diff --git a/clients/AGENTS.md b/clients/AGENTS.md
index c493b0c63a4..0b34d2d9316 100644
--- a/clients/AGENTS.md
+++ b/clients/AGENTS.md
@@ -195,7 +195,9 @@ 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.
-- **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`, `HighlightedTextView.CodeTextView`, `VCodeView.VCodeTextView`.
+- **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).
+- **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.
 <details>
 <summary><strong>Per-entity observation and dictionary patterns</strong></summary>
 
@@ -448,7 +450,8 @@ Swift's type checker has quadratic complexity with chained view modifiers. Compl
 | `Timer.publish` / `Timer.scheduledTimer` for UI updates | Timer continues firing when the view is off-screen, wastes energy, and requires manual lifecycle management (invalidate, cancellable storage) | Use `TimelineView(.periodic(from: .now, by: interval))` for fixed-rate progress displays or `TimelineView(.animation)` for frame-rate animations. TimelineView auto-pauses when the view is off-screen and requires no manual teardown. |
 | `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. See § "View Bodies and Rendering". |
+| 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". |
 
 </details>
 
diff --git a/clients/macos/vellum-assistant/Features/Chat/ComposerTextEditor.swift b/clients/macos/vellum-assistant/Features/Chat/ComposerTextEditor.swift
index fcfdf10251c..6f568b1f7e0 100644
--- a/clients/macos/vellum-assistant/Features/Chat/ComposerTextEditor.swift
+++ b/clients/macos/vellum-assistant/Features/Chat/ComposerTextEditor.swift
@@ -102,6 +102,11 @@ struct ComposerTextEditor: NSViewRepresentable {
         // Reference: https://developer.apple.com/documentation/appkit/nstextview/1449309-layoutmanager
         let textStorage = NSTextStorage()
         let layoutManager = NSLayoutManager()
+        // Confine glyph generation to the requested bounding rect so that
+        // attaching the text view to its scroll view does not force full
+        // document layout on the main thread.
+        // https://developer.apple.com/documentation/appkit/nslayoutmanager/allowsnoncontiguouslayout
+        layoutManager.allowsNonContiguousLayout = true
         textStorage.addLayoutManager(layoutManager)
         let textContainer = NSTextContainer(size: NSSize(
             width: 0,
diff --git a/clients/macos/vellum-assistant/Features/MainWindow/Panels/HighlightedTextView.swift b/clients/macos/vellum-assistant/Features/MainWindow/Panels/HighlightedTextView.swift
index 3c0b93252ee..d28adddc8d5 100644
--- a/clients/macos/vellum-assistant/Features/MainWindow/Panels/HighlightedTextView.swift
+++ b/clients/macos/vellum-assistant/Features/MainWindow/Panels/HighlightedTextView.swift
@@ -222,6 +222,11 @@ private struct CodeTextView: NSViewRepresentable {
         // TextKit 1 stack — matches the gutter's NSLayoutManager.defaultLineHeight computation
         let textStorage = NSTextStorage()
         let layoutManager = NSLayoutManager()
+        // Confine glyph generation to the requested bounding rect so that
+        // attaching the text view to its scroll view does not force full
+        // document layout on the main thread.
+        // https://developer.apple.com/documentation/appkit/nslayoutmanager/allowsnoncontiguouslayout
+        layoutManager.allowsNonContiguousLayout = true
         textStorage.addLayoutManager(layoutManager)
         let textContainer = NSTextContainer(size: NSSize(
             width: CGFloat.greatestFiniteMagnitude,
@@ -271,6 +276,7 @@ private struct CodeTextView: NSViewRepresentable {
         textView.onEscape = onEscape
         textView.onCommandF = onCommandF
         textView.string = text
+        context.coordinator.lineCount = VCodeView.countLines(in: text)
 
         let scrollView = VCodeHorizontalScrollView()
         scrollView.documentView = textView
@@ -287,7 +293,7 @@ private struct CodeTextView: NSViewRepresentable {
         context.coordinator.parent = self
         guard let textView = scrollView.documentView as? CodeNSTextView else { return }
         if textView.string != text {
-            context.coordinator.invalidateMeasurementCache()
+            context.coordinator.lineCount = VCodeView.countLines(in: text)
             let selectedRanges = textView.selectedRanges
             textView.string = text
             let length = (text as NSString).length
@@ -310,33 +316,20 @@ private struct CodeTextView: NSViewRepresentable {
         nsView: VCodeHorizontalScrollView,
         context: Context
     ) -> CGSize? {
-        guard let textView = nsView.documentView as? CodeNSTextView,
-              let layoutManager = textView.layoutManager,
-              let textContainer = textView.textContainer,
-              let textStorage = textView.textStorage else { return nil }
+        guard let textView = nsView.documentView as? CodeNSTextView else { return nil }
 
