The stacked mount reads as one document in view mode — title, body and the thread share a single scroll

Preview, stacked (over/under): title, the rendered body and the comment
thread now stack in one continuous document with one scroll, instead of
the fixed ≈3:2 split with each pane keeping its own. Edit mode keeps the
split unchanged (an editor needs a stable scroll of its own), and the
beside mount is untouched.

CardBodySurface gains a `scrolls` flag: false switches off the hosted
NSScrollView's scroller and elasticity and reports the NSTextView's own
height for the proposed width via `sizeThatFits`, the layout-manager
height-fit trick CommentBodyView already uses one level up. CardCommentsPane
gains an `embeddedProxy`: supplied, it renders the same header, find bar,
rows and composer without wrapping them in a second ScrollView, driving
scrollTo off the outer document's proxy instead of its own. CardWindowView
composes the two behind a new pure predicate, CommentsMount.showsContinuousDocument(mode:),
tested in CardCommentsLayoutTests.

The continuous↔split swap within stacked mount is a genuine remount of the
body pane (two independent scrolls can't become one shared scroll by
reconfiguration) — the same accepted cost the raw-source outlet already
takes elsewhere in this window. No new animation on that swap, matching
this file's existing precedent (the raw-source swap and the beside↔stacked
mount switch are both instant cuts today). Decisions recorded on the card's
thread, flagged for owner review where they're user-visible.

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

Kanban/UI/Card/CardBodySurface.swift

@@ -64,6 +64,21 @@ struct CardBodySurface: NSViewRepresentable {
     let isTaskToggleEnabled: Bool
     /// Byte offset and the state the user saw — straight through to `BoardStore.toggleTaskMarker`.
     let onToggleTask: (Int, Bool) -> Void
+    /// Whether this surface scrolls on its own — `true`, the default, for every mount and mode
+    /// today. `false` is the stacked mount's continuous Preview arrangement
+    /// (`CardWindowView.continuousStackedContent`), where this surface's rendered content is one
+    /// section of a single shared document scroll rather than its own scrolling region: the hosted
+    /// `NSScrollView` loses its scroller and elasticity (the outer `ScrollView` owns wheel/trackpad
+    /// scrolling instead) and `sizeThatFits(_:nsView:context:)` below reports the text's own height
+    /// for whatever width it is proposed, rather than the view answering nothing and falling back to
+    /// a frame nothing constrains.
+    var scrolls: Bool = true
+
+    /// The height a measurement pass lays out into while `scrolls` is `false` — tall enough that no
+    /// card body reaches it, finite so the arithmetic stays well-defined. `CommentBodyView`'s own
+    /// constant, applied one level down through the hosting scroll view rather than straight to the
+    /// represented view.
+    private static let embeddedLayoutCeiling: CGFloat = 100_000
 
     func makeCoordinator() -> Coordinator {
         Coordinator()
@@ -133,6 +148,17 @@ struct CardBodySurface: NSViewRepresentable {
         scrollView.autohidesScrollers = true
         scrollView.drawsBackground = false
         scrollView.findBarPosition = .aboveContent
+        // **`scrolls == false` switches this scroll view off** — no scroller to grab, no elastic
+        // bounce to fight the outer `ScrollView`'s own wheel/trackpad scrolling, which is what
+        // "embedded in an outer scroll" (the continuous arrangement) actually means at the AppKit
+        // layer. `sizeThatFits` below is the other half: without it this view would still have
+        // nothing to report and would collapse to zero height inside an unbounded `ScrollView`
+        // proposal.
+        if !scrolls {
+            scrollView.hasVerticalScroller = false
+            scrollView.verticalScrollElasticity = .none
+            scrollView.horizontalScrollElasticity = .none
+        }
 
         context.coordinator.textView = textView
         context.coordinator.session = session
@@ -158,6 +184,33 @@ struct CardBodySurface: NSViewRepresentable {
         return scrollView
     }
 
