ios: experiment with inverted Agent transcript layout

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

@@ -1,30 +1,37 @@
 import SwiftUI
+import Observation
 
 struct SybilAgentView: View {
-    @Bindable var room: SybilAgentRoom
+    var room: SybilAgentRoom
     var topInset: CGFloat = 0
-    @Environment(\.scenePhase) private var scenePhase
+    @State private var latestRequest = 0
+
+    var body: some View {
+        VStack(spacing: 0) {
+            AgentTranscriptView(room: room, topInset: topInset, latestRequest: latestRequest)
+            AgentStatusBar(room: room)
+            AgentComposer(room: room) { latestRequest += 1 }
+        }
+        .onAppear { room.isSelected = true }
+        .onDisappear { room.isSelected = false }
+    }
+}
+
+// Draft edits and keyboard focus belong to the composer. The transcript observes
+// only room/render/scroll state; SwiftUI handles keyboard avoidance.
+private struct AgentTranscriptView: View {
+    var room: SybilAgentRoom
+    var topInset: CGFloat
+    var latestRequest: Int
     @Environment(\.accessibilityReduceMotion) private var reduceMotion
-    @FocusState private var composerFocused: Bool
     @State private var atLatest = true
-    @State private var isScrolling = false
+    @State private var scrollPhase: ScrollPhase = .idle
     @State private var historyLimit = 100
     @State private var historyStartID: String?
-    @State private var scrollPosition = ScrollPosition()
+    private static let bottomID = "agent-bottom"
     @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):\(atBottom):\(isScrolling):\(room.isObscured):\(room.isModalPresented):\(room.isAttachmentModalPresented):\(scenePhase == .active):\(room.isLoading):\(room.snapshot.revision)"
-    }
-    private var visibleReadingAnchor: AgentReadingAnchor? {
-        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) }
-    }
+    private var isScrolling: Bool { scrollPhase.isScrolling }
 