         let width = proposal.width ?? 400
 
-        // SwiftUI calls `sizeThatFits` for every cell on every `LazyVStack`
-        // layout pass. Returning the cached size when
-        // `(textStorage.length, width)` matches the last measurement avoids
-        // rerunning `ensureLayout`, which is O(n) in glyph count.
-        // `updateNSView` invalidates on text replacement and `textDidChange`
-        // invalidates on user edits.
-        let coordinator = context.coordinator
-        let length = textStorage.length
-        if length == coordinator.lastMeasuredLength,
-           width == coordinator.lastMeasuredWidth {
-            return CGSize(width: width, height: coordinator.lastMeasuredHeight)
-        }
-
-        layoutManager.ensureLayout(for: textContainer)
-        let usedRect = layoutManager.usedRect(for: textContainer)
-        let height = usedRect.height + textView.textContainerInset.height * 2
-
-        coordinator.lastMeasuredLength = length
-        coordinator.lastMeasuredWidth = width
-        coordinator.lastMeasuredHeight = height
+        // Height is derived directly from line count and the pinned
+        // per-line height, bypassing `NSLayoutManager.ensureLayout(for:)`.
+        // The text container is unbounded horizontally, so lines never
+        // wrap; `paragraphStyle.minimumLineHeight == maximumLineHeight`
+        // clamps every line to the same `defaultLineHeight`. That makes
+        // the geometry exactly `lineCount * lineHeight + insets`, and
+        // removes an O(glyph count) main-thread layout pass that SwiftUI
+        // would otherwise re-run on every cell during `LazyVStack` layout.
+        let height = CGFloat(context.coordinator.lineCount) * VCodeView.lineHeight
+            + textView.textContainerInset.height * 2
         return CGSize(width: width, height: height)
     }
 
@@ -347,25 +340,17 @@ private struct CodeTextView: NSViewRepresentable {
     class Coordinator: NSObject, NSTextViewDelegate {
         var parent: CodeTextView
 
-        // Last successful `sizeThatFits` measurement. Invalidated whenever
-        // the text storage is replaced (`updateNSView`, `textDidChange`).
-        var lastMeasuredLength: Int = -1
-        var lastMeasuredWidth: CGFloat = -1
-        var lastMeasuredHeight: CGFloat = 0
+        // Newline count for the current text. Drives `sizeThatFits`
+        // geometry directly, avoiding `NSLayoutManager.ensureLayout(for:)`.
+        var lineCount: Int = 1
 
         init(parent: CodeTextView) {
             self.parent = parent
         }
 
-        func invalidateMeasurementCache() {
-            lastMeasuredLength = -1
-            lastMeasuredWidth = -1
-            lastMeasuredHeight = 0
-        }
-
         func textDidChange(_ notification: Notification) {
             guard let textView = notification.object as? NSTextView else { return }
-            invalidateMeasurementCache()
+            lineCount = VCodeView.countLines(in: textView.string)
             parent.text = textView.string
             parent.onTextChange?(textView.string)
         }
diff --git a/clients/shared/DesignSystem/Components/Display/SelectableTextView.swift b/clients/shared/DesignSystem/Components/Display/SelectableTextView.swift
index f27a742260f..be445ffe48f 100644
--- a/clients/shared/DesignSystem/Components/Display/SelectableTextView.swift
+++ b/clients/shared/DesignSystem/Components/Display/SelectableTextView.swift
@@ -177,6 +177,11 @@ public struct VSelectableTextView: NSViewRepresentable {
         // Reference: https://developer.apple.com/documentation/appkit/nstextview/1449309-layoutmanager
         let textStorage = NSTextStorage()
         let layoutManager = NSLayoutManager()
+        // Confine glyph generation to the requested bounding rect so that
+        // attaching the text view to its hosting view does not force full
+        // document layout on the main thread.
+        // https://developer.apple.com/documentation/appkit/nslayoutmanager/allowsnoncontiguouslayout
+        layoutManager.allowsNonContiguousLayout = true
         textStorage.addLayoutManager(layoutManager)
         let textContainer = NSTextContainer(size: NSSize(
             width: 0,
diff --git a/clients/shared/DesignSystem/Components/Display/VCodeView.swift b/clients/shared/DesignSystem/Components/Display/VCodeView.swift
index 1a9a2c8b593..47599507e1d 100644
--- a/clients/shared/DesignSystem/Components/Display/VCodeView.swift
+++ b/clients/shared/DesignSystem/Components/Display/VCodeView.swift
@@ -239,6 +239,14 @@ struct VCodeTextView: NSViewRepresentable {
     func makeNSView(context: Context) -> VCodeHorizontalScrollView {
         let textStorage = NSTextStorage()
         let layoutManager = NSLayoutManager()
+        // Non-contiguous layout confines glyph generation to the requested
+        // bounding rect. Without this, adding the NSTextView to its scroll
+        // view triggers full-document glyph layout from character 0 on the
+        // main thread (via `_setSuperview:` → `setNeedsDisplayInRect:` →
+        // `_glyphRangeForBoundingRect:`), producing multi-second hangs for
+        // large code blocks.
+        // https://developer.apple.com/documentation/appkit/nslayoutmanager/allowsnoncontiguouslayout
+        layoutManager.allowsNonContiguousLayout = true
         textStorage.addLayoutManager(layoutManager)
         let textContainer = NSTextContainer(size: NSSize(
             width: CGFloat.greatestFiniteMagnitude,
@@ -326,33 +334,21 @@ struct VCodeTextView: NSViewRepresentable {
         nsView: VCodeHorizontalScrollView,
         context: Context
     ) -> CGSize? {
-        guard let textView = nsView.documentView as? ClickReportingTextView,
-              let layoutManager = textView.layoutManager,
-              let textContainer = textView.textContainer,
-              let textStorage = textView.textStorage else { return nil }
+        guard let textView = nsView.documentView as? ClickReportingTextView else { return nil }
 
