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

A defendant is presumed competent to stand trial. (People v. Campbell (1976) 63 Cal.App.3d 599, 608; see also Pen. Code, 1369, subd. (f) ["It shall be presumed that the defendant is mentally competent unless it is proved by a preponderance of the evidence that the defendant is mentally incompetent."].) "A defendant is deemed incompetent to stand trial if he lacks ' " 'sufficient present ability to consult with his lawyer with a reasonable degree of rational understanding [or] a rational as well as factual understanding of the proceedings against him [or her].' " ' " (People v. Lightsey (2012) 54 Cal.4th 668, 690; accord, People v. Mickel (2016) 2 Cal.5th 181, 194-195 (Mickel); see also Pen. Code, 1367, subd. (a) ["A defendant is mentally incompetent . . . if . . . 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."].)