+    /// **The intrinsic height, while `scrolls` is `false`.** `nil` otherwise — the default
+    /// (unimplemented) answer, which lets a scrolling instance keep taking whatever frame its
+    /// `.frame(maxHeight: .infinity)` modifier proposes exactly as it always has.
+    ///
+    /// `CommentBodyView.sizeThatFits`'s own trick, one level down through the hosting scroll view:
+    /// the container tracks the text view's own frame width (`widthTracksTextView`), so forcing that
+    /// frame to the proposed width *is* how the width is proposed at all, and `ensureLayout` is not
+    /// optional — `usedRect` only means something once the glyphs are laid, and an unlaid container
+    /// answers a zero-height rect, which would collapse the whole body to nothing.
+    func sizeThatFits(_ proposal: ProposedViewSize, nsView scrollView: NSScrollView, context: Context) -> CGSize? {
+        guard !scrolls else { return nil }
+        guard let textView = scrollView.documentView as? NSTextView,
+            let container = textView.textContainer,
+            let layoutManager = textView.layoutManager
+        else { return nil }
+        guard let width = proposal.width, width > 0, width.isFinite else { return nil }
+
+        textView.frame = NSRect(x: 0, y: 0, width: width, height: Self.embeddedLayoutCeiling)
+        layoutManager.ensureLayout(for: container)
+        let usedHeight = layoutManager.usedRect(for: container).height
+        // `usedRect` is the text alone; the gutter inset on both edges is this surface's own, added
+        // back in rather than folded into the container width above (`textContainerInset` already
+        // does that subtraction for `widthTracksTextView`, so doing it twice here would double it).
+        let totalHeight = usedHeight + textView.textContainerInset.height * 2
+        return CGSize(width: width, height: totalHeight.rounded(.up))
+    }
+
     func updateNSView(_ scrollView: NSScrollView, context: Context) {
         let coordinator = context.coordinator
         coordinator.onToggleTask = onToggleTask

Kanban/UI/Card/CardCommentsLayout.swift

@@ -57,6 +57,20 @@ public enum CommentsMount: Sendable, Equatable {
     /// Stacked, the comments pane takes the width the body already had, so a narrow display keeps the
     /// minimum it has always had. That is the whole reason the option exists.
     public var widensWindow: Bool { self == .beside }
+
+    /// **Whether Preview shows the stacked mount's continuous, single-scroll arrangement** — title,
+    /// body and the comment thread stacked directly atop each other in one document, rather than the
+    /// fixed ≈3:2 split with each pane keeping its own scroll.
+    ///
+    /// Pure and mode-driven, so "continuous is Preview-only; Edit keeps the split; beside is
+    /// untouched" is a fact a test can hold rather than something read off a running window
+    /// (`CardWindowView.contentPanes` is the one caller). Edit needs its own stable scroll — a text
+    /// editor's caret, selection and undo session cannot share a document scroll with the thread
+    /// beneath it and stay usable — and the beside mount never had a split to begin with, so neither
+    /// case answers `true` regardless of `mode`.
+    public func showsContinuousDocument(mode: CardBodyMode) -> Bool {
+        self == .stacked && mode == .preview
+    }
 }
 
 // MARK: - Which way the thread runs

Kanban/UI/Card/CardCommentsPane.swift

@@ -30,6 +30,17 @@ struct CardCommentsPane: View {
     /// The window's thumbnail memory, shared with the sidebar's attachment rows so a file shown in
     /// both places is rendered once.
     let thumbnails: AttachmentThumbnailCache
+    /// **The proxy of an outer `ScrollViewReader` this pane's content already lives inside** — the
+    /// stacked mount's continuous Preview arrangement (`CardWindowView.continuousStackedContent`),
+    /// where title, body and the thread share one document scroll rather than this pane owning its
+    /// own. `nil`, the default, is every other mount and mode: this pane wraps its own content in a
+    /// `ScrollView`/`ScrollViewReader` exactly as it always has.
+    ///
+    /// **Nothing about a comment row, the composer, the find bar or the header changes with this** —
+    /// the type's own doc above ("it does not know where it is mounted") extended one step further:
+    /// the one thing that does change is which `ScrollViewProxy` a scrollTo call reaches for, and
+    /// whether this view supplies the `ScrollView` those calls need a proxy over in the first place.
+    var embeddedProxy: ScrollViewProxy? = nil
 
