FB21792467: Kill The Window Server with MATH

[MrTheSaw]

Submission Date

2026-28-01

Status

Open

Area

SwiftUI

Operating System Version

macOS 26.2

Type

Incorrect/Unexpected Behavior

Description

am developing a program which draws a bunch of things in SwiftUI. When I set the starting angle to tau*3/4 radians, the Preview didn't.

In fact, the WindowServer hanged. (Not the underlying OS). Eventually the OS noted the lack of response, and killed the Window server.

I have included a minimized example file: ContentView.swift. Note that I commented out the #Preview, as I got tired of blowing up my session. I tested in a VM running 26.2.

I'm pretty sure it's a problem with using cos() and or sin() from the Core Graphics library. I haven't tested it, but I note in a recent FB that sometimes regular sin() sometimes doesn't compile. See: FB21779423

//
//  ContentView.swift
//  WindowServerKillerApp
//
//  Created by Hacksaw on 1/28/26.
//

import SwiftUI
import Foundation

@inline(__always) func pt(_ x: Double, _ y: Double) -> CGPoint {
  CGPoint(x: x, y: y)
}

let tau = 2.0 * Double.pi

let dashStyle = StrokeStyle (
  lineWidth: 2,
  lineCap: .round,
  lineJoin: .miter,
  miterLimit: 0,
  dash: [5, 10],
  dashPhase: 0
)


  struct ContentView: View {
  @State private var spin = false
  @State private var angleUnitsRadians = false
  @State private var angle: Angle = Angle(radians: (tau*3.0)/4.0)
  @State private var size: CGSize = CGSize(width: 800, height: 600)
  @State private var qdt: (qx: Double, qy: Double) = (-1,-1)
  var body: some View {
    let sH = (0.8 * size.height)/2.0

    ZStack {

      let ctr = pt(size.width/2.0, size.height/2.0)

      // draw Secant line indicator
       Path { path in
         path.move(to: pt(0, qdt.qy * -20.0))
          // DIV BY ZERO
         path.addLine(to: pt(qdt.qx * (sH)/cos(angle.radians), qdt.qy * -20))
          }
      .stroke(Color.pink, style: dashStyle)
      .transformEffect(.init(1, 0, 0, -1, ctr.x, ctr.y))


      //Cosecant Line indicator
      Path { path in
        path.move(to: pt(qdt.qx * -20, 0))
        // DIV BY ZERO
        path.addLine(to: pt(qdt.qx * -20, qdt.qy * (sH)/cos(angle.radians)))
      }
      .stroke(Color.orange, style: dashStyle
      ).transformEffect(.init(1, 0, 0, -1, ctr.x, ctr.y))

    }
    // sets the main size @State variable
    .onGeometryChange(for: CGSize.self) {
      proxy in proxy.size
    } action: { newValue, someValue in size = newValue }
      .frame(width: size.width, height: size.height)
      .padding()
  }
}

//#Preview {
//    ContentView()
//
//}

Keywords

WindowServer crash

Prerequisites

• The title follows the format FB<number>: <title>
• I will keep this issue updated with Apple's responses

[MrTheSaw]

The problem is localized to the Secant Line indictor Path and/or the Cosecant line indicator Path.

[MrTheSaw]

Really weirdly, it seems like the problem is in the dashstyle!

If I remove

.stroke(Color.orange, style: dashStyle)

replacing it with just

.stroke(Color.orange)

It doesn't crash.

But only when the initial value of angle is tau*3.0/4.0 or tau/4.0, which would be when cos(angle) would equal 0.0.