What is the test for whether a defendant is mentally competent to stand trial?

"A person cannot be tried . . . while that person is mentally incompetent. A defendant is mentally incompetent . . . if, as a result of mental disorder or developmental disability, the defendant is unable to understand the nature of the criminal proceedings or to assist counsel in the conduct of a defense in a rational manner." ( 1367, subd. (a).) "'"Both federal due process and state law require a trial judge to suspend trial proceedings and conduct a competency hearing whenever the court is presented with substantial evidence of incompetence, that is, evidence that raises a reasonable or bona fide doubt concerning the defendant's competence to stand trial."'" (People v. Sattiewhite (2014) 59 Cal.4th 446, 464-465.)