perf(chat): reduce transcript render churn

Sources/Halos/Views/ActivityListView.swift

@@ -19,6 +19,7 @@ struct ActivityListView: View {
                     } else {
                         ForEach(messages) { message in
                             ActivityRowView(message: message, resolveVeyraApproval: resolveVeyraApproval)
+                                .equatable()
                                 .id(message.id)
                         }
                     }
@@ -77,24 +78,15 @@ struct ActivityListView: View {
                 guard viewportHeight > 0 else { return }
                 isPinnedToBottom = bottomOffset <= viewportHeight + bottomFollowThreshold
             }
-            .onChange(of: contentFingerprint) { _, _ in
+            .onChange(of: contentToken) { _, _ in
                 guard isPinnedToBottom else { return }
                 scrollToBottom(proxy, animated: true)
             }
         }
     }
 
-    private var contentFingerprint: String {
-        messages
-            .map { message in
-                [
-                    message.id,
-                    message.title,
-                    message.body,
-                    message.streamState.rawValue,
-                ].joined(separator: "\u{1F}")
-            }
-            .joined(separator: "\u{1E}")
+    private var contentToken: [ActivityListContentToken] {
+        messages.map(ActivityListContentToken.init)
     }
 
     private func scrollToBottom(_ proxy: ScrollViewProxy, animated: Bool) {
@@ -124,6 +116,18 @@ struct ActivityListView: View {
     }
 }
 
+private struct ActivityListContentToken: Equatable {
+    let id: String
+    let bodyByteCount: Int
+    let streamState: MessageStreamState
+
+    init(message: ActivityMessage) {
+        self.id = message.id
+        self.bodyByteCount = message.body.utf8.count
+        self.streamState = message.streamState
+    }
+}
+
 private enum ActivityListCoordinateSpace {
     static let name = "activity-list-scroll"
 }

Sources/Halos/Views/ActivityRowView.swift

@@ -1,6 +1,6 @@
 import SwiftUI
 
-struct ActivityRowView: View {
+struct ActivityRowView: View, Equatable {
     let message: ActivityMessage
     let resolveVeyraApproval: ((String, VeyraApprovalDecision) -> Void)?
     @Environment(\.accessibilityReduceMotion) private var reduceMotion
@@ -15,6 +15,10 @@ struct ActivityRowView: View {
         self.resolveVeyraApproval = resolveVeyraApproval
     }
 
+    nonisolated static func == (lhs: ActivityRowView, rhs: ActivityRowView) -> Bool {
+        lhs.message == rhs.message
+    }
+
     var body: some View {
         if let approval = message.veyraApproval {
             veyraApprovalCard(approval)
@@ -295,8 +299,8 @@ struct ActivityRowView: View {
 
             if isWorkSummaryExpanded {
                 VStack(alignment: .leading, spacing: 7) {
-                    ForEach(workSummaryLines, id: \.self) { line in
-                        markdownText(line)
+                    ForEach(workSummaryLines) { line in
+                        markdownText(line.text)
                             .font(.system(size: 11.5, weight: .medium))
                             .foregroundStyle(HalosTheme.tertiaryText)
                             .fixedSize(horizontal: false, vertical: true)
@@ -314,10 +318,11 @@ struct ActivityRowView: View {
         .frame(maxWidth: .infinity, alignment: .leading)
     }
 
-    private var workSummaryLines: [String] {
+    private var workSummaryLines: [WorkSummaryLine] {
         message.body
             .split(separator: "\n", omittingEmptySubsequences: true)
-            .map(String.init)
+            .enumerated()
+            .map { index, line in WorkSummaryLine(index: index, text: String(line)) }
     }
 
     private func syncWorkSummaryExpansionIfNeeded() {
@@ -336,6 +341,15 @@ struct ActivityRowView: View {
     }
 }
 
+private struct WorkSummaryLine: Identifiable {
+    let index: Int
+    let text: String
+
+    var id: Int {
+        index
+    }
+}
+
 struct HalosThinkingEyesView: View {
     let isActive: Bool
     @Environment(\.accessibilityReduceMotion) private var reduceMotion