     var body: some View {
         #if DEBUG
@@ -35,149 +42,146 @@ struct SybilAgentView: View {
         let allMessages = transcript.messages
         let startIndex = transcript.startIndex
         let displayed = transcript.displayed
-        VStack(spacing: 0) {
-            ScrollViewReader { proxy in
-                ScrollView {
-                    VStack(alignment: .leading, spacing: 15) {
-                        if allMessages.count > displayed.count {
-                            Button("Load earlier messages") {
-                                if var anchor = visibleReadingAnchor {
-                                    anchor.expectedCount = min(allMessages.count, displayed.count + 100)
-                                    restoreAnchor = anchor
-                                }
-                                atLatest = false
-                                historyLimit += 100
-                                historyStartID = allMessages[max(0, startIndex - 100)].id
-                            }
-                            .frame(maxWidth: .infinity)
-                            .padding(.vertical, 10)
+        ScrollViewReader { proxy in
+            ScrollView {
+                LazyVStack(alignment: .leading, spacing: 15) {
+                    // The native scroll origin is the visual bottom. Keep model
+                    // and cursor order ascending; reverse only presentation.
+                    Color.clear.frame(height: 17).id(Self.bottomID)
+                    if rendered.isTyping {
+                        HStack(spacing: 9) {
+                            AgentAvatar()
+                            ProgressView().controlSize(.small)
+                            Text("Sybil is thinking…").font(.sybil(.caption))
                         }
-                        if room.isLoading { ProgressView("Loading messages…").frame(maxWidth: .infinity) }
-                        ForEach(Array(displayed.enumerated()), id: \.element.id) { index, message in
-                            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) {
-                                AgentAvatar()
-                                ProgressView().controlSize(.small)
-                                Text("Sybil is thinking…").font(.sybil(.caption))
-                            }
-                            .foregroundStyle(SybilTheme.textMuted)
-                            .accessibilityIdentifier("agent-typing")
-                        }
-                        Color.clear.frame(height: 1).id(bottomID)
+                        .foregroundStyle(SybilTheme.textMuted)
+                        .accessibilityIdentifier("agent-typing")
+                        .scaleEffect(x: 1, y: -1)
                     }
-                    .scrollTargetLayout()
-                    .padding(.horizontal, 16)
-                    .padding(.bottom, 16)
-                    // Match normal chat: content clears the header, while the
-                    // viewport extends beneath the shared navigation fade.
-                    .padding(.top, 16 + topInset)
-                    .frame(maxWidth: SybilLayout.webContentMaxWidth)
-                    .frame(maxWidth: .infinity)
-                }
-                .coordinateSpace(name: "agent-scroll")
-                .scrollPosition($scrollPosition)
-                .defaultScrollAnchor(.bottom, for: .initialOffset)
-                .defaultScrollAnchor(.bottom, for: .alignment)
-                .scrollDismissesKeyboard(.interactively)
-                .onScrollPhaseChange { _, phase in
-                    isScrolling = phase.isScrolling
-                    if phase == .interacting || phase == .tracking { restoreAnchor = nil }
-                }
-                .onScrollGeometryChange(for: AgentScrollMetrics.self) { geometry in
-                    AgentScrollMetrics(offset: geometry.contentOffset.y,
-                        height: geometry.containerSize.height,
-                        contentHeight: geometry.contentSize.height,
-                        bottomInset: geometry.contentInsets.bottom, topInset: geometry.contentInsets.top,
-                        visibleBottom: geometry.visibleRect.maxY)
-                } action: { previous, next in
-                    #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 {
-                        proxy.scrollTo(bottomID, anchor: .bottom)
+                    ForEach(Array(displayed.enumerated().reversed()), id: \.element.id) { index, message in
+                        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))
+                            .scaleEffect(x: 1, y: -1)
+                            .id(message.id)
+                    }
+                    if room.isLoading {
+                        ProgressView("Loading messages…").frame(maxWidth: .infinity)
+                            .scaleEffect(x: 1, y: -1)
+                    }
+                    if allMessages.count > displayed.count {
+                        Button("Load earlier messages") {
+                            atLatest = false
+                            historyLimit += 100
+                            historyStartID = allMessages[max(0, startIndex - 100)].id
+                        }
+                        .frame(maxWidth: .infinity)
+                        .padding(.vertical, 10)
+                        .scaleEffect(x: 1, y: -1)
                     }
                 }
-                .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 = 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 + scrollTracking.metrics.topInset)
-                        }
-                        restoreAnchor = nil
-                    } else {
-                        scrollTracking.rowFrames = frames
-                        if let expected = restoreAnchor?.expectedCount, frames.count >= expected { restoreAnchor = nil }
-                    }
-                }
-                .overlay(alignment: .bottomTrailing) {
-                    if !atLatest {
-                        Button {
-                            atLatest = true
-                            withAnimation(reduceMotion ? nil : .easeOut(duration: 0.24)) { proxy.scrollTo(bottomID, anchor: .bottom) }
-                        } label: {
-                            Label(room.unreadCount > 0 ? "\(room.unreadCount) unread" : "Latest", systemImage: "arrow.down")
-                                .font(.sybil(.caption, weight: .semibold))
-                                .padding(12)
-                                .background(SybilTheme.surfaceStrong, in: Capsule())
-                                .overlay(Capsule().stroke(SybilTheme.border))
-                        }
-                        .padding(14)
-                        .accessibilityIdentifier("agent-jump-latest")
-                    }
-                }
-                .onAppear {
-                    appeared = true
-                    if !room.isLoading { historyStartID = displayed.first?.id }
-                }
-                .onChange(of: room.isLoading) { _, loading in
-                    if !loading && historyStartID == nil { historyStartID = displayed.first?.id }
-                }
-                .onChange(of: allMessages.count) { _, _ in
-                    if atLatest { proxy.scrollTo(bottomID, anchor: .bottom) }
-                }
-                .onChange(of: composerFocused) { _, _ in
-                    if atLatest { proxy.scrollTo(bottomID, anchor: .bottom) }
+                // Keep native lazy-row targets without binding scroll position.
+                .scrollTargetLayout()
+                .padding(.horizontal, 16)
+                // Match normal chat: content clears the header, while the
+                // viewport extends beneath the shared navigation fade.
+                .padding(.bottom, 16 + topInset)
+                .frame(maxWidth: SybilLayout.webContentMaxWidth)
+                .frame(maxWidth: .infinity)
+            }
+            .scaleEffect(x: 1, y: -1)
+            .accessibilityIdentifier("agent-transcript")
+            .scrollDismissesKeyboard(.interactively)
+            .onScrollPhaseChange { _, phase in
+                scrollPhase = phase
+            }
+            .onScrollGeometryChange(for: Bool.self) { geometry in
+                geometry.containerSize.height > 0 &&
+                    geometry.contentOffset.y + geometry.contentInsets.top <= 24
+            } action: { _, atBottom in
+                #if DEBUG
+                AgentRenderDiagnostics.record("scrollGeometry")
+                #endif
+                scrollTracking.atBottom = atBottom
+                // Only user scrolling changes follow intent. Animated Latest
+                // navigation and native keyboard resizing must not cancel it.
+                if isScrolling && scrollPhase != .animating {
+                    atLatest = atBottom
                 }
             }
-            statusBar
-            composer
+            .overlay(alignment: .bottomTrailing) {
+                if !atLatest {
+                    Button {
+                        atLatest = true
+                        withAnimation(reduceMotion ? nil : .easeOut(duration: 0.24)) { proxy.scrollTo(Self.bottomID, anchor: .top) }
+                    } label: {
+                        Label(room.unreadCount > 0 ? "\(room.unreadCount) unread" : "Latest", systemImage: "arrow.down")
+                            .font(.sybil(.caption, weight: .semibold))
+                            .padding(12)
+                            .background(SybilTheme.surfaceStrong, in: Capsule())
+                            .overlay(Capsule().stroke(SybilTheme.border))
+                    }
+                    .padding(14)
+                    .accessibilityIdentifier("agent-jump-latest")
+                }
+            }
+            .onAppear {
+                appeared = true
+                if !room.isLoading {
+                    historyStartID = displayed.first?.id
+                }
+            }
+            .onChange(of: room.isLoading) { _, loading in
+                if !loading && historyStartID == nil {
+                    historyStartID = displayed.first?.id
+                }
+            }
+            .onChange(of: latestRequest) { _, _ in
+                if !atLatest {
+                    atLatest = true
+                    proxy.scrollTo(Self.bottomID, anchor: .top)
+                }
+            }
+            .modifier(AgentReadAcknowledgement(room: room, tracking: scrollTracking,
+                                              atLatest: atLatest, isScrolling: isScrolling))
         }
-        .onAppear { room.isSelected = true }
-        .onDisappear { room.isSelected = false }
+    }
+}
+
+// Bottom proximity can change twice per entrance/keyboard frame while the
+// scroll view follows the growing content. Only the read-settling task needs
+// those transitions; they must not project and rebuild the transcript hierarchy.
+private struct AgentReadAcknowledgement: ViewModifier {
+    var room: SybilAgentRoom
+    var tracking: AgentScrollTracking
+    var atLatest: Bool
+    var isScrolling: Bool
+    @Environment(\.scenePhase) private var scenePhase
+    @Environment(\.accessibilityReduceMotion) private var reduceMotion
+
+    private var readKey: String {
+        "\(room.snapshot.room.cursor):\(atLatest):\(tracking.atBottom):\(isScrolling):\(room.isObscured):\(room.isModalPresented):\(room.isAttachmentModalPresented):\(scenePhase == .active):\(room.isLoading):\(room.snapshot.revision)"
+    }
+
+    func body(content: Content) -> some View {
+        let cursor = room.snapshot.room.cursor
+        content
         .task(id: readKey) {
-            // 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, scrollTracking.metrics.atBottom, !isScrolling, !room.isObscured, !room.isModalPresented, !room.isAttachmentModalPresented, scenePhase == .active, !room.isLoading else { return }
+            // Capture the rendered cursor before scheduling. New network input
+            // cancels this task and must settle in layout before acknowledgement.
+            guard atLatest, tracking.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, scrollTracking.metrics.atBottom, !isScrolling, !room.isObscured, !room.isModalPresented, !room.isAttachmentModalPresented, scenePhase == .active else { return }
+            guard !Task.isCancelled, atLatest, tracking.atBottom, !isScrolling, !room.isObscured, !room.isModalPresented, !room.isAttachmentModalPresented, scenePhase == .active else { return }
             room.markRead(renderedCursor: cursor)
         }
     }
+}
 
