diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 251f6a7..a349a6c 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -29,6 +29,11 @@ on:
       - "tools/build_macos.py"
       - "tools/make_icon.py"
       - ".github/workflows/release.yml"
+      # The front ends themselves. Neither is built by CI, so a change here
+      # reaches a compiler for the first time in this workflow - and the macOS
+      # one cannot be built anywhere else in this project at all.
+      - "macos/**"
+      - "winui/**"
   workflow_dispatch:
 
 permissions:
diff --git a/macos/Package.swift b/macos/Package.swift
index 5338113..5826807 100644
--- a/macos/Package.swift
+++ b/macos/Package.swift
@@ -13,10 +13,12 @@ let package = Package(
     name: "MThreadDraw",
     platforms: [.macOS(.v13)],
     targets: [
+        // No file here is called main.swift, which is what lets @main in
+        // MThreadDrawApp.swift stand: a file by that name is top-level code,
+        // and the two cannot coexist.
         .executableTarget(
             name: "MThreadDraw",
-            path: "Sources/MThreadDraw",
-            swiftSettings: [.unsafeFlags(["-parse-as-library"])]
+            path: "Sources/MThreadDraw"
         )
     ]
 )
diff --git a/macos/Sources/MThreadDraw/ContentView.swift b/macos/Sources/MThreadDraw/ContentView.swift
index d9a0fb9..f520257 100644
--- a/macos/Sources/MThreadDraw/ContentView.swift
+++ b/macos/Sources/MThreadDraw/ContentView.swift
@@ -183,6 +183,7 @@ final class Model: ObservableObject {
     func stop() { engine.post("stop") }
 }
 
+@MainActor
 struct ContentView: View {
     @StateObject private var model = Model()
 
@@ -227,7 +228,9 @@ struct ContentView: View {
                             Text("Portraits, animals, nature").tag("flow")
                         }
                         .labelsHidden()
-                        .onChange(of: model.tracer) { Task { await model.preview() } }
+                        // The closure takes the new value: the argument-less form of onChange
+                        // is macOS 14, and this app runs on 13.
+                        .onChange(of: model.tracer) { _ in Task { await model.preview() } }
 
                         slider("How much detail", value: $model.detail, range: 1...10,
                                caption: detailWord(model.detail)) {
@@ -294,7 +297,7 @@ struct ContentView: View {
                 .padding(.horizontal, 10).padding(.vertical, 6)
                 .background {
                     RoundedRectangle(cornerRadius: 8)
-                        .fill(index == model.current ? .white.opacity(0.14) : .clear)
+                        .fill(Color.white.opacity(index == model.current ? 0.14 : 0))
                 }
                 .opacity(layer.visible ? 1 : 0.45)
                 .onTapGesture { Task { await model.run("layer_select", ["index": index]) } }
diff --git a/macos/Sources/MThreadDraw/Engine.swift b/macos/Sources/MThreadDraw/Engine.swift
index 444ac77..9d3dd4f 100644
--- a/macos/Sources/MThreadDraw/Engine.swift
+++ b/macos/Sources/MThreadDraw/Engine.swift
@@ -90,7 +90,14 @@ final class Engine {
         process.arguments = arguments
         process.standardInput = input
         process.standardOutput = output
-        process.standardError = Pipe()
+        // Drained, not ignored. A pipe nobody reads fills at 64 KB and the
+        // next write blocks for ever, so the engine would hang rather than
+        // fail - and the last few lines are what says why it did.
+        let errors = Pipe()
+        process.standardError = errors
+        errors.fileHandleForReading.readabilityHandler = { [weak self] handle in
+            self?.absorbErrors(handle.availableData)
+        }
         if let repository = Engine.findRepository() {
             process.currentDirectoryURL = repository
         }
@@ -108,9 +115,12 @@ final class Engine {
                     self?.readyContinuation = continuation
                 }
             }
-            group.addTask {
+            group.addTask { [weak self] in
                 try await Task.sleep(for: .seconds(30))
-                throw EngineError.notStarted("It produced no output in thirty seconds.")
+                let detail = self?.lastErrors ?? ""
+                let complaint = "It produced no output in thirty seconds."
+                throw EngineError.notStarted(
+                    detail.isEmpty ? complaint : complaint + "\n\n" + detail)
             }
             try await group.next()
             group.cancelAll()
@@ -119,6 +129,14 @@ final class Engine {
 
     // MARK: - reading
 
+    /// The tail of the engine's standard error, for when it dies without a word.
+    private(set) var lastErrors = ""
+
+    private func absorbErrors(_ data: Data) {
+        guard !data.isEmpty, let text = String(data: data, encoding: .utf8) else { return }
+        lastErrors = String((lastErrors + text).suffix(4000))
+    }
+
     private func absorb(_ data: Data) {
         guard !data.isEmpty else { return }
         buffer.append(data)
@@ -179,10 +197,7 @@ final class Engine {
     /// Send one request and wait for its reply.
     @discardableResult
     func call(_ operation: String, _ arguments: [String: Any] = [:]) async throws -> [String: Any] {
-        lock.lock()
-        nextId += 1
-        let id = nextId
-        lock.unlock()
+        let id = claimId()
 
         var request: [String: Any] = arguments
         request["id"] = id
@@ -190,13 +205,29 @@ final class Engine {
         let line = try JSONSerialization.data(withJSONObject: request) + Data([0x0A])
 
         return try await withCheckedThrowingContinuation { continuation in
-            lock.lock()
-            pending[id] = continuation
-            lock.unlock()
+            register(id, continuation)
             input.fileHandleForWriting.write(line)
         }
     }
 
+    // Both of these exist to be synchronous. Taking a lock directly in an async
+    // function is a warning today and an error under Swift 6, because the
+    // thread that resumes after an await need not be the one that took it;
+    // inside a plain method there is no await to move, so the pair is safe.
+    private func claimId() -> Int {
+        lock.lock()
+        defer { lock.unlock() }
+        nextId += 1
+        return nextId
+    }
+
+    private func register(_ id: Int,
+                          _ continuation: CheckedContinuation<[String: Any], Error>) {
+        lock.lock()
+        defer { lock.unlock() }
+        pending[id] = continuation
+    }
+
     /// Send a request without waiting - for stop, which cannot queue.
     func post(_ operation: String) {
         let line = "{\"op\": \"\(operation)\"}\n"
diff --git a/macos/Sources/MThreadDraw/main.swift b/macos/Sources/MThreadDraw/MThreadDrawApp.swift
similarity index 100%
rename from macos/Sources/MThreadDraw/main.swift
rename to macos/Sources/MThreadDraw/MThreadDrawApp.swift
diff --git a/tools/github.sh b/tools/github.sh
index 1fa2e67..a8b4de4 100644
--- a/tools/github.sh
+++ b/tools/github.sh
@@ -55,14 +55,17 @@ if [ -z "$TOKEN" ]; then
     exit 1
 fi
 
+# --location because the log endpoints answer with a redirect to wherever the
+# logs are actually stored, and a job log is the only way to see why a build
+# failed on a machine you do not have.
 if [ -n "$BODY" ]; then
-    curl -sS -X "$METHOD" \
+    curl -sS --location -X "$METHOD" \
         -H "Authorization: Bearer ${TOKEN}" \
         -H "Accept: application/vnd.github+json" \
         -d "$BODY" \
         "${API}${PATH_UNDER_REPO}"
 else
-    curl -sS -X "$METHOD" \
+    curl -sS --location -X "$METHOD" \
         -H "Authorization: Bearer ${TOKEN}" \
         -H "Accept: application/vnd.github+json" \
         "${API}${PATH_UNDER_REPO}"