External Bossbar
data class ExternalBossbar(val id: UUID, val title: String, val progress: Float, val color: String = "PURPLE", val overlay: String = "PROGRESS", val flags: Int = 0, val progressFrom: Float = progress, val progressSince: Long = 0)(source)
Immutable copy of an external BossBar. No packet buffer, Adventure or Bukkit object crosses threads.