ios: experiment with inverted Agent transcript layout

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")
@@ -48,28 +42,12 @@ private struct AgentTranscriptView: View {
         let allMessages = transcript.messages
         let startIndex = transcript.startIndex
         let displayed = transcript.displayed
+        ScrollViewReader { proxy in
             ScrollView {
                 LazyVStack(alignment: .leading, spacing: 15) {
-                if allMessages.count > displayed.count {
+                    // The native scroll origin is the visual bottom. Keep model
-                    Button("Load earlier messages") {
+                    // and cursor order ascending; reverse only presentation.
-                        // Loading older history may move the viewport as lazy
+                    Color.clear.frame(height: 17).id(Self.bottomID)
-                        // row estimates settle; do not chase the old position.
-                        scrollTracking.rowFrames = [:]
-                        atLatest = false
-                        historyLimit += 100
-                        historyStartID = allMessages[max(0, startIndex - 100)].id
-                    }
-                    .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()
@@ -78,75 +56,65 @@ private struct AgentTranscriptView: View {
                         }
                         .foregroundStyle(SybilTheme.textMuted)
                         .accessibilityIdentifier("agent-typing")
+                        .scaleEffect(x: 1, y: -1)
                     }
-                Color.clear.frame(height: 1)
+                    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)
+                    }
+                }
+                // Keep native lazy-row targets without binding scroll position.
                 .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)
+                .padding(.bottom, 16 + topInset)
                 .frame(maxWidth: SybilLayout.webContentMaxWidth)
                 .frame(maxWidth: .infinity)
             }
+            .scaleEffect(x: 1, y: -1)
             .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
+            .onScrollGeometryChange(for: Bool.self) { geometry in
-            AgentScrollMetrics(offset: geometry.contentOffset.y,
+                geometry.containerSize.height > 0 &&
-                height: geometry.containerSize.height,
+                    geometry.contentOffset.y + geometry.contentInsets.top <= 24
-                contentHeight: geometry.contentSize.height,
+            } action: { _, atBottom in
-                bottomInset: geometry.contentInsets.bottom, topInset: geometry.contentInsets.top,
-                visibleBottom: geometry.visibleRect.maxY)
-        } action: { _, next in
                 #if DEBUG
                 AgentRenderDiagnostics.record("scrollGeometry")
                 #endif
-            scrollTracking.metrics = next
+                scrollTracking.atBottom = atBottom
-            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 {
+                if isScrolling && scrollPhase != .animating {
-                atLatest = next.atBottom
+                    atLatest = atBottom
-            }
-        }
-        .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)
-                }
-            } else {
-                scrollTracking.rowFrames = frames
                 }
             }
             .overlay(alignment: .bottomTrailing) {
                 if !atLatest {
                     Button {
                         atLatest = true
-                    withAnimation(reduceMotion ? nil : .easeOut(duration: 0.24)) { scrollPosition.scrollTo(edge: .bottom) }
+                        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))
@@ -160,22 +128,26 @@ private struct AgentTranscriptView: View {
             }
             .onAppear {
                 appeared = true
-            if !room.isLoading { historyStartID = displayed.first?.id }
+                if !room.isLoading {
+                    historyStartID = displayed.first?.id
+                }
             }
             .onChange(of: room.isLoading) { _, loading in
-            if !loading && historyStartID == nil { historyStartID = displayed.first?.id }
+                if !loading && historyStartID == nil {
+                    historyStartID = displayed.first?.id
                 }
-        .onChange(of: allMessages.count) { _, _ in
-            if atLatest { scrollPosition.scrollTo(edge: .bottom) }
             }
             .onChange(of: latestRequest) { _, _ in
+                if !atLatest {
                     atLatest = true
-            scrollPosition.scrollTo(edge: .bottom)
+                    proxy.scrollTo(Self.bottomID, anchor: .top)
+                }
             }
             .modifier(AgentReadAcknowledgement(room: room, tracking: scrollTracking,
                                               atLatest: atLatest, isScrolling: isScrolling))
         }
     }
+}
 
 // Bottom proximity can change twice per entrance/keyboard frame while the
 // scroll view follows the growing content. Only the read-settling task needs
@@ -198,9 +170,9 @@ 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 +247,14 @@ 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 +270,6 @@ 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 +306,24 @@ private struct AgentMessageRow: View {
     }
 }
 
-// Animate both occupied height and content, so incoming rows push older history up.
+// Keep the row's measured height stable for lazy layout. Animate its appearance
-private struct AgentGrowingLayout: Layout {
+// without collapsing the scroll target while the composer or keyboard resizes.
-    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))
-    }
-}
 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
-            content.opacity(preparing ? 0 : 1).offset(y: preparing ? 16 : 0)
+        .opacity(preparing ? 0 : 1)
-        }
+        .offset(y: preparing ? 16 : 0)
-        .clipped()
         .onAppear {
             withAnimation(animate && !reduceMotion ? .easeOut(duration: 0.24) : nil) { entered = true }
         }
     }
 }
 
-// Pixel geometry is imperative anchor bookkeeping, not rendered content. Keeping
+// Only the bottom boundary invalidates read acknowledgement, not the transcript.
-// it outside observation avoids rebuilding the entire transcript on every scroll
-// frame. Only the at-bottom boundary invalidates the read acknowledgement.
 @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 })
-    }
 }

ios/Tests/SybilUITests/TranscriptFadeUITests.swift

@@ -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