What is the test for a defendant to be present at the trial of a defendant in a criminal case?

In People v. Waidla (2000) 22 Cal.4th 690, 741-742, the court discussed the rights of defendants to be present at the various stages of a criminal trial:

In order to show prejudice from actions taken in a defendant's absence the person must demonstrate that his or her presence was required because it " 'bears a reasonable and substantial relation to his full opportunity to defend against the charges.' [Citation.] The defendant must show that any violation of this right resulted in prejudice or violated the defendant's right to a fair and impartial trial." (People v. Hines (1997) 15 Cal.4th 997, 1039.)