A SwiftUI view modifier that fits a sheet height to its contents.

ReadHeightModifier.swift

import SwiftUI

fileprivate struct ReadHeightModifier: ViewModifier {
    let action: (Double) -> Void

    func body(content: Content) -> some View {
        content
            .onGeometryChange(for: Double.self) { proxy in
                proxy.size.height
            } action: { newValue in
                action(newValue)
            }
    }
}

struct FittedPresentationDetentModifier: ViewModifier {
    @State private var height = 0.0

    func body(content: Content) -> some View {
        content
            .readHeight { height in
                /*
                 this guard is to avoid the following layout error that will
                 crash in future releases:

                 A presentation preference is rapidly switching between values,
                 possibly because the presentation's preferences depend on its size.

                 -- Previous Preferences --
                 presentationsDetents: Height 389.3333333333333

                 -- Most Recent Preferences --
                 presentationsDetents: Height 376.6666666666667

                 The most recent value was ignored to avoid cyclic layout.
                 Please update your code to avoid this issue.
                 This may become a crash in a future release.
                 */
                guard height >= self.height else { return }

                self.height = height
            }
            .presentationDetents([.height(height)])
    }
}

extension View {
    func readHeight(_ perform: @escaping (Double) -> Void) -> some View {
        modifier(ReadHeightModifier(action: perform))
    }

    func fittedPresentationDetent() -> some View {
        modifier(FittedPresentationDetentModifier())
    }
}

#Preview {
    Color.clear
        .sheet(isPresented: .constant(true)) {
            DatePicker("", selection: .constant(.now), displayedComponents: .date)
                .datePickerStyle(.graphical)
                .fittedPresentationDetent()
        }
}

#Preview {
    Color.clear
        .sheet(isPresented: .constant(true)) {
            VStack {
                Text("Example")
                Text("Example")
                Text("Example")
            }
            .fittedPresentationDetent()
        }
}