perf(chat): cache isPaginatedEmpty to decouple outer ChatView.body from message-array observation (LUM-1330)

clients/AGENTS.md

@@ -197,6 +197,7 @@ Prefer migrating the parent to `@Observable` so the bridge becomes unnecessary (
 - **Lazy containers for large collections.** Use `LazyVStack`, `LazyHStack`, `LazyVGrid` instead of eager equivalents when the item count is unbounded or large. In particular, avoid `VStack`/`HStack` inside a `ScrollView` for large or unbounded data-driven lists — eager loading kills scroll performance. Small, fixed-size lists where visibility-sensitive logic (e.g., `onAppear` pagination triggers) matters may use eager containers intentionally. See `clients/macos/AGENTS.md` §§ "No `.frame(maxWidth:)` …" for the FlexFrame anti-pattern that compounds with lazy containers (mechanically enforced in CI via `clients/scripts/check-flexframe.sh`).
 - **Keep modifier chains lean.** Every SwiftUI modifier wraps its content in a new view value. Long chains of redundant or duplicated modifiers deepen the view tree and increase diffing cost. Group related modifiers, remove redundant ones (e.g., don't set `.font` on every row when the parent already sets it), and extract heavily-modified subtrees into standalone `@ViewBuilder` methods.
 - **Scope observation narrowly.** Only observe the specific properties a view needs. Prefer granular `@Observable` properties or `withObservationTracking` over observing an entire store that publishes on unrelated changes. When a view reads `@Observable` properties from many different objects, extract logical sections (toolbar, overlays, content) into standalone child `View` structs — each child creates its own [observation tracking scope](https://developer.apple.com/videos/play/wwdc2023/10149/) so changes to one section's state don't re-evaluate the entire parent body. **Note:** private `@ViewBuilder` functions do NOT create separate observation scopes — they execute during the caller's `body` evaluation, so all their `@Observable` reads are tracked in the caller's scope. Use standalone `View` structs or [`ObservationBoundaryView`](https://developer.apple.com/videos/play/wwdc2023/10149/) (deferred `@escaping @ViewBuilder` closure) when extraction into a full `View` struct is impractical.
+- **Cache derived booleans from high-frequency collections.** Reading `.isEmpty` or `.count` on an `@Observable` stored property [tracks the property, not the value](https://developer.apple.com/videos/play/wwdc2023/10149/) — any mutation invalidates the view even when the derived boolean hasn't changed. For frequently-mutated collections, maintain a separate stored boolean with a write-only-on-change guard (`if new != old { prop = new }`). This also applies to modifiers like `.onChange(of:)` — if the `of:` expression reads a collection property, the outer body observes it.
 - **Use `.task` + async sequences instead of `.onReceive` for `NotificationCenter`.** Each `.onReceive` modifier adds a generic wrapper layer to the view type and exists only for side effects — no rendering value. Use a single [`.task`](https://developer.apple.com/documentation/swiftui/view/task(priority:_:)) with [`NotificationCenter.notifications(named:)`](https://developer.apple.com/documentation/foundation/notificationcenter/notifications(named:object:)) async sequences instead. `.task` has identical lifecycle semantics (auto-cancelled on view disappear) without inflating the view type.
 - **Enable non-contiguous layout on TextKit 1 stacks in `NSViewRepresentable`.** When constructing an `NSLayoutManager` for an `NSTextView` hosted via `NSViewRepresentable`, set `layoutManager.allowsNonContiguousLayout = true`. The default (`false`) forces full-document glyph layout from character 0 on the main thread whenever a glyph range is queried — which happens the first time the text view is attached to its hosting view (`setDocumentView:` / `addSubview:` → `_setSuperview:` → `setNeedsDisplayInRect:` → `_glyphRangeForBoundingRect:`). Reference: [`NSLayoutManager.allowsNonContiguousLayout`](https://developer.apple.com/documentation/appkit/nslayoutmanager/allowsnoncontiguouslayout). **Exception — streaming text with a separate measurement stack:** when the representable measures height via a sibling TextKit stack that always fully lays out every glyph (e.g. `ensureLayout(for:)` + `usedRect(for:)`), the measured frame assumes the render stack will also lay out every glyph in that rect. Non-contiguous layout leaves glyphs pending until the view scrolls or draws them, which races with streaming updates — producing a gap (render paints a smaller laid-out region inside the correct frame) and then overlap (the next sibling is placed via the measured frame and the lazy glyphs later paint outside it). In that case set `allowsNonContiguousLayout = false` and document why. See `clients/shared/DesignSystem/Components/Display/SelectableTextView.swift` (lines 180–205) for the canonical example; `VCodeView` / `HighlightedTextView` still opt into non-contiguous layout independently because they do not share this measurement model.
 - **Prefer line-count math over `ensureLayout(for:)` for `sizeThatFits` when line wrapping is disabled and line height is pinned.** If the TextKit 1 stack uses an unbounded-width `NSTextContainer` (so lines never wrap) and the paragraph style pins `minimumLineHeight == maximumLineHeight`, the total height is exactly `lineCount * lineHeight + insets` — no glyph layout required. This avoids an O(glyph count) main-thread layout pass that SwiftUI would otherwise re-run on every cell during `LazyVStack` layout. Reference: [`NSLayoutManager.defaultLineHeight(for:)`](https://developer.apple.com/documentation/appkit/nslayoutmanager/defaultlineheight(for:)).

