Is there any prejudice sustained under the "harmless beyond a reasonable doubt" standard?

Defendant asserts that we must assess the prejudice he sustained under the "harmless beyond a reasonable doubt" standard from Chapman v. California (1967) 386 U.S. 18, 87 S.Ct. 824, 17 L.Ed.2d 705 ( Chapman ) because the error violated his constitutional right to due process and a fair trial by requiring him to defend against another homicide allegation which, in turn, deprived him of the opportunity to present a viable defense. He asserts that the introduction of this evidence was conducive to irreparable mistake, and therefore its admission violated his federal due process rights. "The beyond-a-reasonable-doubt standard of Chapman requir[es] the beneficiary of a [federal] constitutional error to prove beyond a reasonable doubt that the error complained of did not contribute to the verdict obtained. [Citation.] To say

[56 Cal.App.5th 1164]

that an error did not contribute to the ensuing verdict is ... to find that error unimportant

[271 Cal.Rptr.3d 141]

in relation to everything else the jury considered on the issue in question, as revealed in the record. [Citation.] Thus, the focus is what the jury actually decided and whether the error might have tainted its decision. That is to say, the issue is whether the ... verdict actually rendered in this trial was surely unattributable to the error." ( People v. Neal (2003) 31 Cal.4th 63, 86, 1 Cal.Rptr.3d 650, 72 P.3d 280.)