound(this.getFlopSound(), this.getSoundVolume(), this.getVoicePitch());
}
super.aiStep();
}
Fixed Code
net/minecraft/world/entity/animal/AbstractFish.java
public void aiStep() {
if (!this.isInWater() && this.onGround && this.verticalCollision) {
//Adding the jump boost modifyer on the setDeltaMovement fixes the bug
this.setDeltaMovement(this.getDeltaMovement().add((double)((this.random.nextFloat() * 2.0F - 1.0F) * 0.05F), (double)0.4F + this.getJumpBoostPower(), (double)((this.random.nextFloat() * 2.0F - 1.0F) * 0.05F)));
this.onGround = false;
this.hasImpulse = true;
this.playSound(this.getFlopSound(), this.getSoundVolume(), this.getVoicePitch());
}
super.aiStep();
}
Linked issues
relates to
Comments

Requesting review of this issue.
MC-163853 has been triaged by Mojang, and the issue specifically relates to an aquatic mob's abiltiy to jump when on honey, and how that should be prevented. If aquatic mobs bouncing around is seen as jumping in relation to honey blocks, this behavior should remain consistent across other features that modify jumping (in this example, jump boost).
Additionally; jumping is not specifically exclusive to mobs with legs, as in the case with the slime entity it is affected by jump boost.
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