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

@@ -112,11 +112,11 @@ struct ChatView: View {
     @State private var diskPressureDismissalRefreshTask: Task<Void, Never>? = nil
 
     private var isEmptyState: Bool {
-        viewModel.paginatedVisibleMessages.isEmpty && viewModel.isHistoryLoaded
+        viewModel.isPaginatedEmpty && viewModel.isHistoryLoaded
     }
 
     private var shouldShowSkeleton: Bool {
-        viewModel.paginatedVisibleMessages.isEmpty && !viewModel.isHistoryLoaded
+        viewModel.isPaginatedEmpty && !viewModel.isHistoryLoaded
     }
 
     /// Message IDs whose text contains the search query, ordered chronologically.

clients/shared/Features/Chat/ChatPaginationState.swift

@@ -71,6 +71,11 @@ public final class ChatPaginationState {
     /// Updated when either the message list or `displayedMessageCount` changes.
     public private(set) var paginatedVisibleMessages: [ChatMessage] = []
 
+    /// Whether `paginatedVisibleMessages` is empty. Cached O(1) boolean so
+    /// views needing only emptiness (skeleton/empty-state routing) observe this
+    /// instead of the full array — avoiding invalidation on every message mutation.
+    public private(set) var isPaginatedEmpty: Bool = true
+
     // MARK: - Daemon History Pagination
 
     /// Timestamp of the oldest loaded message (ms since epoch). Used as the
@@ -173,6 +178,10 @@ public final class ChatPaginationState {
     /// `windowOldestIndex` (or the newest slice when that is `nil`); all
     /// other cases fall back to a grow-only suffix.
     func recomputePaginatedSuffix() {
+        defer {
+            let newEmpty = paginatedVisibleMessages.isEmpty
+            if newEmpty != isPaginatedEmpty { isPaginatedEmpty = newEmpty }
+        }
         let visible = displayedMessages
         if isShowAllMode {
             let cap = Self.maxPaginatedWindowSize

clients/shared/Features/Chat/ChatViewModel.swift

@@ -1037,6 +1037,12 @@ public final class ChatViewModel: MessageSendCoordinatorDelegate {
         paginationState.paginatedVisibleMessages
     }
 
+    /// Whether `paginatedVisibleMessages` is empty. Prefer over
+    /// `paginatedVisibleMessages.isEmpty` to avoid observing the full array.
+    public var isPaginatedEmpty: Bool {
+        paginationState.isPaginatedEmpty
+    }
+
     public var historyCursor: Double? {
         get { paginationState.historyCursor }
         set { paginationState.historyCursor = newValue }