diff --git a/cmuxUITests/DisplayResolutionRegressionUITests.swift b/cmuxUITests/DisplayResolutionRegressionUITests.swift
index 5131e82c..1df755df 100644
--- a/cmuxUITests/DisplayResolutionRegressionUITests.swift
+++ b/cmuxUITests/DisplayResolutionRegressionUITests.swift
@@ -1,5 +1,6 @@
 import XCTest
 import Foundation
+import AppKit
 
 final class DisplayResolutionRegressionUITests: XCTestCase {
     private let defaultDisplayHarnessManifestPath = "/tmp/cmux-ui-test-display-harness.json"
@@ -11,7 +12,7 @@ final class DisplayResolutionRegressionUITests: XCTestCase {
     private var displayDonePath = ""
     private var helperBinaryPath = ""
     private var helperLogPath = ""
-    private var appProcess: Process?
+    private var launchedRunningApp: NSRunningApplication?
     private var helperProcess: Process?
 
     override func setUp() {
@@ -35,7 +36,7 @@ final class DisplayResolutionRegressionUITests: XCTestCase {
     }
 
     override func tearDown() {
-        terminateAppProcess()
+        terminateLaunchedApp()
         helperProcess?.terminate()
         helperProcess?.waitUntilExit()
         helperProcess = nil
@@ -239,69 +240,71 @@ final class DisplayResolutionRegressionUITests: XCTestCase {
         helperProcess = proc
     }
 
-    // Launch the app binary directly via Process, matching what smoke-test-ci.sh
-    // does. XCUIApplication.launch() and NSWorkspace.openApplication both require
-    // foreground activation which fails on headless WarpBuild CI runners.
-    // Running the binary directly works because it doesn't need WindowServer activation.
+    // Launch via NSWorkspace.openApplication which goes through LaunchServices,
+    // escaping the test runner's sandbox. XCUIApplication.launch() blocks for 60s
+    // on headless CI trying to foreground-activate. Process inherits the test
+    // runner's sandbox, preventing file writes. NSWorkspace with activates=false
+    // avoids both problems.
     private func launchAppProcess(targetDisplayID: String) throws {
-        let binaryPath = try resolveAppBinaryPath()
-        let appEnv = launchEnvironment(targetDisplayID: targetDisplayID)
+        let appBundlePath = try resolveAppBundlePath()
+        let appURL = URL(fileURLWithPath: appBundlePath)
 
-        // Build a clean environment with only system essentials + our test vars.
-        // Do NOT pass the test runner's full environment — it contains XCTest
-        // variables (DYLD_INSERT_LIBRARIES, XCInjectBundle, etc.) that cause the
-        // app to hang when launched via Process.
-        let runnerEnv = ProcessInfo.processInfo.environment
-        var cleanEnv = appEnv
-        for key in ["HOME", "PATH", "TMPDIR", "USER", "SHELL", "LANG",
-                     "TERM", "LOGNAME", "DISPLAY", "__CF_USER_TEXT_ENCODING"] {
-            if let val = runnerEnv[key] { cleanEnv[key] = val }
+        let config = NSWorkspace.OpenConfiguration()
+        config.environment = launchEnvironment(targetDisplayID: targetDisplayID)
+        config.activates = false
+        config.addsToRecentItems = false
+
+        let semaphore = DispatchSemaphore(value: 0)
+        var launchError: Error?
+        var runningApp: NSRunningApplication?
+
+        NSWorkspace.shared.openApplication(at: appURL, configuration: config) { app, error in
+            runningApp = app
+            launchError = error
+            semaphore.signal()
         }
 
-        let proc = Process()
-        proc.executableURL = URL(fileURLWithPath: binaryPath)
-        proc.environment = cleanEnv
+        let waitResult = semaphore.wait(timeout: .now() + 30.0)
+        if waitResult == .timedOut {
+            throw NSError(domain: "DisplayResolutionRegressionUITests", code: 2, userInfo: [
+                NSLocalizedDescriptionKey: "NSWorkspace.openApplication timed out after 30s for \(appBundlePath)"
+            ])
+        }
 
-        let logPath = "/tmp/cmux-ui-test-app-\(launchTag).log"
-        FileManager.default.createFile(atPath: logPath, contents: nil)
-        let logHandle = FileHandle(forWritingAtPath: logPath)
-        proc.standardOutput = logHandle
-        proc.standardError = logHandle
+        if let error = launchError {
+            throw NSError(domain: "DisplayResolutionRegressionUITests", code: 2, userInfo: [
+                NSLocalizedDescriptionKey: "NSWorkspace.openApplication failed: \(error.localizedDescription) path=\(appBundlePath)"
+            ])
+        }
 
