ios: isolate Agent scrolling and activity rendering

ios/Packages/Sybil/Sources/Sybil/AgentRenderDiagnostics.swift

@@ -0,0 +1,14 @@
+#if DEBUG
+/// Opt-in operation counters for the simulator presentation benchmark. These
+/// measure work, not frames; CADisplayLink sampling lives in the test target.
+@MainActor
+enum AgentRenderDiagnostics {
+    static var enabled = false
+    static var counts: [String: Int] = [:]
+
+    static func record(_ operation: String) {
+        guard enabled else { return }
+        counts[operation, default: 0] += 1
+    }
+}
+#endif

ios/Packages/Sybil/Sources/Sybil/SybilAgentRoom.swift

@@ -74,6 +74,9 @@ final class SybilAgentRoom {
     var isBusy: Bool { snapshot.room.activity.isBusy }
     var unreadCount: Int { snapshot.room.unreadCount }
     var messages: [Message] {
+        #if DEBUG
+        AgentRenderDiagnostics.record("messageProjection")
+        #endif
         let committed = Set(snapshot.room.messages.map(\.id))
         let queued = snapshot.room.activity.queuedMessages.filter { !committed.contains($0.id) }
         var messages = AgentRoomState.merge(snapshot.room.messages, queued)
@@ -337,6 +340,9 @@ final class SybilAgentRoom {
     }
 
     func flush() {
+        #if DEBUG
+        AgentRenderDiagnostics.record("snapshotPublish")
+        #endif
         batchTask?.cancel(); batchTask = nil
         typingTask?.cancel(); typingTask = nil
         snapshot = Snapshot(room: pending, isTyping: presenter.isTyping(now: Date()), entranceIDs: pendingEntrances, revision: snapshot.revision + 1)

ios/Packages/Sybil/Sources/Sybil/SybilAgentView.swift

@@ -11,22 +11,25 @@ struct SybilAgentView: View {
     @State private var historyLimit = 100
     @State private var historyStartID: String?
     @State private var scrollPosition = ScrollPosition()
-    @State private var scrollMetrics = AgentScrollMetrics()
-    @State private var rowFrames: [String: CGRect] = [:]
+    @State private var scrollTracking = AgentScrollTracking()
+    @State private var atBottom = false
     @State private var restoreAnchor: AgentReadingAnchor?
     @State private var appeared = false
     private let bottomID = "agent-bottom"
 
     private var readKey: String {
-        "\(room.snapshot.room.cursor):\(atLatest):\(scrollMetrics.atBottom):\(isScrolling):\(room.isObscured):\(room.isModalPresented):\(room.isAttachmentModalPresented):\(scenePhase == .active):\(room.isLoading):\(room.snapshot.revision)"
+        "\(room.snapshot.room.cursor):\(atLatest):\(atBottom):\(isScrolling):\(room.isObscured):\(room.isModalPresented):\(room.isAttachmentModalPresented):\(scenePhase == .active):\(room.isLoading):\(room.snapshot.revision)"
     }
     private var visibleReadingAnchor: AgentReadingAnchor? {
-        rowFrames.filter { $0.value.maxY > topInset && $0.value.minY < scrollMetrics.height }
+        scrollTracking.rowFrames.filter { $0.value.maxY > topInset && $0.value.minY < scrollTracking.metrics.height }
             .min { $0.value.minY < $1.value.minY }
             .map { AgentReadingAnchor(id: $0.key, offset: $0.value.minY) }
     }
 