     /// **App-wide and persisted** (05 ▸ The comments column; 11-command-nexus.md files the header
     /// control under Configuration controls). Read here rather than mirrored onto the window's handle
@@ -59,7 +70,14 @@ struct CardCommentsPane: View {
 
             thread
         }
-        .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)
+        // **`maxHeight` drops out while embedded.** The standalone mounts hand this pane a *fixed*
+        // remaining height to fill (the beside `HStack`'s column, the stacked split's own row under
+        // its `GeometryReader`), so `.infinity` there is exactly what claims all of it. Embedded, the
+        // outer document `ScrollView` proposes an *unbounded* height instead, and a pane that still
+        // claimed `.infinity` of an unbounded proposal would grow to its content's ideal height
+        // anyway — but stating `nil` here is the honest version of that rather than a coincidence
+        // this file would have to keep re-deriving by eye.
+        .frame(maxWidth: .infinity, maxHeight: embeddedProxy == nil ? .infinity : nil, alignment: .topLeading)
         // 10-accessibility.md's container label for the pane ("Comments, N") — the count is the
         // thread's, so the spoken container and the visible header can never disagree.
         .accessibilityElement(children: .contain)
@@ -118,9 +136,26 @@ struct CardCommentsPane: View {
 
     // MARK: - The thread
 
+    /// The rows, the composer, and their own `ScrollView` — or, embedded, the same rows and composer
+    /// without one, since `embeddedProxy`'s owner already supplies the scroll they sit inside.
+    @ViewBuilder
     private var thread: some View {
+        if let embeddedProxy {
+            threadRows(proxy: embeddedProxy)
+        } else {
             ScrollViewReader { proxy in
                 ScrollView(.vertical) {
+                    threadRows(proxy: proxy)
+                }
+            }
+        }
+    }
+
+    /// The rows and the composer, at whichever end `direction` puts it — the one place either is
+    /// laid out, standalone or embedded. `proxy` is this pane's own when standalone and the outer
+    /// document's when embedded; the scrollTo calls below do not know which.
+    @ViewBuilder
+    private func threadRows(proxy: ScrollViewProxy) -> some View {
         LazyVStack(alignment: .leading, spacing: CardWindowMetrics.commentSpacing(bodyPointSize: pointSize)) {
             if direction.placesComposerFirst {
                 composer.id(Self.composerAnchor)
@@ -140,11 +175,15 @@ struct CardCommentsPane: View {
         }
         .padding(CardWindowMetrics.gutter(bodyPointSize: pointSize))
         .frame(maxWidth: .infinity, alignment: .leading)
-            }
-            // **The window opens scrolled to the composer** (05). Deferred one turn rather than run
-            // inline: `scrollTo` needs the content laid out to have somewhere to scroll to, and a
-            // thread's rows measure their own rendered height (`CommentBodyView`).
+        // **The window opens scrolled to the composer** (05) — **standalone only**. Embedded, that
+        // would mean skipping past the title and the body to the thread's newest end on every open,
+        // which reads wrong for a single continuous document; the composer still gets the reader
+        // brought to it by an explicit request (`focusComposerRequests`, just below), only the
+        // *opening* auto-scroll is standalone-only. Deferred one turn rather than run inline:
+        // `scrollTo` needs the content laid out to have somewhere to scroll to, and a thread's rows
+        // measure their own rendered height (`CommentBodyView`).
         .task {
+            guard embeddedProxy == nil else { return }
             await Task.yield()
             proxy.scrollTo(Self.composerAnchor, anchor: direction.placesComposerFirst ? .top : .bottom)
         }
@@ -163,7 +202,6 @@ struct CardCommentsPane: View {
             proxy.scrollTo(match.comment, anchor: .center)
         }
     }
-    }
 
     // MARK: - The composer
 

Kanban/UI/Card/CardWindowView.swift

@@ -267,6 +267,13 @@ struct CardWindowView: View {
     @ViewBuilder
     private var contentPanes: some View {
         if showComments {
+            // **Preview, stacked: one continuous document** — checked ahead of the mount switch
+            // below because it is the one case that is not a fixed division of the body and the
+            // comments pane at all; everything under the switch still divides *something* (a width
+            // in `.beside`, a height in `.stacked`), and this arrangement doesn't.
+            if mount.showsContinuousDocument(mode: bodyPresentation.mode) {
+                continuousStackedContent
+            } else {
                 switch mount {
                 case .beside:
                     // The `GeometryReader` is new here for the divider's sake — a resizable column needs
@@ -274,7 +281,7 @@ struct CardWindowView: View {
                     // exactly the reason the stacked branch below already reaches for one.
                     GeometryReader { proxy in
                         HStack(spacing: 0) {
-                        bodyColumn
+                            bodyColumn()
                                 .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)
 
