nvrsion: Add, fix, and refactor iOS session detail view logic to prevent UI lock-ups and high CPU usage during scroll operations.

Nucleic-Promote: 1
Co-authored-by: Nucleic <[email protected]>

NucleicRemote/NucleicRemote/Views/SessionDetailView.swift

@@ -284,15 +284,6 @@ struct SessionDetailView: View {
     /// pending. Content scrolls beneath it; nothing renders when there's nothing to act on.
     private var actionArea: some View {
         VStack(spacing: 8) {
-            // The jump-to-bottom chevron rides here, immediately above the chat bar — not as a
-            // transcript overlay, which aligned to the scroll view's full-height bounds and so sat
-            // behind this floating bar at the screen's bottom edge. Shown only while scrolled up; a
-            // tap bumps the same scroll request the send button uses, so following resumes once the
-            // transcript reaches the bottom. Mirrors the Mac's `JumpToBottomButton`.
-            if !isScrolledToBottom {
-                JumpToBottomButton { scrollToBottomRequest += 1 }
-                    .transition(.move(edge: .bottom).combined(with: .opacity))
-            }
             // Interacting with a chat while the owning Mac is unreachable surfaces this first, so a
             // disabled composer reads as "offline / read-only history" rather than broken.
             if !store.connectivity.isLive { disconnectedBanner }
@@ -300,6 +291,27 @@ struct SessionDetailView: View {
         }
         .padding(.horizontal, 12)
         .padding(.bottom, 8)
+        // The jump-to-bottom chevron floats just above the chat bar as an *overlay* — deliberately
+        // not a stack member. This whole action area is the transcript's bottom `safeAreaInset`,
+        // so a stack-member chevron changed the scroll view's bottom inset by ~38pt every time it
+        // appeared — and its visibility is *decided by* that same scroll geometry
+        // (`isScrolledToBottom`). That geometry→chevron→geometry cycle could oscillate every
+        // frame near the follow threshold, re-layouting the whole eager transcript each time and
+        // pinning the main thread at 100% with the UI locked. An overlay contributes nothing to
+        // the inset height, so showing or hiding it can't move the scroll geometry at all.
+        // (It's not a transcript overlay either — that aligned to the scroll view's full-height
+        // bounds and sat behind this floating bar at the screen's bottom edge.) A tap bumps the
+        // same scroll request the send button uses, so following resumes once the transcript
+        // reaches the bottom. Mirrors the Mac's `JumpToBottomButton`.
+        .overlay(alignment: .top) {
+            if !isScrolledToBottom {
+                JumpToBottomButton { scrollToBottomRequest += 1 }
+                    // Hang the button *above* the bar: report its bottom edge (plus a gap) as its
+                    // top guide, so the overlay's top alignment places it outside the bar's bounds.
+                    .alignmentGuide(.top) { $0[.bottom] + 8 }
+                    .transition(.move(edge: .bottom).combined(with: .opacity))
+            }
+        }
         .animation(.easeInOut(duration: 0.15), value: isScrolledToBottom)
     }
 
@@ -576,6 +588,26 @@ struct TranscriptList: View {
     /// the Mac's `bottomFollowThreshold`.
     private let bottomFollowThreshold: CGFloat = 24
 