     var body: some View {
+        #if DEBUG
+        let _ = AgentRenderDiagnostics.record("agentBody")
+        #endif
         let rendered = room.snapshot
         let transcript = AgentTranscript(messages: room.messages, startID: historyStartID, limit: historyLimit)
         let allMessages = transcript.messages
@@ -51,21 +54,11 @@ struct SybilAgentView: View {
                         }
                         if room.isLoading { ProgressView("Loading messages…").frame(maxWidth: .infinity) }
                         ForEach(Array(displayed.enumerated()), id: \.element.id) { index, message in
-                            VStack(spacing: 14) {
-                                if index == 0 || !Calendar.current.isDate(message.createdAt, inSameDayAs: displayed[index - 1].createdAt) {
-                                    Text(message.createdAt.formatted(date: .abbreviated, time: .omitted))
-                                        .font(.sybil(.caption2))
-                                        .foregroundStyle(SybilTheme.textMuted)
-                                        .frame(maxWidth: .infinity)
-                                        .padding(.vertical, 6)
-                                }
-                                AgentMessageRow(message: message, room: room, retry: { room.retry(message.id) })
-                                .background(GeometryReader { geometry in
-                                    Color.clear.preference(key: AgentRowFrames.self, value: [message.id: geometry.frame(in: .named("agent-scroll"))])
-                                })
-                            }
-                            .modifier(AgentEntrance(animate: appeared && rendered.entranceIDs.contains(message.id), reduceMotion: reduceMotion))
-                            .id(message.id)
+                            AgentTranscriptRow(message: message, room: room,
+                                startsDay: index == 0 || !Calendar.current.isDate(message.createdAt, inSameDayAs: displayed[index - 1].createdAt))
+                                .equatable()
+                                .modifier(AgentEntrance(animate: appeared && rendered.entranceIDs.contains(message.id), reduceMotion: reduceMotion))
+                                .id(message.id)
                         }
                         if rendered.isTyping {
                             HStack(spacing: 9) {
@@ -103,7 +96,11 @@ struct SybilAgentView: View {
                         bottomInset: geometry.contentInsets.bottom, topInset: geometry.contentInsets.top,
                         visibleBottom: geometry.visibleRect.maxY)
                 } action: { previous, next in
-                    scrollMetrics = next
+                    #if DEBUG
+                    AgentRenderDiagnostics.record("scrollGeometry")
+                    #endif
+                    scrollTracking.metrics = next
+                    if atBottom != next.atBottom { atBottom = next.atBottom }
                     if isScrolling || next.contentHeight <= next.height { atLatest = next.atBottom }
                     if atLatest, !isScrolling, restoreAnchor == nil,
                        previous.height != next.height || previous.contentHeight != next.contentHeight || previous.bottomInset != next.bottomInset {
@@ -111,23 +108,26 @@ struct SybilAgentView: View {
                     }
                 }
                 .onPreferenceChange(AgentRowFrames.self) { frames in
+                    #if DEBUG
+                    AgentRenderDiagnostics.record("rowPreferences")
+                    #endif
                     let anchor = restoreAnchor ?? (!atLatest && !isScrolling ? visibleReadingAnchor : nil)
                     if let anchor, let frame = frames[anchor.id], abs(frame.minY - anchor.offset) > 1 {
                         // Restore the exact pixel offset after insertion or text
                         // reflow. Normalize the saved frames to
                         // the requested offset so the next layout cannot undo it.
-                        let requested = scrollMetrics.offset + frame.minY - anchor.offset
-                        let target = min(max(requested, -scrollMetrics.topInset), scrollMetrics.maximumOffset)
-                        let appliedShift = target - scrollMetrics.offset
-                        rowFrames = frames.mapValues { $0.offsetBy(dx: 0, dy: -appliedShift) }
+                        let requested = scrollTracking.metrics.offset + frame.minY - anchor.offset
+                        let target = min(max(requested, -scrollTracking.metrics.topInset), scrollTracking.metrics.maximumOffset)
+                        let appliedShift = target - scrollTracking.metrics.offset
+                        scrollTracking.rowFrames = frames.mapValues { $0.offsetBy(dx: 0, dy: -appliedShift) }
                         if abs(appliedShift) > 1 {
                             // Geometry reports the raw offset; ScrollPosition uses
                             // zero at the top inset, including the safe area.
-                            scrollPosition.scrollTo(y: target + scrollMetrics.topInset)
+                            scrollPosition.scrollTo(y: target + scrollTracking.metrics.topInset)
                         }
                         restoreAnchor = nil
                     } else {
-                        rowFrames = frames
+                        scrollTracking.rowFrames = frames
                         if let expected = restoreAnchor?.expectedCount, frames.count >= expected { restoreAnchor = nil }
                     }
                 }
@@ -170,9 +170,9 @@ struct SybilAgentView: View {
             // This cursor belongs to the rendered snapshot. New network input cancels
             // the task, and cannot be acknowledged while still entering the layout.
             let cursor = rendered.room.cursor
-            guard atLatest, scrollMetrics.atBottom, !isScrolling, !room.isObscured, !room.isModalPresented, !room.isAttachmentModalPresented, scenePhase == .active, !room.isLoading else { return }
+            guard atLatest, scrollTracking.metrics.atBottom, !isScrolling, !room.isObscured, !room.isModalPresented, !room.isAttachmentModalPresented, scenePhase == .active, !room.isLoading else { return }
             try? await Task.sleep(for: .milliseconds(reduceMotion ? 80 : 360))
-            guard !Task.isCancelled, atLatest, scrollMetrics.atBottom, !isScrolling, !room.isObscured, !room.isModalPresented, !room.isAttachmentModalPresented, scenePhase == .active else { return }
+            guard !Task.isCancelled, atLatest, scrollTracking.metrics.atBottom, !isScrolling, !room.isObscured, !room.isModalPresented, !room.isAttachmentModalPresented, scenePhase == .active else { return }
             room.markRead(renderedCursor: cursor)
         }
     }
@@ -236,10 +236,41 @@ struct AgentAvatar: View {
     }
 }
 
