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Apple has shared documentation that explains how developers can fully enable faster ProMotion refresh rates for their apps on ‌iPhone 13 Pro‌ models. Specifically, the documentation provides a key that developers can add to an app's Info.plist file to enable the full range of refresh rates for custom animations.
In addition, a forthcoming software update will resolve a bug where animations driven by Core Animation were not able to reach the maximum 120Hz refresh rates.