-    @ViewBuilder private var statusBar: some View {
+private struct AgentStatusBar: View {
+    var room: SybilAgentRoom
+
+    @ViewBuilder var body: some View {
         let activity = room.snapshot.room.activity
         if let error = room.error ?? activity.error {
             HStack(alignment: .top) {
@@ -199,12 +203,18 @@ struct SybilAgentView: View {
             .padding(.horizontal, 16).padding(.vertical, 6)
         }
     }
+}
+
+private struct AgentComposer: View {
+    @Bindable var room: SybilAgentRoom
+    var onSubmit: () -> Void
+    @FocusState private var composerFocused: Bool
 
     private var canSendComposer: Bool {
         room.attachmentDraft.canSend
     }
 
-    private var composer: some View {
+    var body: some View {
         AgentAttachmentComposer(room: room) {
             SybilPromptField(
                 text: $room.composer,
@@ -223,7 +233,7 @@ struct SybilAgentView: View {
 
     private func submitComposer() {
         guard canSendComposer, room.sendComposer() else { return }
-        atLatest = true
+        onSubmit()
     }
 }
 
@@ -236,9 +246,8 @@ 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).
+// Activity/read publications still update the transcript. Completed rows depend
+// only on message values and heading; scrolling needs no per-row geometry.
 private struct AgentTranscriptRow: View, Equatable {
     var message: Message
     var room: SybilAgentRoom
@@ -261,9 +270,6 @@ private struct AgentTranscriptRow: View, Equatable {
                     .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"))])
-                })
         }
     }
 }
@@ -300,63 +306,24 @@ private struct AgentMessageRow: View {
     }
 }
 
-// Animate both occupied height and content, so incoming rows push older history up.
-private struct AgentGrowingLayout: Layout {
-    var progress: CGFloat
-    var animatableData: CGFloat { get { progress } set { progress = newValue } }
-    func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGSize {
-        let size = subviews.first?.sizeThatFits(ProposedViewSize(width: proposal.width, height: nil)) ?? .zero
-        return CGSize(width: proposal.width ?? size.width, height: size.height * progress)
-    }
-    func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) {
-        subviews.first?.place(at: bounds.origin, proposal: ProposedViewSize(width: bounds.width, height: nil))
-    }
-}
+// Keep the row's measured height stable for lazy layout. Animate its appearance
+// without collapsing the scroll target while the composer or keyboard resizes.
 private struct AgentEntrance: ViewModifier {
     var animate: Bool
     var reduceMotion: Bool
     @State private var entered = false
     func body(content: Content) -> some View {
         let preparing = animate && !entered && !reduceMotion
-        AgentGrowingLayout(progress: preparing ? 0 : 1) {
-            content.opacity(preparing ? 0 : 1).offset(y: preparing ? 16 : 0)
-        }
-        .clipped()
+        content
+        .opacity(preparing ? 0 : 1)
+        .offset(y: preparing ? 16 : 0)
         .onAppear {
             withAnimation(animate && !reduceMotion ? .easeOut(duration: 0.24) : nil) { entered = true }
         }
     }
 }
 
