What is the test for determining whether a juror committed misconduct by failing to follow a trial's instruction not to discuss a defendant's failure to testify?

When ruling on a motion for new trial, "'"[i]t is the trial court's function to resolve conflicts in the evidence, to assess the credibility of the declarants, and to evaluate the prejudicial effect of the alleged misconduct."'" (People v. Cumpian (1991) 1 Cal.App.4th 307, 311.) "We accept the trial court's credibility determinations and findings on questions of historical fact if supported by substantial evidence. [Citations.]" (People v. Nesler (1997) 16 Cal.4th 561, 582.) An appellate court reviews de novo whether there was misconduct. (People v. Cumpian, supra, 1 Cal.App.4th at p. 311.) "'"Whether prejudice arose from juror misconduct . . . is a mixed question of law and fact subject to an appellate court's independent determination."' [Citation.]" (People v. Avila (2009) 46 Cal.4th 680, 726-727.)

A jury commits misconduct by violating a trial court's instruction not to discuss a defendant's failure to testify. (People v. Leonard (2007) 40 Cal.4th 1370, 1425.) Such "misconduct gives rise to a presumption of prejudice, which 'may be rebutted . . . by a reviewing court's determination, upon examining the entire record, that there is no substantial likelihood that the complaining party suffered actual harm.' [Citations.]" (Ibid.)

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