-        try proc.run()
-        appProcess = proc
+        launchedRunningApp = runningApp
 
         if !waitForAppLaunchDiagnostics(timeout: 15.0) {
-            let isAlive = proc.isRunning
-            let appLog = (try? String(contentsOfFile: logPath, encoding: .utf8))?.suffix(2000) ?? "<empty>"
-            let envDump = cleanEnv.sorted(by: { $0.key < $1.key }).map { "\($0.key)=\($0.value.prefix(80))" }.joined(separator: " | ")
+            let isAlive = launchedRunningApp?.isTerminated == false
             throw NSError(domain: "DisplayResolutionRegressionUITests", code: 2, userInfo: [
-                NSLocalizedDescriptionKey: "App failed to write launch diagnostics. alive=\(isAlive) diagnostics=\(loadDiagnostics() ?? [:]) binary=\(binaryPath) env=[\(envDump)] appLog=[\(appLog)]"
+                NSLocalizedDescriptionKey: "App failed to write launch diagnostics. alive=\(isAlive) diagnostics=\(loadDiagnostics() ?? [:]) pid=\(runningApp?.processIdentifier ?? -1)"
             ])
         }
     }
 
-    private func resolveAppBinaryPath() throws -> String {
+    private func resolveAppBundlePath() throws -> String {
         // UI test bundle is at:
         //   .../Build/Products/Debug/cmuxUITests-Runner.app/Contents/PlugIns/cmuxUITests.xctest
-        // The app binary is at:
-        //   .../Build/Products/Debug/cmux DEV.app/Contents/MacOS/cmux DEV
+        // The app is at:
+        //   .../Build/Products/Debug/cmux DEV.app
         let testBundle = Bundle(for: Self.self)
         let productsDir = testBundle.bundleURL
             .deletingLastPathComponent()  // -> .../Contents/PlugIns
             .deletingLastPathComponent()  // -> .../Contents
             .deletingLastPathComponent()  // -> .../cmuxUITests-Runner.app
             .deletingLastPathComponent()  // -> .../Debug
-        let binaryPath = productsDir
-            .appendingPathComponent("cmux DEV.app")
-            .appendingPathComponent("Contents/MacOS/cmux DEV")
-            .path
-        if FileManager.default.fileExists(atPath: binaryPath) {
-            return binaryPath
+        let appPath = productsDir.appendingPathComponent("cmux DEV.app").path
+        if FileManager.default.fileExists(atPath: appPath) {
+            return appPath
         }
 
         throw NSError(domain: "DisplayResolutionRegressionUITests", code: 4, userInfo: [
-            NSLocalizedDescriptionKey: "App binary not found at \(binaryPath). testBundle=\(testBundle.bundleURL.path)"
+            NSLocalizedDescriptionKey: "App bundle not found at \(appPath). testBundle=\(testBundle.bundleURL.path)"
         ])
     }
 
@@ -315,24 +318,24 @@ final class DisplayResolutionRegressionUITests: XCTestCase {
         ]
     }
 
-    private func terminateAppProcess() {
-        guard let proc = appProcess else { return }
-        defer { appProcess = nil }
+    private func terminateLaunchedApp() {
+        guard let app = launchedRunningApp else { return }
+        defer { launchedRunningApp = nil }
 
-        if !proc.isRunning { return }
-        proc.terminate()
+        if app.isTerminated { return }
+        app.terminate()
         let deadline = Date().addingTimeInterval(5.0)
-        while proc.isRunning && Date() < deadline {
+        while !app.isTerminated && Date() < deadline {
             RunLoop.current.run(until: Date().addingTimeInterval(0.1))
         }
-        if proc.isRunning {
-            proc.interrupt()
+        if !app.isTerminated {
+            app.forceTerminate()
         }
     }
 
     private func launchedAppDiagnostics() -> String {
-        guard let proc = appProcess else { return "not-launched" }
-        return "pid=\(proc.processIdentifier) running=\(proc.isRunning)"
+        guard let app = launchedRunningApp else { return "not-launched" }
+        return "pid=\(app.processIdentifier) terminated=\(app.isTerminated)"
     }
 
     private func waitForAppLaunchDiagnostics(timeout: TimeInterval) -> Bool {