+// Activity/read publications still update the surrounding view. Completed rows
+// depend only on their message values and date heading, so progress changes
+// must not rebuild the eager history (whose exact heights preserve anchors).
+private struct AgentTranscriptRow: View, Equatable {
+    var message: Message
+    var room: SybilAgentRoom
+    var startsDay: Bool
+
+    nonisolated static func == (lhs: Self, rhs: Self) -> Bool {
+        lhs.message == rhs.message && lhs.room === rhs.room && lhs.startsDay == rhs.startsDay
+    }
+
+    var body: some View {
+        #if DEBUG
+        let _ = AgentRenderDiagnostics.record("transcriptRow")
+        #endif
+        VStack(spacing: 14) {
+            if startsDay {
+                Text(message.createdAt.formatted(date: .abbreviated, time: .omitted))
+                    .font(.sybil(.caption2))
+                    .foregroundStyle(SybilTheme.textMuted)
+                    .frame(maxWidth: .infinity)
+                    .padding(.vertical, 6)
+            }
+            AgentMessageRow(message: message, room: room)
+                .background(GeometryReader { geometry in
+                    Color.clear.preference(key: AgentRowFrames.self, value: [message.id: geometry.frame(in: .named("agent-scroll"))])
+                })
+        }
+    }
+}
+
 private struct AgentMessageRow: View {
     var message: Message
     var room: SybilAgentRoom
-    var retry: () -> Void
     private var own: Bool { message.role == .user }
     private var status: String { message.metadata?.objectValue?["status"]?.stringValue ?? "complete" }
     var body: some View {
@@ -257,7 +288,7 @@ private struct AgentMessageRow: View {
                     if own && status == "queued" { Label("Queued", systemImage: "clock") }
                     if own && status == "failed" {
                         Text("Not confirmed").foregroundStyle(SybilTheme.danger)
-                        Button("Retry", action: retry).accessibilityIdentifier("agent-retry")
+                        Button("Retry") { room.retry(message.id) }.accessibilityIdentifier("agent-retry")
                     }
                 }
                 .font(.sybil(.caption2)).foregroundStyle(SybilTheme.textMuted)
@@ -297,6 +328,14 @@ private struct AgentEntrance: ViewModifier {
     }
 }
 
+// Pixel geometry is imperative anchor bookkeeping, not rendered content. Keeping
+// it outside observation avoids rebuilding the entire transcript on every scroll
+// frame. Only the at-bottom boundary above invalidates the read acknowledgement.
+private final class AgentScrollTracking {
+    var metrics = AgentScrollMetrics()
+    var rowFrames: [String: CGRect] = [:]
+}
+
 private struct AgentScrollMetrics: Equatable {
     var offset: CGFloat = 0
     var height: CGFloat = 0

ios/Packages/Sybil/Sources/Sybil/SybilMessageText.swift

@@ -28,6 +28,9 @@ struct SybilMessageText: View {
     }
 
     var body: some View {
+        #if DEBUG
+        let _ = AgentRenderDiagnostics.record("messageTextBuild")
+        #endif
         Markdown(format == .literal ? Self.literalContent(content) : MarkdownContent(content))
             .tint(SybilTheme.primary)
             .foregroundStyle(format == .literal ? SybilTheme.text : SybilTheme.text.opacity(0.95))