fix: replace AlignmentBarrierLayout with _FrameLayout to stop alignment cascade and message disappearing

clients/macos/AGENTS.md

@@ -259,12 +259,7 @@ All design system types use the `V` prefix (VButton, VColor, VFont, etc.). Alway
   - [`.containerRelativeFrame(.horizontal)`](https://developer.apple.com/documentation/swiftui/view/containerrelativeframe(_:alignment:)) — width constraint without FlexFrame.
 
   Never trade `HStack+Spacer` for `.frame(alignment:)` in lazy containers — fewer layout nodes is not worth O(n) recursive alignment queries per node.
-- **`AlignmentBarrierLayout` for LazyVStack protection**: Wrap the `ScrollView` (from the **outside**) with `AlignmentBarrierLayout { ScrollView { ... } }` to block `explicitAlignment` queries from cascading into the lazy container. The barrier returns `nil` for all alignment queries while passing through sizing and placement unchanged. Three critical constraints:
-  1. **The barrier MUST wrap the ScrollView, not its content.** Placing a custom `Layout` _inside_ a `ScrollView` (between the `ScrollView` and a `LazyVStack`) breaks `LazyVStack`'s viewport tracking — cells fail to materialize on resize, send, or content changes until the user scrolls. Custom `Layout` containers disrupt the internal communication channel `ScrollView` uses to tell `LazyVStack` which cells are visible.
-  2. **`.id()` MUST be outside the barrier, not inside.** A custom `Layout` container does not propagate child identity changes to the outer modifier chain. If `.id(conversationId)` is inside the `Layout`, `.onAppear`/`.onDisappear` on the outer chain will not fire on identity changes (e.g. conversation switch), breaking lifecycle handlers like `handleAppear()`. Place `.id()` after the barrier in the modifier chain: `AlignmentBarrierLayout { ScrollView { ... } }.id(conversationId)`.
-  3. **Scroll-specific modifiers MUST be inside the barrier, on the ScrollView.** `.scrollPosition`, `.defaultScrollAnchor`, `.onScrollPhaseChange`, and `.scrollIndicators` must be applied directly to the `ScrollView` inside the barrier's content closure. Placing them outside forces SwiftUI to traverse the custom `Layout` container to find the `ScrollView`, which can intermittently fail and cause cells to not materialize.
-
-  See [`Layout.explicitAlignment`](https://developer.apple.com/documentation/swiftui/layout/explicitalignment(of:in:proposal:subviews:cache:)-3iqmu) and `clients/shared/DesignSystem/Layout/AlignmentBarrierLayout.swift`.
+- **Use `.frame(width:)` (not `.frame(maxWidth:)`) on ScrollView containers with LazyVStack**: `.frame(width:)` creates [`_FrameLayout`](https://developer.apple.com/documentation/swiftui/view/frame(width:height:alignment:)) which returns `bounds.midX` for alignment without querying children — the alignment cascade stops here. `.frame(maxWidth:)` creates [`_FlexFrameLayout`](https://developer.apple.com/documentation/swiftui/view/frame(minwidth:idealwidth:maxwidth:minheight:idealheight:maxheight:alignment:)) which queries `explicitAlignment` on children, cascading O(n) through the entire `LazyVStack` subtree on every layout pass. Use a computed width (e.g. `min(containerWidth, maxWidth)`) to get responsive behavior without `_FlexFrameLayout`. Do NOT use a custom `Layout` barrier — custom `Layout` containers between the outer modifier chain and the `ScrollView` disrupt SwiftUI's internal scroll infrastructure, causing intermittent cell materialization failures.
 - **Gallery**: When adding or modifying a design system primitive/component, update the corresponding Gallery section file (`Gallery/Sections/`) so the visual catalog stays current.
 - **Accessibility**: See `clients/AGENTS.md` § [Accessibility](../AGENTS.md#accessibility) for the full checklist (labels, hidden elements, custom interactions, AppKit panels). All rules there apply to macOS components.
 