-// 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
-    var contentHeight: CGFloat = 0
-    var bottomInset: CGFloat = 0
-    var topInset: CGFloat = 0
-    var visibleBottom: CGFloat = 0
-    // visibleRect includes the viewport extended into the safe area, as well as
-    // the alignment inset used when a short transcript is pinned to the bottom.
-    var distanceFromBottom: CGFloat { contentHeight + bottomInset - visibleBottom }
-    var atBottom: Bool { height > 0 && distanceFromBottom <= 24 }
-    var maximumOffset: CGFloat { max(-topInset, offset + distanceFromBottom) }
-}
-private struct AgentReadingAnchor {
-    var id: String
-    var offset: CGFloat
-    var expectedCount: Int? = nil
-}
-private struct AgentRowFrames: PreferenceKey {
-    static let defaultValue: [String: CGRect] = [:]
-    static func reduce(value: inout [String: CGRect], nextValue: () -> [String: CGRect]) {
-        value.merge(nextValue(), uniquingKeysWith: { _, next in next })
-    }
+// Only the bottom boundary invalidates read acknowledgement, not the transcript.
+@Observable private final class AgentScrollTracking {
+    var atBottom = false
 }

ios/Tests/AgentPerformance.md

@@ -1,4 +1,4 @@
-# Agent scrolling benchmark
+# Agent interaction benchmarks
 
 Run the production Agent view in an app-hosted XCTest on the iPhone 16e simulator:
 
