` UIBezierPath(roundedRect:cornerRadius:)` renders Inconsistently at Specific Size-to-Radius Ratios

Hello everyone,

I've encountered a fascinating and perplexing rendering anomaly when using UIBezierPath(roundedRect:cornerRadius:) to create a CGPath.

Summary of the Issue:

When the shortest side of the rectangle (min(width, height)) is just under a certain multiple of the cornerRadius (empirically, around 3x), the algorithm for generating the path seems to change entirely. This results in a path with visually different (and larger) corners than when the side is slightly longer, even with the same cornerRadius parameter.

How to Reproduce:

The issue is most clearly observed with a fixed cornerRadius while slightly adjusting the rectangle's height or width across a specific threshold.

1. Create a UIView (contentView) and another UIView (shadowView) behind it.
2. Set the shadowView.layer.shadowPath using UIBezierPath(roundedRect: contentView.bounds, cornerRadius: 16).cgPath.
3. Adjust the height of the contentView.
4. Observe the shadowPath at height 48 vs. height 49

Minimal Reproducible Example:

Here is a simple UIViewController to demonstrate the issue. You can drop this into a project. Tapping the "Toggle Height" button will switch between the two states and print the resulting CGPath to the console.

import UIKit
 
class PathTestViewController: UIViewController {
 
    private let contentView = UIView()
    private let shadowView = UIView()
    private var heightConstraint: NSLayoutConstraint!
    
    private let cornerRadius: CGFloat = 16.0
    private let normalHeight: CGFloat = 49
    private let anomalyHeight: CGFloat = 48
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .systemGray5
        setupViews()
        setupButton()
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        updateShadowPath()
    }
    
    private func updateShadowPath() {
        let newPath = UIBezierPath(roundedRect: contentView.bounds, cornerRadius: cornerRadius).cgPath
        shadowView.layer.shadowPath = newPath
    }
 
    private func setupViews() {
        // ContentView (the visible rect)
        contentView.backgroundColor = .systemBlue
        contentView.translatesAutoresizingMaskIntoConstraints = false
        contentView.isHidden = true
        
        // ShadowView (to render the path)
        shadowView.layer.shadowColor = UIColor.black.cgColor
        shadowView.layer.shadowOpacity = 1
        shadowView.layer.shadowRadius = 2
        shadowView.layer.shadowOffset = .zero
        shadowView.translatesAutoresizingMaskIntoConstraints = false
 
        view.addSubview(shadowView)
        view.addSubview(contentView)
        
        heightConstraint = contentView.heightAnchor.constraint(equalToConstant: normalHeight)
 
        NSLayoutConstraint.activate([
            contentView.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            contentView.centerYAnchor.constraint(equalTo: view.centerYAnchor),
            contentView.widthAnchor.constraint(equalToConstant: 300),
            heightConstraint,
            
            shadowView.topAnchor.constraint(equalTo: contentView.topAnchor),
            shadowView.bottomAnchor.constraint(equalTo: contentView.bottomAnchor),
            shadowView.leadingAnchor.constraint(equalTo: contentView.leadingAnchor),
            shadowView.trailingAnchor.constraint(equalTo: contentView.trailingAnchor),
        ])
    }
    
    private func setupButton() {
        let button = UIButton(type: .system, primaryAction: UIAction(title: "Toggle Height", handler: { [unowned self] _ in
            let newHeight = self.heightConstraint.constant == self.normalHeight ? self.anomalyHeight : self.normalHeight
            self.heightConstraint.constant = newHeight
            
            UIView.animate(withDuration: 0.3) {
                self.view.layoutIfNeeded()
            }
        }))
        button.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(button)
        NSLayoutConstraint.activate([
            button.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            button.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor, constant: -20)
        ])
    }
}

Evidence: CGPath Analysis

Note: The CGPath data below is from my initial observation. At that time, height 48.7 produced a path with straight edges. Now, this "correct" path is only produced at height 49.0 or greater. The inconsistency now occurs at 48.7.*

The key difference lies in the raw CGPath data.

1. Path for Height = 48.7 (Expected Behavior)

The path is constructed with lineto commands for the straight edges between the curved corners.

// Path for Height 48.7
Path 0x60000300a0a0:
  moveto (24.4586, 0)
    lineto (24.5414, 0) // <-- Straight line on top edge
    curveto (31.5841, 0) (35.1055, 0) (38.8961, 1.19858)
    ...

1. Path for Height = 48.6 (Anomalous Behavior)

The lineto commands for the short edges disappear. The path is composed of continuous curveto commands, as if the two corners have merged into a single, larger curve. This creates the visual discrepancy.

// Path for Height 48.6
Path 0x600003028630:
  moveto (24.1667, 0)
    lineto (24.1667, 0) // <-- Zero-length line
    curveto (24.1667, 0) (24.1667, 0) (24.1667, 0)
    lineto (25.375, 1.44329e-15)
    curveto (34.8362, -2.77556e-16) (43.2871, 5.9174) (46.523, 14.808) // <-- First curve
    curveto (48.3333, 20.5334) (48.3333, 25.8521) (48.3333, 36.4896) // <-- Second curve, no straight line in between
    ...

My Questions:

1. Is this change in the path-generation algorithm at this specific size/radius threshold an intended behavior, or is it a bug?
2. Is this behavior documented anywhere? The threshold doesn't seem to be a clean side/radius == 2.0, so it's hard to predict.
3. Is there a recommended workaround to ensure consistent corner rendering across these small size thresholds?

Any insight would be greatly appreciated. Thank you!

Environment:

• Xcode: 16.4
• iOS: 16.5.1(iPad), 18.4(iphone simulator)