Source

File: wp-admin/includes/class-wp-list-table.php

protected function get_default_primary_column_name() {
		$columns = $this->get_columns();
		$column  = '';

		if ( empty( $columns ) ) {
			return $column;
		}

		// We need a primary defined so responsive views show something,
		// so let's fall back to the first non-checkbox column.
		foreach ( $columns as $col => $column_name ) {
			if ( 'cb' === $col ) {
				continue;
			}

			$column = $col;
			break;
		}

		return $column;
	}