docs(clients/AGENTS): document allowsNonContiguousLayout exception for streaming text views

clients/AGENTS.md

@@ -195,7 +195,7 @@ 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.
 - **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.
-- **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).
+- **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:)).
 - **Memoize `sizeThatFits` (and sibling measurement calls) in `NSViewRepresentable` views placed in lazy containers.** Any `NSViewRepresentable` that hosts a TextKit stack (`NSLayoutManager` + `NSTextContainer`) and can appear inside a `LazyVStack` / `LazyHStack` / `List` cell must cache its last measurement. [`NSLayoutManager.ensureLayout(for:)`](https://developer.apple.com/documentation/uikit/nslayoutmanager/ensurelayout(for:)) is O(glyph count) and runs synchronously on the main thread, and SwiftUI calls [`sizeThatFits(_:nsView:context:)`](https://developer.apple.com/documentation/swiftui/nsviewrepresentable/sizethatfits(_:nsview:context:)) on every visible cell on every layout pass (scroll, resize, streaming edit). Cache at minimum on `(textStorage.length, proposalWidth)`; for attribute-sensitive paths (static measurement helpers, highlighted content) key on the `NSAttributedString` itself and fall through to [`NSAttributedString.isEqual(_:)`](https://developer.apple.com/documentation/foundation/nsattributedstring/isequal(_:)) for identity — `NSAttributedString.hash` alone admits collisions and surfaces as wrong-height cells. Invalidate at every text-storage mutation site (text apply, edit callbacks, async highlight/formatting completions). Reference: `SelectableTextView`. When line wrapping is disabled and line height is pinned (as in `VCodeView.VCodeTextView` and `HighlightedTextView.CodeTextView`), prefer the line-count math above over a cached `ensureLayout` call.
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@@ -451,7 +451,7 @@ Swift's type checker has quadratic complexity with chained view modifiers. Compl
 | `DispatchQueue.main.sync` from `@MainActor` or main thread | Deadlocks. Ref: [Apple — DispatchQueue.sync](https://developer.apple.com/documentation/dispatch/dispatchqueue/sync(execute:)-3gef0) | Use `Thread.isMainThread` guard, or `await MainActor.run {}` from async contexts. Prefer thread-safe APIs that don't need the main thread. |
 | CPU-bound work inside `@MainActor` type without offloading | Blocks UI (JSON decode, image resize, compression). Ref: [WWDC25 — Embracing Swift concurrency](https://developer.apple.com/videos/play/wwdc2025/268/) | Offload the expensive call via `Task.detached`. Keep the type on `@MainActor`. See § "@MainActor Isolation Boundaries". |
 | Unguarded `NSLayoutManager.ensureLayout(for:)` in `NSViewRepresentable.sizeThatFits` inside `LazyVStack`/`List` cells | SwiftUI calls `sizeThatFits` on every visible cell on every layout pass; `ensureLayout` is O(glyphs) on the main thread. Caches keyed only on text content (not width) silently skip on same-text-new-width queries, running layout again. | Cache last measurement on `(textStorage.length, proposalWidth)` at minimum; use `NSAttributedString.isEqual(_:)` when attributes matter. Invalidate at every text-storage mutation site. When line wrapping is off and line height is pinned, use `lineCount * lineHeight + insets` directly. See § "View Bodies and Rendering". |
-| Default `NSLayoutManager.allowsNonContiguousLayout` (`false`) on TextKit 1 stacks in `NSViewRepresentable` | The first time the `NSTextView` is attached to its hosting view, AppKit's `setNeedsDisplayInRect:` asks the layout manager for a glyph range, which forces full-document layout from character 0 on the main thread. Multi-second hangs on large documents. | Set `layoutManager.allowsNonContiguousLayout = true` when constructing the TextKit 1 stack. See § "View Bodies and Rendering". |
+| Default `NSLayoutManager.allowsNonContiguousLayout` (`false`) on TextKit 1 stacks in `NSViewRepresentable` | The first time the `NSTextView` is attached to its hosting view, AppKit's `setNeedsDisplayInRect:` asks the layout manager for a glyph range, which forces full-document layout from character 0 on the main thread. Multi-second hangs on large documents. | Set `layoutManager.allowsNonContiguousLayout = true` when constructing the TextKit 1 stack. **Exception:** representables that measure height via a separate TextKit stack which always fully lays out every glyph (e.g. `VSelectableTextView`, used for streaming chat text) must keep `allowsNonContiguousLayout = false` on the render stack — otherwise the render lags the measured frame during streaming, producing a visible gap and then overlap with the next sibling. See § "View Bodies and Rendering" and `clients/shared/DesignSystem/Components/Display/SelectableTextView.swift:180-205`. |
 
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