@@ -7,62 +7,101 @@ just generate
 xcodebuild test -scheme SybilPresentation \
   -destination 'platform=iOS Simulator,name=iPhone 16e,OS=latest' \
   -derivedDataPath build/DerivedData -parallel-testing-enabled NO \
-  -only-testing:SybilPresentationTests/AgentPerformanceTests \
-  -resultBundlePath build/agent-performance.xcresult
+  -only-testing:SybilPresentationTests/AgentPerformanceTests/testInputAndEntranceResponsiveness \
+  -resultBundlePath build/agent-input-performance.xcresult
 ```
 
-`AgentPerformanceTests` seeds a room through its normal event reducer with 160
-local messages (100 initially displayed). User text is literal; assistant text
-contains paragraphs, lists, links, emphasis and code. The real Agent view,
-composer, avatar, entrance layout and thinking spinner remain enabled. No server,
-production credentials, notification delivery or network images are needed.
+`testInputAndEntranceResponsiveness` seeds 160 local mixed literal/Markdown
+messages through the normal room reducer (100 initially displayed), then:
 
-Three approximately 12-second phases run in order: idle scrolling, thinking at
-the bottom, and thinking while scrolling. The test explicitly resets the offset
-to the bottom before the stationary thinking phase (including in the baseline).
-Thinking activity changes queue depth, worker count and tool progress every
-500 ms through the normal 100 ms publication
-batch. The display link sweeps a 2400-point range at 600 points/second, reversing
-every four seconds. Use the **same test source, simulator, build configuration
-and host load** for before/after runs. Don't run other builds or profilers during
-a measured run. Warm builds and repeat runs help distinguish stable effects from
-host noise.
+- Receives 12 more messages at 550 ms intervals through the production 100 ms
+  publication batch and 240 ms entrance animation, checking bottom following.
+- Presents and dismisses the actual software keyboard six times, 600 ms per
+  transition. It asserts that the transcript viewport shrinks by over 100 points
+  and remains at the bottom; a hardware-keyboard-only run is invalid.
+- Calls UIKit `insertText` on the actual focused composer every 30 ms, including
+  enough text to wrap and grow to its six-line limit. It verifies the draft,
+  bottom following, and that keyboard/draft changes don't reproject the history.
 
-The geometry assertions use iPhone 16e, iOS 26.5, and the default text size. They
-retain the unoptimized transcript heights (39181.333 points idle, 39223.333 while
-thinking), so an isolation wrapper cannot silently change row spacing. Rebaseline
-these explicitly when changing simulator/font configuration or the fixture.
+The real composer, Markdown rows, avatars and entrance animations remain enabled.
+No server, production credentials, notification delivery or network images are
+needed. The log prints `AGENT_INPUT_PERFORMANCE_JSON`; the result bundle retains
+JSON and screenshots of the actual hosted view before and after typing.
+Screenshots capture the hosting view, not the separate system keyboard window.
 
-The log prints `AGENT_PERFORMANCE_JSON`; the result bundle retains the JSON as an
-attachment. Delivery-gap p50/p95/p99/max and counts over 25/50 ms measure actual
-main-run-loop responsiveness. `missedBudgetIntervals` estimates gaps relative to
-the reported display-link budget. These are **not compositor FPS, GPU hitches,
-touch latency or physical-device results**. Scrolling is programmatic; native
-swipe/anchor/read behavior needs the local-fixture UI regression tests as well.
-The hosted view excludes the surrounding app shell. Physical iPhone scrolling,
-streaming and thinking animation remain a required follow-up.
+Use the same test source, simulator, build configuration and host load for
+before/after measurements. Don't run other builds or profilers during sampling.
+Functional assertions about observation boundaries should fail against the old
+implementation; use the same sampling procedure when collecting a baseline.
 
-Opt-in `AgentRenderDiagnostics` counters are compiled only in Debug and are off
-outside this test. They explain work (Agent body evaluations, message projection,
-message text construction, isolated row evaluations, geometry callbacks and
-snapshot publications). A drop in these counts alone does not establish improved
-frame responsiveness.
+## Measurement limits
 
-## Reference result
+Delivery-gap p50/p95/p99/max and counts over 25/50 ms measure **CADisplayLink
+main-run-loop callback delivery**, not compositor FPS, GPU hitches, touch-to-key
+latency or physical-device performance. The hosted view excludes the surrounding
+app shell. UIKit input exercises editing/layout without synthesizing key taps.
+The first keyboard presentation includes system warmup; residual system and
+simulator scheduling gaps remain. Native gestures, read settling, header fade,
+attachments and reading anchors are covered separately by the loopback UI tests.
 
-On monsoon, Xcode 26.5, Debug, iPhone 16e / iOS 26.5, clean repeat runs against
-`f534f76` plus counters and against the geometry/row isolation fix gave:
+`AgentRenderDiagnostics` counters are Debug-only and opt-in. They explain work;
+a drop in counts alone does not establish improved responsiveness.
 
-| Phase | Before p95 / p99 (ms) | After p95 / p99 (ms) | Gaps >50 ms, before / after |
+The transcript now uses `LazyVStack` with the observation boundaries introduced
+in `bbaa5e8`. Loading older history may move the viewport; this is the only
+waived position guarantee. Native regressions still assert reading anchors for
+incoming messages, bottom following, keyboard behavior, read settling, fade,
+and attachments. There is no saved prepend anchor or convergence loop.
+
+Lazy content height estimates change as offscreen rows materialize. The earlier
+43869→35870 point difference was not shortened content: after-arrival screenshots
+were byte-identical. Compare visible geometry and native behavior, not estimated
+whole-history height, when assessing lazy performance.
+
+## Scrolling harness
+
+`testThinkingScrollResponsiveness` retains the earlier three approximately
+12-second phases: idle scroll, stationary thinking, and scrolling with changing
+queue/worker/tool activity every 500 ms. Its display link sweeps 2400 points at
+600 points/second. This programmatic sweep does not reproduce native drag phases;
+use the UI tests for reading-anchor geometry and interactive keyboard dismissal.
+