clients/macos/vellum-assistant/Features/Chat/MessageListView.swift

@@ -110,70 +110,53 @@ struct MessageListView: View {
         #if DEBUG
         let _ = os_signpost(.event, log: PerfSignposts.log, name: "MessageListView.body")
         #endif
-            // AlignmentBarrierLayout stops explicitAlignment queries from the
-            // outer .frame(maxWidth:) modifiers from cascading into the
-            // LazyVStack. It MUST wrap the ScrollView from the outside —
-            // placing it inside the ScrollView breaks LazyVStack's viewport
-            // tracking, causing cells to not materialize until scrolled.
-            // See AGENTS.md and AlignmentBarrierLayout.swift for details.
-            //
-            // Scroll-specific modifiers (.scrollPosition, .defaultScrollAnchor,
-            // .onScrollPhaseChange, .scrollIndicators) are applied INSIDE the
-            // barrier directly on the ScrollView. Placing them outside forces
-            // SwiftUI to traverse a custom Layout container to find the
-            // ScrollView, which can intermittently fail and cause cells to
-            // not materialize.
-            AlignmentBarrierLayout {
-                ScrollView {
-                    scrollViewContent
-                        .background(
-                            MessageListScrollObserver { newState in
-                                enqueueScrollGeometryUpdate(newState)
-                            }
-                        )
-                }
-                .scrollContentBackground(.hidden)
-                .scrollDisabled(messages.isEmpty && !isSending)
-                // Apply only to .initialOffset — where the scroll view starts
-                // when first displayed (including .id() recreation on switch).
-                // Deliberately NOT using the all-roles overload (.sizeChanges)
-                // because it fights user scroll-up during streaming: SwiftUI's
-                // definition of "at bottom" for anchor purposes can differ from
-                // our hysteresis-based isAtBottom, causing the viewport to snap
-                // back to bottom on every content-height change even after the
-                // user has entered freeBrowsing. Our explicit content-height
-                // auto-follow handles streaming growth with proper mode checks.
-                // https://developer.apple.com/documentation/swiftui/view/defaultscrollanchor(_:for:)
-                .defaultScrollAnchor(.bottom, for: .initialOffset)
-                .scrollPosition($scrollPosition)
-                .environment(\.suppressAutoScroll, { [self] in
-                    os_signpost(.event, log: PerfSignposts.log, name: "scrollSuppressionChanged", "on reason=manualExpansionDetach")
-                    let intents = scrollCoordinator.handle(.manualExpansion)
-                    executeCoordinatorIntents(intents)
-                    // Keep scrollState in sync as runtime executor.
-                    scrollState.handleManualExpansionInteraction()
-                })
-                .onScrollPhaseChange { oldPhase, newPhase in
-                    let coordinatorPhase = ScrollCoordinator.Phase.from(newPhase)
-                    let intents = scrollCoordinator.handle(.scrollPhaseChanged(phase: coordinatorPhase))
-                    executeCoordinatorIntents(intents)
-                    // Keep scrollState in sync as runtime executor.
-                    scrollState.scrollPhase = newPhase
-                    if newPhase == .idle && oldPhase != .idle && scrollState.isAtBottom {
-                        scrollState.handleReachedBottom()
-                    }
+            // .frame(width:) creates _FrameLayout (not _FlexFrameLayout). FrameLayout
+            // returns bounds.midX for alignment without querying children, stopping the
+            // alignment cascade. The old .frame(maxWidth:) pattern created FlexFrameLayout
+            // which queried explicitAlignment on the entire LazyVStack subtree — O(n) per
+            // layout pass, causing 34-70s hangs. See AGENTS.md.
+            ScrollView {
+                scrollViewContent
+                    .background(
+                        MessageListScrollObserver { newState in
+                            enqueueScrollGeometryUpdate(newState)
+                        }
+                    )
+            }
+            .scrollContentBackground(.hidden)
+            .scrollDisabled(messages.isEmpty && !isSending)
+            // Apply only to .initialOffset — where the scroll view starts
+            // when first displayed (including .id() recreation on switch).
+            // Deliberately NOT using the all-roles overload (.sizeChanges)
+            // because it fights user scroll-up during streaming: SwiftUI's
+            // definition of "at bottom" for anchor purposes can differ from
+            // our hysteresis-based isAtBottom, causing the viewport to snap
+            // back to bottom on every content-height change even after the
+            // user has entered freeBrowsing. Our explicit content-height
+            // auto-follow handles streaming growth with proper mode checks.
+            // https://developer.apple.com/documentation/swiftui/view/defaultscrollanchor(_:for:)
+            .defaultScrollAnchor(.bottom, for: .initialOffset)
+            .scrollPosition($scrollPosition)
+            .environment(\.suppressAutoScroll, { [self] in
+                os_signpost(.event, log: PerfSignposts.log, name: "scrollSuppressionChanged", "on reason=manualExpansionDetach")
+                let intents = scrollCoordinator.handle(.manualExpansion)
+                executeCoordinatorIntents(intents)
+                // Keep scrollState in sync as runtime executor.
+                scrollState.handleManualExpansionInteraction()
+            })
+            .onScrollPhaseChange { oldPhase, newPhase in
+                let coordinatorPhase = ScrollCoordinator.Phase.from(newPhase)
+                let intents = scrollCoordinator.handle(.scrollPhaseChanged(phase: coordinatorPhase))
+                executeCoordinatorIntents(intents)
+                // Keep scrollState in sync as runtime executor.
+                scrollState.scrollPhase = newPhase
+                if newPhase == .idle && oldPhase != .idle && scrollState.isAtBottom {
+                    scrollState.handleReachedBottom()
                 }
-                .scrollIndicators(scrollState.scrollIndicatorsHidden ? .hidden : .automatic)
             }
-            // .id() MUST be outside AlignmentBarrierLayout. A custom Layout
-            // container does not propagate child identity changes to the outer
-            // modifier chain — .onAppear would not fire on conversation switch
-            // if .id() were inside the Layout, breaking handleAppear() and all
-            // scroll state that depends on it (configureScrollCallbacks,
-            // handleConversationSwitched). See MessageListView+Lifecycle.swift.
+            .scrollIndicators(scrollState.scrollIndicatorsHidden ? .hidden : .automatic)
             .id(conversationId)
-            .frame(maxWidth: VSpacing.chatColumnMaxWidth)
-            .frame(maxWidth: .infinity)
+            .frame(width: containerWidth > 0 ? min(containerWidth, VSpacing.chatColumnMaxWidth) : VSpacing.chatColumnMaxWidth)
             .overlay(alignment: .bottom) {
                 ScrollToLatestOverlayView(scrollState: scrollState)
             }