ios: simplify Agent transcript positioning and entrances

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

@@ -28,17 +28,11 @@ private struct AgentTranscriptView: View {
     @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 appeared = false
     private var isScrolling: Bool { scrollPhase.isScrolling }
 
-    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) }
-    }
-
     var body: some View {
         #if DEBUG
         let _ = AgentRenderDiagnostics.record("agentBody")
@@ -48,132 +42,115 @@ private struct AgentTranscriptView: View {
         let allMessages = transcript.messages
         let startIndex = transcript.startIndex
         let displayed = transcript.displayed
-        ScrollView {
-            LazyVStack(alignment: .leading, spacing: 15) {
-                if allMessages.count > displayed.count {
-                    Button("Load earlier messages") {
-                        // Loading older history may move the viewport as lazy
-                        // row estimates settle; do not chase the old position.
-                        scrollTracking.rowFrames = [:]
-                        atLatest = false
-                        historyLimit += 100
-                        historyStartID = allMessages[max(0, startIndex - 100)].id
+        ScrollViewReader { proxy in
+            ScrollView {
+                LazyVStack(alignment: .leading, spacing: 15) {
+                    if allMessages.count > displayed.count {
+                        Button("Load earlier messages") {
+                            // Loading older history may move the viewport as lazy
+                            // row estimates settle; do not chase the old position.
+                            atLatest = false
+                            historyLimit += 100
+                            historyStartID = allMessages[max(0, startIndex - 100)].id
+                        }
+                        .frame(maxWidth: .infinity)
+                        .padding(.vertical, 10)
                     }
-                    .frame(maxWidth: .infinity)
-                    .padding(.vertical, 10)
-                }
-                if room.isLoading { ProgressView("Loading messages…").frame(maxWidth: .infinity) }
-                ForEach(Array(displayed.enumerated()), id: \.element.id) { index, message in
-                    AgentTranscriptRow(message: message, room: room, tracksReadingAnchor: !atLatest,
-                        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))
+                    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)
                     }
-                    .foregroundStyle(SybilTheme.textMuted)
-                    .accessibilityIdentifier("agent-typing")
+                    if rendered.isTyping {
+                        HStack(spacing: 9) {
+                            AgentAvatar()
+                            ProgressView().controlSize(.small)
+                            Text("Sybil is thinking…").font(.sybil(.caption))
+                        }
+                        .foregroundStyle(SybilTheme.textMuted)
+                        .accessibilityIdentifier("agent-typing")
+                    }
+                    // Include bottom padding in the target so Latest reaches
+                    // the actual content end before native keyboard avoidance.
+                    Color.clear.frame(height: 17).id(Self.bottomID)
                 }
-                Color.clear.frame(height: 1)
+                // 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(.top, 16 + topInset)
+                .frame(maxWidth: SybilLayout.webContentMaxWidth)
+                .frame(maxWidth: .infinity)
             }
-            .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)
-        }
-        .accessibilityIdentifier("agent-transcript")
-        .coordinateSpace(name: "agent-scroll")
-        .scrollPosition($scrollPosition)
-        .defaultScrollAnchor(.bottom, for: .initialOffset)
-        // Let SwiftUI follow size changes, including keyboard avoidance,
-        // without competing scrollTo calls from geometry updates.
-        .defaultScrollAnchor(atLatest ? .bottom : nil, for: .sizeChanges)
-        .defaultScrollAnchor(.bottom, for: .alignment)
-        .scrollDismissesKeyboard(.interactively)
-        .onScrollPhaseChange { _, phase in
-            scrollPhase = phase
-        }
-        .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: { _, next in
-            #if DEBUG
-            AgentRenderDiagnostics.record("scrollGeometry")
-            #endif
-            scrollTracking.metrics = next
-            if scrollTracking.atBottom != next.atBottom { scrollTracking.atBottom = next.atBottom }
-            // Only user scrolling changes follow intent. Animated Latest
-            // navigation and native keyboard resizing must not cancel it.
-            if (isScrolling && scrollPhase != .animating) || next.contentHeight <= next.height {
-                atLatest = next.atBottom
+            .accessibilityIdentifier("agent-transcript")
+            .defaultScrollAnchor(.bottom, for: .initialOffset)
+            // Let SwiftUI follow size changes, including keyboard avoidance,
+            // without competing scrollTo calls from geometry updates.
+            .defaultScrollAnchor(atLatest ? .bottom : nil, for: .sizeChanges)
+            .defaultScrollAnchor(.bottom, for: .alignment)
+            .scrollDismissesKeyboard(.interactively)
+            .onScrollPhaseChange { _, phase in
+                scrollPhase = phase
             }
-        }
-        .onPreferenceChange(AgentRowFrames.self) { frames in
-            #if DEBUG
-            AgentRenderDiagnostics.record("rowPreferences")
-            #endif
-            let anchor = !atLatest && !isScrolling ? visibleReadingAnchor : nil
-            if let anchor, let frame = frames[anchor.id], abs(frame.minY - anchor.offset) > 1 {
-                // Preserve ordinary reading position after new messages 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)
+            .onScrollGeometryChange(for: Bool.self) { geometry in
+                geometry.containerSize.height > 0 &&
+                    geometry.contentSize.height + geometry.contentInsets.bottom - geometry.visibleRect.maxY <= 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
                 }
-            } else {
-                scrollTracking.rowFrames = frames
             }
-        }
-        .overlay(alignment: .bottomTrailing) {
-            if !atLatest {
-                Button {
+            .overlay(alignment: .bottomTrailing) {
+                if !atLatest {
+                    Button {
+                        atLatest = true
+                        withAnimation(reduceMotion ? nil : .easeOut(duration: 0.24)) { proxy.scrollTo(Self.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
+                    proxy.scrollTo(Self.bottomID, anchor: .bottom)
+                }
+            }
+            .onChange(of: room.isLoading) { _, loading in
+                if !loading && historyStartID == nil {
+                    historyStartID = displayed.first?.id
+                    proxy.scrollTo(Self.bottomID, anchor: .bottom)
+                }
+            }
+            .onChange(of: allMessages.count) { _, _ in
+                if atLatest { proxy.scrollTo(Self.bottomID, anchor: .bottom) }
+            }
+            .onChange(of: latestRequest) { _, _ in
+                if !atLatest {
                     atLatest = true
-                    withAnimation(reduceMotion ? nil : .easeOut(duration: 0.24)) { scrollPosition.scrollTo(edge: .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))
+                    proxy.scrollTo(Self.bottomID, anchor: .bottom)
                 }
-                .padding(14)
-                .accessibilityIdentifier("agent-jump-latest")
             }
+            .modifier(AgentReadAcknowledgement(room: room, tracking: scrollTracking,
+                                              atLatest: atLatest, isScrolling: isScrolling))
         }
-        .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 { scrollPosition.scrollTo(edge: .bottom) }
-        }
-        .onChange(of: latestRequest) { _, _ in
-            atLatest = true
-            scrollPosition.scrollTo(edge: .bottom)
-        }
-        .modifier(AgentReadAcknowledgement(room: room, tracking: scrollTracking,
-                                          atLatest: atLatest, isScrolling: isScrolling))
     }
 }
 
