portrait_wear_no_headgear_trigger

portrait_wear_no_headgear_trigger = {
	OR = {
		AND = {
			trigger_if = { # No headgear for children under 16
				limit = {
					portrait_wear_no_crown_trigger = yes
				}
				scope:age < 16
			}
			trigger_else = { # Unless it's a crown
				scope:age <= coronation_minimum_age_value
			}
			NOR = {
				portrait_sami_clothing_trigger = yes
				portrait_ugro_permian_clothing_trigger = yes
			}
		}
		has_character_flag = no_hat
		portrait_prisoner_trigger = yes
		has_character_modifier = hajj_halq_modifier # To show shaved hair

		has_character_flag = roaming_character

		AND = {
			OR = {
				should_be_naked_trigger = yes # Deliberately not the "fully naked" one since dropping your crown's intended even if nudity is disabled

				AND = {
					highest_held_title_tier < tier_duchy # Should wear ducal or royal crown instead.
					OR = {
						has_short_disease_trigger = yes # Affliction that would plausibly leave them bedridden for treatment
						has_character_flag = use_sickness_clothes
					}
				}

				activity_tournament_shirtless_trigger = yes
			}

			NOR = { # Exceptions for dukes, kings, popes
				highest_held_title_tier >= tier_duchy # Should wear ducal or royal crown instead.
				portrait_religious_head_popes_trigger = yes
				culture = { has_cultural_pillar = heritage_japonic } # Showing the top of your head is shameful
				culture = { has_cultural_pillar = heritage_chinese } # Showing the top of your head is shameful
			}
		}

		AND = { # No headgear for vanity characters with the no_headgear flag, unless they are dukes or higher
			exists = this
			has_character_flag = no_headgear
			highest_held_title_tier < tier_duchy
		}

		portrait_ep2_ihram_clothes_trigger = yes # Remove headgear if you are undertaking the Hajj and meet the requirements for the Ihram clothes (Halq modifier and male)

		AND = { # No headgear for charioteers
			exists = this
			has_any_charioteer_trait = yes
		}
	}
}

As written in game/common/scripted_triggers/00_clothing_triggers.txt.