+The eager-stack reference in `b9a6f5d` has exact heights of 39181.333
+points idle and 39223.333 while thinking. That reference harness retains its
+eager-only total-height assertions; run it at `bbaa5e8` for the eager comparison.
+Those totals are not valid assertions about lazy estimated content size. Use the
+input harness and native gesture tests above for the current lazy implementation.
+That earlier scrolling fix reduced busy-scroll p95 from 88.8 to 19.4 ms; it did
+not address the input/entrance paths measured here. Physical phone validation
+remains necessary for hardware performance claims.
+
+## 2026-10-16 comparison before the prepend waiver
+
+Single Debug simulator runs on the same iPhone 16e / iOS 26.5, starting at
+`7510603` (which includes `b9a6f5d`). Values are callback-gap p95 milliseconds:
+
+| Path | Before | Eager isolation (`bbaa5e8`) | Earlier lazy experiment |
 | --- | ---: | ---: | ---: |
-| Idle scroll | 50.01 / 50.02 | 16.68 / 16.71 | 128 / 0 |
-| Thinking stationary | 33.33 / 81.92 | 28.91 / 38.35 | 22 / 2 |
-| Thinking scroll | 88.78 / 90.89 | 19.41 / 30.42 | 123 / 1 |
+| 12 message entrances | 40.8 | 33.2 | 16.8 |
+| Six keyboard cycles | 33.2 | 33.1 | 29.2 |
+| Multiline composer typing | 24.8 | 25.2 | 21.3 |
 
-Each phase lasted approximately 12 seconds. Content heights matched exactly.
-Busy-scroll Agent body/projection evaluations fell from 261 to 27; unchanged
-text constructions were zero in both versions. Geometry isolation alone removed
-the idle-scroll stalls; individual row isolation reduced activity-related gaps.
-These results support those two boundaries, without a Markdown cache or lazy
-layout change. Occasional simulator gaps remain; this does not replace device
-validation.
+Eager transcript projections were 12/0/0 versus 220/24/10, with no row-frame
+preference callbacks in these pinned-bottom phases. Eager content height stayed
+43869.333 points. Entrance p99 improved 83.4→57.1 ms; keyboard p99 55.9→44.5;
+typing p99 33.2→26.7. Residual gaps remain: entrance/keyboard had 10/3 gaps over
+50 ms; typing had 30 over 25 ms (baseline 19). Keyboard p95 was essentially flat
+and typing p95 did not improve. These single runs establish removed transcript
+work and a safer entrance improvement, not universally smooth input or device FPS.
+The lazy option was initially rejected because strict prepend anchors failed.
+The subsequent release accepts that prepend tradeoff; all other guarantees remain.
+
+## Lazy release after the prepend waiver
+
+The same input harness passes unchanged: p95 entrance/keyboard/typing gaps are
+16.9/28.1/20.9 ms, versus the eager run's 33.2/33.1/25.2 ms. Projections stay
+12/0/0; pinned row-frame preference callbacks stay zero. The after-arrival PNG
+is byte-identical to the eager baseline. Residual gaps remain (one keyboard gap
+over 50 ms and one typing gap over 25 ms); these are single simulator samples. Five
+focused native regressions pass with only the two prepend-offset assertions
+replaced by checks that the remaining history loaded.

ios/Tests/SybilPresentationTests/AgentPerformanceTests.swift

@@ -72,6 +72,116 @@ final class AgentPerformanceTests: XCTestCase {
         attachment.name = "agent-performance"; attachment.lifetime = .keepAlways; add(attachment)
     }
 
+    /// Exercises UIKit editing/focus and the real message reducer; no synthetic
+    /// view rebuild loop or replacement composer/animation is used.
+    func testInputAndEntranceResponsiveness() async throws {
+        let room = SybilAgentRoom()
+        try room.receive(.messages(AgentPage(conversationId: "performance",
+            messages: (1...160).map(Self.message), cursor: "160", beforeCursor: nil, hasMore: false)), replay: true)
+        room.flush()
+        let host = UIHostingController(rootView: SybilAgentView(room: room, topInset: 64)
+            .background(SybilTheme.background).environment(\.colorScheme, .dark))
+        let scene = try XCTUnwrap(UIApplication.shared.connectedScenes.compactMap { $0 as? UIWindowScene }.first)
+        let window = UIWindow(windowScene: scene)
+        window.rootViewController = host
+        window.makeKeyAndVisible()
+        defer {
+            host.view.endEditing(true)
+            AgentRenderDiagnostics.enabled = false
+            window.isHidden = true; window.rootViewController = nil
+            room.stop()
+        }
+        host.view.frame = window.bounds
+        host.view.layoutIfNeeded()
+        try await Task.sleep(for: .seconds(2))
+        let scroll = try XCTUnwrap(findScroll(host.view))
+        let input = try XCTUnwrap(findInput(host.view))
+        AgentRenderDiagnostics.enabled = true
+        var reports: [[String: Any]] = []
+        func report(_ sampler: AgentFrameSampler, phase: String) {
+            var value = sampler.stop()
+            value["phase"] = phase
+            value["procedure"] = "mixed160-input-entrance-v1"
+            value["operations"] = AgentRenderDiagnostics.counts
+            value["contentHeight"] = scroll.contentSize.height
+            value["viewportHeight"] = scroll.bounds.height
+            reports.append(value)
+        }
+
+        AgentRenderDiagnostics.counts = [:]
+        let entrance = AgentFrameSampler(scroll: nil)
+        entrance.start()
+        for index in 161...172 {
+            try room.receive(.messages(AgentPage(conversationId: "performance",
+                messages: [Self.message(index)], cursor: String(index), beforeCursor: nil, hasMore: false)))
+            // Use the production 100 ms publication batch and 240 ms entrance.
+            try await Task.sleep(for: .milliseconds(550))
+            assertAtBottom(scroll)
+        }
+        XCTAssertLessThanOrEqual(AgentRenderDiagnostics.counts["messageProjection", default: 0], 24,
+                                 "Entrance frames must not repeatedly project the history")
+        report(entrance, phase: "message-entrance")
+        XCTAssertEqual(room.snapshot.room.messages.count, 172)
+        assertAtBottom(scroll)
+        capture(host, "input-entrance-latest")
+
+        let unfocusedHeight = scroll.bounds.height
+        AgentRenderDiagnostics.counts = [:]
+        let keyboard = AgentFrameSampler(scroll: nil)
+        keyboard.start()
+        for _ in 0..<6 {
+            XCTAssertTrue(input.becomeFirstResponder())
+            try await Task.sleep(for: .milliseconds(600))
+            XCTAssertLessThan(scroll.bounds.height, unfocusedHeight - 100, "Software keyboard must actually resize the transcript")
