.buttonStyle(.glass) background changes abruptly between 50pt and 51pt in dark mode

[Submitted as FB22612121]

A SwiftUI Button using .buttonStyle(.glass) with .buttonBorderShape(.capsule) changes its background abruptly when its size goes from 50×50 to 51×51 points in dark mode. This appears to be a threshold in opacity/material rather than a smooth size-based change.

The sample shows identical buttons at 40, 50, 51, and 60 points, with a clear jump between 50 and 51. Measured RGB values shift from 19,19,19 to 30,30,30. The effect also varies with the background, which points to a material/opacity change rather than a fixed fill.

ENVIRONMENT

• iOS 26.4.1 (23E254a)
• iOS 26.5 (23F5059e)

REPRO STEPS

1. Create a new iOS SwiftUI project.
2. Replace ContentView with the sample code below.
3. Run the app or open ContentView in SwiftUI Preview (dark mode).
4. Observe the buttons at 40×40, 50×50, 51×51, and 60×60.
5. Compare the 50pt and 51pt buttons.

ACTUAL

The background changes abruptly between 50pt and 51pt. The 51pt button uses a noticeably different opacity/material, producing a visible jump in dark mode.

EXPECTED

The glass background should remain visually consistent or change smoothly as size changes by 1 point. 50pt and 51pt buttons should not have a discontinuous difference.

SCREENSHOT

SAMPLE CODE

struct ContentView: View {
    private let sizes: [CGFloat] = [40, 50, 51, 60]

    var body: some View {
        ScrollView {
            VStack(alignment: .leading, spacing: 16) {
                Text("Glass button dark-mode size jump")
                    .font(.headline)

                Text("All buttons use .buttonStyle(.glass). Only the label frame changes.")
                    .font(.footnote)
                    .foregroundStyle(.secondary)

                ForEach(Array(sizes.enumerated()), id: \.offset) { index, size in
                    HStack(spacing: 14) {
                        Button {
                        } label: {
                            Text("\(index + 1)")
                                .font(.system(size: size * 0.42, weight: .medium))
                                .frame(width: size, height: size)
                        }
                        .buttonStyle(.glass)
                        .buttonBorderShape(.capsule)

                        Text("label frame: \(Int(size)) x \(Int(size))")
                            .font(.callout.monospacedDigit())
                            .foregroundStyle(.secondary)
                    }
                }
            }
            .padding(24)
        }
        .preferredColorScheme(.dark)
    }
}