Fishing line appears in opposite hand when switching slots

Relates to MC-116379.

Can confirm in 1.16.5. Here are some steps to reproduce this issue:

Steps to Reproduce:

• Cast a fishing rod.

• Switch to another hotbar slot whilst the rod is cast.
  → ❌  Notice how the fishing line appears in the offhand for a split second.

Can't confirm for 21w11a because of MC-219849

Unblocked in 21w14a.

Code Analysis:

The following is based on a decompiled version of Minecraft 1.20.1 using MCP-Reborn.

net.minecraft.client.renderer.entity.FishingHookRenderer.java // Original

public class FishingHookRenderer extends EntityRenderer<FishingHook> {
   ...
   int i = player.getMainArm() == HumanoidArm.RIGHT ? 1 : -1; 
   ItemStack itemstack = player.getMainHandItem(); 
   if (!itemstack.is(Items.FISHING_ROD)) { 
      i = -i; 
   }
   ...
}

We can take a look at this specific part of code in the decompiled version of Minecraft 1.20.1, as we can see, if a fishing rod is NOT present in the player's main arm, as a default it will be the right hand. Then it will automatically move the fishing line towards the player's offhand, without it even checking for a fishing rod in the offhand.

Fix:

To fix this, we can easily add a condition to the current "if" statement in the code above, checking if the player has a fishing rod in the offhand. Therefore, whenever you swap out of the fishing rod, it'll persist on the default side of the mainhand.

net.minecraft.client.renderer.entity.FishingHookRenderer.java // Updated

public class FishingHookRenderer extends EntityRenderer<FishingHook> {
   ...
   int i = player.getMainArm() == HumanoidArm.RIGHT ? 1 : -1;
   ItemStack itemstack = player.getMainHandItem();
   if (!itemstack.is(Items.FISHING_ROD) && player.getOffhandItem().is(Items.FISHING_ROD)) {
      i = -i;
   }
   ...
}

@unknown, 1.20.1 has already been marked as affected!

Can confirm in 1.20.5rc2.

Can confirm in 1.20.5rc2.

Can confirm in 24w21b.