The case of Shinn v. Keyer featured another death row inmate seeking to overturn his conviction through a federal habeas challenge under the Antiterrorism and Effective Death Penalty Act of 1996. Keyer alleged that his counsel was ineffective because he failed to present enough evidence of mitigation at sentencing, although Keyer himself had obstructed efforts of his counsel to develop those defenses and refused to agree to continue sentencing to allow more time for that effort. He lost in the state courts, and the federal district court also denied relief, applying the familiar two-part test in Strickland v. Washington, 466 U.S. 668 (1984). However, a divided Ninth Circuit reversed, finding that the evidence allowed a “reasonable probability” that Keyer would not have been given the death penalty had his full mitigating evidence been presented.
The Court, in a 6-3 per curiam ruling, reversed, holding that the Ninth Circuit had failed to apply the “highly demanding” standard required by the Act and Strickland. The majority held that the Ninth Circuit treated the case as with a de novo review, and did not consider whether fair-minded jurists might come to different conclusion. Justices Breyer, Sotomayor, and Kagan dissented, but did not explain their reasoning.
A link to the opinion is here: https://www.supremecourt.gov/opinions/20pdf/19-1302_8nj9.pdf