+            assertAtBottom(scroll)
+            XCTAssertTrue(input.resignFirstResponder())
+            try await Task.sleep(for: .milliseconds(600))
+        }
+        XCTAssertEqual(AgentRenderDiagnostics.counts["messageProjection", default: 0], 0,
+                       "Keyboard focus and geometry must not project the transcript")
+        report(keyboard, phase: "keyboard-presentation")
+        XCTAssertEqual(scroll.bounds.height, unfocusedHeight, accuracy: 1)
+        XCTAssertTrue(input.becomeFirstResponder())
+        try await Task.sleep(for: .milliseconds(700))
+        let keyInput = try XCTUnwrap(input as? any UIKeyInput)
+        AgentRenderDiagnostics.counts = [:]
+        let typing = AgentFrameSampler(scroll: nil)
+        typing.start()
+        let text = "Typing into the actual composer should keep the long transcript responsive as this sentence wraps across several lines. "
+        for character in text + text {
+            keyInput.insertText(String(character))
+            try await Task.sleep(for: .milliseconds(30))
+        }
+        XCTAssertEqual(AgentRenderDiagnostics.counts["messageProjection", default: 0], 0,
+                       "Draft edits and line wrapping must not project the transcript")
+        assertAtBottom(scroll)
+        report(typing, phase: "composer-typing")
+        XCTAssertEqual(room.composer, text + text)
+        capture(host, "input-multiline-keyboard")
+        let data = try JSONSerialization.data(withJSONObject: reports, options: [.sortedKeys])
+        print("AGENT_INPUT_PERFORMANCE_JSON " + String(decoding: data, as: UTF8.self))
+        let attachment = XCTAttachment(data: data, uniformTypeIdentifier: "public.json")
+        attachment.name = "agent-input-performance"; attachment.lifetime = .keepAlways; add(attachment)
+    }
+
+    private func assertAtBottom(_ scroll: UIScrollView, file: StaticString = #filePath, line: UInt = #line) {
+        XCTAssertEqual(scroll.contentOffset.y + scroll.bounds.height - scroll.adjustedContentInset.bottom,
+                       scroll.contentSize.height, accuracy: 25, file: file, line: line)
+    }
+
+    private func findInput(_ view: UIView) -> UIView? {
+        if view is UITextView || view is UITextField { return view }
+        return view.subviews.lazy.compactMap { self.findInput($0) }.first
+    }
+
+    private func capture(_ host: UIViewController, _ name: String) {
+        let image = UIGraphicsImageRenderer(bounds: host.view.bounds).image { _ in
+            host.view.drawHierarchy(in: host.view.bounds, afterScreenUpdates: true)
+        }
+        let attachment = XCTAttachment(image: image)
+        attachment.name = name; attachment.lifetime = .keepAlways; add(attachment)
+    }
+
     private static func message(_ index: Int) -> Message {
         let own = index % 3 == 0
         let content = own

ios/Tests/SybilUITests/AgentUITests.swift

@@ -321,15 +321,15 @@ final class AgentUITests: XCTestCase {
         XCTAssertEqual(anchor.frame.minY, before, accuracy: 2)
         capture("agent-reading-anchor")
         for _ in 0..<25 {
-            if app.buttons["Load earlier messages"].isHittable { break }
+            if app.buttons["Load earlier messages"].exists && app.buttons["Load earlier messages"].isHittable { break }
             scroll.swipeDown(velocity: .fast)
         }
         XCTAssertTrue(app.buttons["Load earlier messages"].isHittable)
-        let first = app.staticTexts["History message 61: local simulator fixture."]
-        let beforeLoad = first.frame.minY
         app.buttons["Load earlier messages"].tap()
-        XCTAssertEqual(first.frame.minY, beforeLoad, accuracy: 3)
-        capture("agent-older-history-anchor")
+        // Loading older lazy history may move the viewport. It must still load
+        // the remaining history and preserve Latest/keyboard behavior below.
+        XCTAssertFalse(app.buttons["Load earlier messages"].exists)
+        capture("agent-older-history-loaded")
         app.buttons["agent-jump-latest"].tap()
         XCTAssertTrue(app.staticTexts["Reply while reading earlier history"].isHittable)
         composer.tap(); composer.typeText("A multiline draft\nwith a second line\nand a third line")

ios/Tests/SybilUITests/TranscriptFadeUITests.swift

@@ -37,8 +37,8 @@ final class TranscriptFadeUITests: XCTestCase {
     }
 
     private var transcript: XCUIElement {
-        // The tablet sidebar is also a scroll view; select the detail by its content.
-        app.scrollViews.containing(.staticText, identifier: "History message 160: local simulator fixture.").firstMatch
+        // Identify the transcript independently of any particular message row.
+        app.scrollViews["agent-transcript"]
     }
 
     private func capture(_ name: String) {
@@ -55,6 +55,79 @@ final class TranscriptFadeUITests: XCTestCase {
         return try XCTUnwrap(JSONSerialization.jsonObject(with: data) as? [String: Any])
     }
 
+    private func reloadMixedHistory() async throws {
+        app.terminate()
+        let rows = (1...160).map { index in
+            let short = "History message \(index): local simulator fixture."
+            return ["role": index % 3 == 0 ? "user" : "assistant",
+                    "content": index == 160 || index % 3 == 0 ? short :
+                        "### Investigation \(index)\n\n" + String(repeating: "A longer transcript paragraph with **emphasis** and `inline code`.\n\n", count: 4) +
+                        "- Preserve visible content.\n- Keep the composer available.\n\n```swift\nrender(messages[\(index)])\n```"]
+        }
+        try await control(["action": "presentation-samples", "messages": rows])
+        app.launch()
+    }
+
+    func testInitialOpenAndTabReturnShowLatestWithoutScrolling() async throws {
+        try await reloadMixedHistory()
+        openAgent()
+        let latest = app.staticTexts["History message 160: local simulator fixture."]
+        XCTAssertTrue(latest.isHittable)
+        XCTAssertLessThan(latest.frame.maxY, composer.frame.minY)
+        capture("agent-initial-open")
+
+        showSidebar()
+        app.buttons["Settings"].tap()
+        XCTAssertFalse(composer.exists)
+        openAgent()
+        XCTAssertTrue(latest.isHittable, "Returning to Agent must not need a scroll to materialize rows")
+        XCTAssertLessThan(latest.frame.maxY, composer.frame.minY)
+        capture("agent-tab-return")
+    }
+
+    func testSendClearsMultilineComposerAndKeepsLatestAboveKeyboard() async throws {
