ios: render Agent history lazily

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

@@ -30,7 +30,6 @@ private struct AgentTranscriptView: View {
     @State private var historyStartID: String?
     @State private var scrollPosition = ScrollPosition()
     @State private var scrollTracking = AgentScrollTracking()
-    @State private var restoreAnchor: AgentReadingAnchor?
     @State private var appeared = false
     private let bottomID = "agent-bottom"
 
@@ -51,16 +50,12 @@ private struct AgentTranscriptView: View {
         let displayed = transcript.displayed
         ScrollViewReader { proxy in
             ScrollView {
-                // Exact eager geometry preserves reading/prepend offsets. Lazy
-                // estimates required repeated corrections and failed those anchors;
-                // isolate observation instead of adding a geometry feedback loop.
-                VStack(alignment: .leading, spacing: 15) {
+                LazyVStack(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
-                            }
+                            // 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
@@ -104,7 +99,6 @@ private struct AgentTranscriptView: View {
             .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,
@@ -119,7 +113,7 @@ private struct AgentTranscriptView: View {
                 scrollTracking.metrics = next
                 if scrollTracking.atBottom != next.atBottom { scrollTracking.atBottom = next.atBottom }
                 if isScrolling || next.contentHeight <= next.height { atLatest = next.atBottom }
-                if atLatest, !isScrolling, restoreAnchor == nil,
+                if atLatest, !isScrolling,
                    previous.height != next.height || previous.contentHeight != next.contentHeight || previous.bottomInset != next.bottomInset {
                     proxy.scrollTo(bottomID, anchor: .bottom)
                 }
@@ -128,10 +122,10 @@ private struct AgentTranscriptView: View {
                 #if DEBUG
                 AgentRenderDiagnostics.record("rowPreferences")
                 #endif
-                let anchor = restoreAnchor ?? (!atLatest && !isScrolling ? visibleReadingAnchor : nil)
+                let anchor = !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
+                    // 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)
@@ -142,16 +136,13 @@ private struct AgentTranscriptView: View {
                         // 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 {
-                        restoreAnchor = nil
                         atLatest = true
                         withAnimation(reduceMotion ? nil : .easeOut(duration: 0.24)) { proxy.scrollTo(bottomID, anchor: .bottom) }
                     } label: {
@@ -176,7 +167,6 @@ private struct AgentTranscriptView: View {
                 if atLatest { proxy.scrollTo(bottomID, anchor: .bottom) }
             }
             .onChange(of: latestRequest) { _, _ in
-                restoreAnchor = nil
                 atLatest = true
                 proxy.scrollTo(bottomID, anchor: .bottom)
             }
@@ -407,7 +397,6 @@ private struct AgentScrollMetrics: Equatable {
 private struct AgentReadingAnchor {
     var id: String
     var offset: CGFloat
-    var expectedCount: Int? = nil
 }
 private struct AgentRowFrames: PreferenceKey {
     static let defaultValue: [String: CGRect] = [:]