MouseDelayFix

MouseDelayFix is a forge mod for Minecraft 1.8.x, 1.9.x and 1.10.x to fix the mouse delay bug introduced in Minecraft 1.8 (https://bugs.mojang.com/browse/MC-67665).

The bug

// EntityLivingBase
public Vec3 getLook(float partialTicks) {
    if (partialTicks == 1.0F) {
        return this.getVectorForRotation(this.rotationPitch, this.rotationYawHead);
    } else {
        float f = this.prevRotationPitch + (this.rotationPitch - this.prevRotationPitch) * partialTicks;
        float f1 = this.prevRotationYawHead + (this.rotationYawHead - this.prevRotationYawHead) * partialTicks;
        return this.getVectorForRotation(f, f1);
    }
}

Since 1.8 entity look direction is being derived from rotationYawHead instead of rotationYaw. This is bad for players because the angle your head faces is not the same as your crosshair location.

Mojang's fix (MC 1.11)

// EntityPlayerSP
public Vec3d getLook(float partialTicks) {
    return this.getVectorForRotation(this.rotationPitch, this.rotationYaw);
}

The fix added in 1.11 is to override getLook in the client player class (EntityPlayerSP) and use rotationYaw.

My fix

 // EntityLivingBase
 public Vec3 getLook(float partialTicks) {
+    if (this instanceof EntityPlayerSP) {
+        return super.getLook(partialTicks);
+    }
     if (partialTicks == 1.0F) {
         return this.getVectorForRotation(this.rotationPitch, this.rotationYawHead);
     } else {
         float f = this.prevRotationPitch + (this.rotationPitch - this.prevRotationPitch) * partialTicks;
         float f1 = this.prevRotationYawHead + (this.rotationYawHead - this.prevRotationYawHead) * partialTicks;
         return this.getVectorForRotation(f, f1);
     }
 }

My fix injects the above code into EntityLivingBase.getLook. This works because Entity.getLook uses rotationYaw (non-living entities don't have heads!). This is equivalent to Mojang's 1.11 fix.