+        try await reloadMixedHistory()
+        openAgent()
+        composer.tap()
+        if let existing = composer.value as? String, !existing.isEmpty, existing != "Enter Prompt" {
+            composer.press(forDuration: 1)
+            let selectAll = app.descendants(matching: .any).matching(NSPredicate(format: "label == %@", "Select All")).firstMatch
+            if selectAll.waitForExistence(timeout: 2) { selectAll.tap() }
+            else { composer.typeText(String(repeating: XCUIKeyboardKey.rightArrow.rawValue, count: existing.count)) }
+            composer.typeText(String(repeating: XCUIKeyboardKey.delete.rawValue, count: existing.count))
+        }
+        let draft = "Keyboard send\nsecond line\nthird line"
+        composer.typeText(draft)
+        XCTAssertTrue(app.keyboards.firstMatch.exists)
+        let multilineHeight = composer.frame.height
+        app.buttons["agent-send"].tap()
+        let cleared = XCTNSPredicateExpectation(predicate: NSPredicate(format: "value == %@ OR value == %@", "", "Enter Prompt"), object: composer)
+        await fulfillment(of: [cleared], timeout: 5)
+        XCTAssertLessThan(composer.frame.height, multilineHeight)
+        XCTAssertTrue(app.keyboards.firstMatch.exists)
+        let sent = app.staticTexts[draft]
+        XCTAssertTrue(sent.waitForExistence(timeout: 5))
+        XCTAssertTrue(sent.isHittable)
+        let reply = app.staticTexts.containing(NSPredicate(format: "label CONTAINS %@", "Finished: Keyboard send")).firstMatch
+        let followed = XCTNSPredicateExpectation(predicate: NSPredicate(format: "hittable == true"), object: reply)
+        await fulfillment(of: [followed], timeout: 10)
+        capture("agent-send-cleared-composer-with-keyboard")
+        XCTAssertGreaterThan(reply.frame.minY, transcript.frame.minY + 64)
+        XCTAssertLessThan(reply.frame.maxY, composer.frame.minY)
+        // Finished: is only the first Markdown paragraph. Measure the final
+        // reply content, leaving the same allowance for metadata and spacing.
+        let replyEnd = app.staticTexts["The local fixture returned three matches."]
+        XCTAssertTrue(replyEnd.isHittable)
+        XCTAssertLessThan(replyEnd.frame.maxY, composer.frame.minY)
+        let bottomGap = composer.frame.minY - replyEnd.frame.maxY
+        print("Agent send settled final-content gap: \(bottomGap)pt")
+        XCTAssertLessThan(bottomGap, 100, "Composer clearing must not leave the transcript shifted above the bottom")
+        XCTAssertTrue(app.buttons["agent-attach"].isHittable)
+        try await Task.sleep(for: .milliseconds(500))
+        let read = try await control(["action": "state"])
+        XCTAssertEqual(read["readCursor"] as? Int, read["cursor"] as? Int)
+    }
+
     func testSharedHeaderFadeGeometry() async throws {
         showSidebar()
         let chat = app.buttons.containing(NSPredicate(format: "label CONTAINS %@", "Transcript fade comparison")).firstMatch
@@ -107,23 +180,23 @@ final class TranscriptFadeUITests: XCTestCase {
         capture("agent-fade-reading-position-and-unread")
 
         for _ in 0..<30 {
-            if app.buttons["Load earlier messages"].isHittable { break }
+            if app.buttons["Load earlier messages"].exists && app.buttons["Load earlier messages"].isHittable { break }
             scroll.swipeDown(velocity: .fast)
         }
         let load = app.buttons["Load earlier messages"]
         XCTAssertTrue(load.isHittable)
-        let first = app.staticTexts["History message 61: local simulator fixture."]
-        let beforeLoad = first.frame.minY
         load.tap()
-        capture("agent-fade-prepend-reading-position")
-        XCTAssertEqual(first.frame.minY, beforeLoad, accuracy: 3,
-                       "Prepending must preserve the readable row's position beneath the header")
+        // Prepend position is intentionally not preserved with lazy history.
+        // New-message reading anchors above and follow/read settling below remain strict.
+        XCTAssertFalse(load.exists)
+        capture("agent-fade-older-history-loaded")
 
         app.buttons["agent-jump-latest"].tap()
         let latest = app.staticTexts["Fade regression reply while reading"]
         let followed = XCTNSPredicateExpectation(predicate: NSPredicate(format: "hittable == true"), object: latest)
         await fulfillment(of: [followed], timeout: 5)
         composer.tap()
+        XCTAssertTrue(app.keyboards.firstMatch.waitForExistence(timeout: 5))
         XCTAssertTrue(latest.isHittable)
         XCTAssertTrue(app.buttons["agent-attach"].isHittable)
         capture("agent-fade-latest-with-keyboard")
@@ -134,5 +207,27 @@ final class TranscriptFadeUITests: XCTestCase {
         try await Task.sleep(for: .milliseconds(500))
         let read = try await control(["action": "state"])
         XCTAssertEqual(read["readCursor"] as? Int, read["cursor"] as? Int)
+
+        // A reply still follows and settles as read while the keyboard is up.
+        try await control(["action": "reply", "content": "Reply with keyboard visible"])
+        let keyboardReply = app.staticTexts["Reply with keyboard visible"]
+        let keyboardFollowed = XCTNSPredicateExpectation(predicate: NSPredicate(format: "hittable == true"), object: keyboardReply)
+        await fulfillment(of: [keyboardFollowed], timeout: 5)
+        try await Task.sleep(for: .milliseconds(500))
+        let keyboardRead = try await control(["action": "state"])
+        XCTAssertEqual(keyboardRead["readCursor"] as? Int, keyboardRead["cursor"] as? Int)
+
+        // Interactive dismissal is user navigation; Latest must still return to
+        // the bottom, and presenting the keyboard again must keep it visible.
+        let dismissStart = scroll.coordinate(withNormalizedOffset: CGVector(dx: 0.5, dy: 0.9))
+        let dismissEnd = app.coordinate(withNormalizedOffset: CGVector(dx: 0.5, dy: 0.98))
+        dismissStart.press(forDuration: 0.1, thenDragTo: dismissEnd)
+        let dismissed = XCTNSPredicateExpectation(predicate: NSPredicate(format: "exists == false"), object: app.keyboards.firstMatch)
+        await fulfillment(of: [dismissed], timeout: 5)
+        if app.buttons["agent-jump-latest"].exists { app.buttons["agent-jump-latest"].tap() }
+        XCTAssertTrue(keyboardReply.isHittable)
+        composer.tap()
+        XCTAssertTrue(app.keyboards.firstMatch.waitForExistence(timeout: 5))
+        XCTAssertTrue(keyboardReply.isHittable)
     }
 }