@@ -198,9 +175,9 @@ private struct AgentReadAcknowledgement: ViewModifier {
         .task(id: readKey) {
             // Capture the rendered cursor before scheduling. New network input
             // cancels this task and must settle in layout before acknowledgement.
-            guard atLatest, tracking.metrics.atBottom, !isScrolling, !room.isObscured, !room.isModalPresented, !room.isAttachmentModalPresented, scenePhase == .active, !room.isLoading else { return }
+            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, tracking.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)
         }
     }
@@ -275,16 +252,14 @@ struct AgentAvatar: View {
 }
 
 // Activity/read publications still update the transcript. Completed rows depend
-// only on message values, heading and whether a reading anchor is needed.
+// only on message values and heading; scrolling needs no per-row geometry.
 private struct AgentTranscriptRow: View, Equatable {
     var message: Message
     var room: SybilAgentRoom
-    var tracksReadingAnchor: Bool
     var startsDay: Bool
 
     nonisolated static func == (lhs: Self, rhs: Self) -> Bool {
         lhs.message == rhs.message && lhs.room === rhs.room && lhs.startsDay == rhs.startsDay
-            && lhs.tracksReadingAnchor == rhs.tracksReadingAnchor
     }
 
     var body: some View {
@@ -300,15 +275,6 @@ private struct AgentTranscriptRow: View, Equatable {
                     .padding(.vertical, 6)
             }
             AgentMessageRow(message: message, room: room)
-                .background {
-                    // Pinned-bottom layout needs only the scroll view's metrics.
-                    // Rows report frames only while preserving a reading anchor.
-                    if tracksReadingAnchor {
-                        GeometryReader { geometry in
-                            Color.clear.preference(key: AgentRowFrames.self, value: [message.id: geometry.frame(in: .named("agent-scroll"))])
-                        }
-                    }
-                }
         }
     }
 }
@@ -345,63 +311,24 @@ 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 invalidates the read acknowledgement.
+// Only the bottom boundary invalidates read acknowledgement, not the transcript.
 @Observable private final class AgentScrollTracking {
     var atBottom = false
-    @ObservationIgnored var metrics = AgentScrollMetrics()
-    @ObservationIgnored 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
-}
-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 })
-    }
 }