         let width = proposal.width ?? 400
 
-        // SwiftUI calls `sizeThatFits` for every cell on every `LazyVStack`
-        // layout pass. Returning the cached size when
-        // `(textStorage.length, width)` matches the last measurement avoids
-        // rerunning `ensureLayout`, which is O(n) in glyph count. `applyText`
-        // invalidates on both the plain-text apply and async syntax-highlight
-        // completion (font variants can change line height).
-        let coordinator = context.coordinator
-        let length = textStorage.length
-        if length == coordinator.lastMeasuredLength,
-           width == coordinator.lastMeasuredWidth {
-            return CGSize(width: width, height: coordinator.lastMeasuredHeight)
-        }
-
-        layoutManager.ensureLayout(for: textContainer)
-        let usedRect = layoutManager.usedRect(for: textContainer)
-        let height = usedRect.height + textView.textContainerInset.height * 2
-
-        coordinator.lastMeasuredLength = length
-        coordinator.lastMeasuredWidth = width
-        coordinator.lastMeasuredHeight = height
+        // Height is derived directly from line count and the pinned
+        // per-line height, bypassing `NSLayoutManager.ensureLayout(for:)`.
+        // The text container is unbounded horizontally, so lines never
+        // wrap; `paragraphStyle.minimumLineHeight == maximumLineHeight`
+        // clamps every line (including bold/italic syntax-highlight runs)
+        // to the same `defaultLineHeight`. That makes the geometry exactly
+        // `lineCount * lineHeight + insets`, and removes an O(glyph count)
+        // main-thread layout pass that SwiftUI would otherwise re-run on
+        // every cell during `LazyVStack` layout.
+        let height = CGFloat(context.coordinator.lineCount) * VCodeView.lineHeight
+            + textView.textContainerInset.height * 2
         return CGSize(width: width, height: height)
     }
 
@@ -366,21 +362,12 @@ struct VCodeTextView: NSViewRepresentable {
         var lastMatchCount: Int = 0
         var paragraphStyle: NSParagraphStyle?
 
-        // Last successful `sizeThatFits` measurement. Invalidated wherever
-        // the text storage is replaced: `applyText` and the async
-        // syntax-highlight completion.
-        var lastMeasuredLength: Int = -1
-        var lastMeasuredWidth: CGFloat = -1
-        var lastMeasuredHeight: CGFloat = 0
+        // Newline count for the current text. Drives `sizeThatFits`
+        // geometry directly, avoiding `NSLayoutManager.ensureLayout(for:)`.
+        var lineCount: Int = 1
 
         private var highlightTask: Task<Void, Never>?
 
-        func invalidateMeasurementCache() {
-            lastMeasuredLength = -1
-            lastMeasuredWidth = -1
-            lastMeasuredHeight = 0
-        }
-
         deinit {
             highlightTask?.cancel()
         }
@@ -397,7 +384,7 @@ struct VCodeTextView: NSViewRepresentable {
             to textView: NSTextView
         ) {
             lastText = text
-            invalidateMeasurementCache()
+            lineCount = VCodeView.countLines(in: text)
 
             // Apply plain text immediately so the view is never empty
             guard let textStorage = textView.textStorage else { return }
@@ -428,10 +415,6 @@ struct VCodeTextView: NSViewRepresentable {
                     storage.setAttributedString(highlighted)
                     storage.endEditing()
 
-                    // Invalidate measurement cache — font variants from
-                    // highlighting (bold/italic) may change line heights.
-                    self?.invalidateMeasurementCache()
-
                     // Re-apply search highlights that were wiped by setAttributedString
                     if let self, !self.currentMatchRanges.isEmpty {
                         self.applySearchHighlights(