[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 Create a new iOS SwiftUI project. Replace ContentView with the sample code below. Run the app or open ContentView in SwiftUI Preview (dark mode). Observe the buttons at 40×40, 50×50, 51×51, and 60×60. 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) } }