iTop/node_modules/tom-select/dist/scss/plugins/optgroup_columns.scss

.#{$select-ns}-dropdown.plugin-optgroup_columns {
	.ts-dropdown-content{
		display: flex;
	}

	.optgroup {
		border-right: 1px solid #f2f2f2;
		border-top: 0 none;
		flex-grow: 1;
		flex-basis: 0;
		min-width: 0;
	}

	.optgroup:last-child {
		border-right: 0 none;
	}

	.optgroup::before {
		display: none;
	}

	.optgroup-header {
		border-top: 0 none;
	}
}