If you want to hide the "Customer" input parameter on a quote line item, you cannot use the function api.customer("attr name"). Instead, you must use api.customer("attr name", "customer id"), with the "customer id" defined. (If it was empty, the system would ask for it.)
Example
def customerId = api.input("Customer")
if (customerId) {
return api.customer("SALESORG",customerId)
} else {
return
}
The api.input() does not trigger the input box creation.