+    /// How far a *user* scroll must move away from the bottom before following disengages. Wider
+    /// than the re-engage threshold above on purpose (hysteresis): the action bar's height isn't
+    /// constant (composer lines grow, the working row appears, the keyboard dismisses
+    /// interactively), and every inset change perturbs the scroll geometry by tens of points. A
+    /// single threshold read both ways let one such perturbation flip the gate, whose reactions
+    /// (anchor toggle, bar animation) perturbed the geometry again — an oscillation that
+    /// re-layouted the whole eager transcript every frame and pinned the main thread at 100%.
+    /// The band is wider than any bar-height delta, so only a deliberate scroll crosses it.
+    private let bottomUnfollowThreshold: CGFloat = 64
+
+    /// The live scroll phase, used to tell *user* scrolling (finger down / flick decelerating)
+    /// from programmatic motion (autoscroll animations, anchor re-pins, inset changes). Only a
+    /// user-driven phase may disengage bottom-following — a programmatic perturbation can only
+    /// ever re-engage it — which structurally breaks every geometry→state→geometry feedback
+    /// cycle: no chain of layout reactions can take the gate false and sustain itself.
+    @State private var scrollPhase: ScrollPhase = .idle
+    private var isUserScrolling: Bool {
+        scrollPhase == .tracking || scrollPhase == .interacting || scrollPhase == .decelerating
+    }
+
     var body: some View {
         ScrollViewReader { proxy in
             ScrollView {
@@ -613,22 +645,28 @@ struct TranscriptList: View {
             // they scroll away, which lets new content land off-screen below instead of dragging
             // the viewport, and restore `.bottom` once they're back at the end.
             .defaultScrollAnchor(isScrolledToBottom ? .bottom : nil)
+            .onScrollPhaseChange { _, newPhase in scrollPhase = newPhase }
             // Track the live scroll position straight from the scroll view's geometry: how much
-            // content still sits below the viewport bottom. Within the slack threshold means the
-            // user is parked at the end (live output keeps following); scrolling up flips this
-            // false, which drops the anchor (above) and reveals the jump-to-bottom chevron.
-            .onScrollGeometryChange(for: Bool.self) { geo in
-                geo.contentSize.height - geo.containerSize.height - geo.contentOffset.y
-                    <= bottomFollowThreshold
-            } action: { _, atBottom in
-                // While the chat is opening the layout grows over a few passes and the scroll
-                // offset lags each growth by a frame — sampling that frame reads "not at bottom"
-                // even though `.defaultScrollAnchor(.bottom)` is about to re-pin. So during the
-                // open window accept only "at bottom" readings; honor real scroll-ups after.
-                if transcriptSettling {
-                    if atBottom { isScrolledToBottom = true }
-                } else {
-                    isScrolledToBottom = atBottom
+            // content still sits below the viewport bottom. Parked within the follow threshold
+            // means live output keeps following; a *user* scroll past the (wider) unfollow
+            // threshold flips it false, which drops the anchor (above) and reveals the chevron.
+            // Asymmetric on purpose — see `bottomUnfollowThreshold` / `isUserScrolling`: a
+            // programmatic geometry change (anchor re-pin, bar resize, keyboard, autoscroll
+            // animation) may re-engage following but can never disengage it, so no layout
+            // feedback cycle through this gate can sustain itself. Tracking the rounded distance
+            // (not a Bool) also means every scroll emits fresh values, so the gate can't latch
+            // against a stale reading (the old settle-window latch bug).
+            .onScrollGeometryChange(for: CGFloat.self) { geo in
+                (geo.contentSize.height - geo.containerSize.height - geo.contentOffset.y).rounded()
+            } action: { _, distance in
+                if distance <= bottomFollowThreshold {
+                    isScrolledToBottom = true
+                } else if !transcriptSettling, distance > bottomUnfollowThreshold, isUserScrolling {
+                    // While the chat is opening the layout grows over a few passes and the offset
+                    // lags each growth by a frame — those frames read "not at bottom" even though
+                    // `.defaultScrollAnchor(.bottom)` is about to re-pin, so settling accepts
+                    // only re-engagement (the `!transcriptSettling` above).
+                    isScrolledToBottom = false
                 }
             }
             // Detect when the opening layout has come to rest: track the content height while
@@ -657,9 +695,15 @@ struct TranscriptList: View {
             }
             // Coalescing means item count lags event count; key the autoscroll on the raw stream
             // so every streamed delta keeps the view pinned to the bottom — but only while the
-            // user is already parked there. Scrolling up to read history is never yanked down.
+            // user is already parked there. Scrolling up to read history is never yanked down,
+            // and a finger actively on the transcript is never fought mid-drag (the drag that
+            // takes them past the unfollow threshold flips the gate; until then the native
+            // bottom anchor alone keeps content pinned, without an animated scroll grabbing the
+            // viewport back out of their hand).
             .onChange(of: events.count) {
-                if isScrolledToBottom { scrollToEnd(proxy, animated: !transcriptSettling) }
+                if isScrolledToBottom, !isUserScrolling {
+                    scrollToEnd(proxy, animated: !transcriptSettling)
+                }
             }
             // An explicit jump — the chevron or sending a message — always wins. The chevron
             // itself lives in the parent's chat bar (above the composer), not as an overlay here,