                             CommentsColumnDivider(
@@ -303,13 +310,14 @@ struct CardWindowView: View {
                     }
 
                 case .stacked:
-                // The ≈3:2 split needs a height to divide, and a `GeometryReader` is the only way to
-                // have one — `layoutPriority` and flexible frames express *preferences*, and this is
-                // a ratio the design fixes. The body takes its share; the comments pane takes the
+                    // Edit only, here — Preview took the branch above. The ≈3:2 split needs a height
+                    // to divide, and a `GeometryReader` is the only way to have one —
+                    // `layoutPriority` and flexible frames express *preferences*, and this is a ratio
+                    // the design fixes. The body takes its share; the comments pane takes the
                     // remainder, so the divider between them can never leave a gap or overlap.
                     GeometryReader { proxy in
                         VStack(spacing: 0) {
-                        bodyColumn
+                            bodyColumn()
                                 .frame(height: mount.bodyHeight(in: proxy.size.height))
                                 .frame(maxWidth: .infinity, alignment: .topLeading)
 
@@ -320,8 +328,9 @@ struct CardWindowView: View {
                         }
                     }
                 }
+            }
         } else {
-            bodyColumn
+            bodyColumn()
                 .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)
         }
     }
@@ -330,10 +339,49 @@ struct CardWindowView: View {
         CardCommentsPane(comments: comments, cardFolder: cardFolder, thumbnails: thumbnails)
     }
 
+    /// **The stacked mount's continuous Preview arrangement** — "title, body, comments stacked
+    /// directly atop each other in a continuous fashion" (the card this milestone implements): one
+    /// `ScrollView`, so the body pane's own scroll switches off in favor of reporting its intrinsic
+    /// height (`CardBodySurface`'s `scrolls: false`), and the comments pane's own `ScrollView`
+    /// switches off the same way (`CardCommentsPane`'s `embeddedProxy`) — both panes' internals
+    /// otherwise untouched: the same header, the same rendering, the same composer, the same row.
+    ///
+    /// **Preview only** (`CommentsMount.showsContinuousDocument(mode:)`), and deliberately so: Edit
+    /// needs its own stable scroll and its own session-scoped undo, which a shared document scroll
+    /// cannot give it. Entering or leaving Edit therefore swaps this arrangement for the fixed ≈3:2
+    /// split above — a genuinely different view of the body pane, not a reconfiguration of the one
+    /// in `bodyColumn(embedded:)`'s doc — the same accepted cost the raw-source outlet already takes
+    /// elsewhere in this window: the buffer survives the swap (`bodySession`, dirty-buffer-wins), the
+    /// scroll position and the find state do not.
+    private var continuousStackedContent: some View {
+        ScrollViewReader { proxy in
+            ScrollView(.vertical) {
+                VStack(alignment: .leading, spacing: 0) {
+                    bodyColumn(embedded: true)
+
+                    Divider()
+
+                    CardCommentsPane(
+                        comments: comments,
+                        cardFolder: cardFolder,
+                        thumbnails: thumbnails,
+                        embeddedProxy: proxy
+                    )
+                }
+                .frame(maxWidth: .infinity, alignment: .leading)
+            }
+        }
+    }
+
     // MARK: - Body column
 
     /// Title, the quiet created/modified line, then the body — 05's top-to-bottom order.
-    private var bodyColumn: some View {
+    ///
+    /// - Parameter embedded: `true` only from `continuousStackedContent`, which mounts this inside an
+    ///   outer document `ScrollView` rather than giving it a fixed or flexible height of its own —
+    ///   see `CardBodySurface.scrolls` for what that switches off. `false`, the default, is every
+    ///   other call site, byte-for-byte the arrangement this had before that mode existed.
+    private func bodyColumn(embedded: Bool = false) -> some View {
         VStack(alignment: .leading, spacing: 0) {
             VStack(alignment: .leading, spacing: bodyPointSize * 0.5) {
                 titleRow
@@ -360,9 +408,14 @@ struct CardWindowView: View {
                 presentation: bodyPresentation,
                 session: bodySession,
                 isTaskToggleEnabled: isEditable,
-                onToggleTask: onToggleTask
+                onToggleTask: onToggleTask,
+                scrolls: !embedded
             )
-            .frame(maxWidth: .infinity, maxHeight: .infinity)
+            // `maxHeight` drops to `nil` while embedded: `sizeThatFits` (`CardBodySurface`) reports
+            // the text's own height for the width it is given, and a modifier still claiming
+            // `.infinity` of the outer scroll's unbounded proposal would ask for an undefined amount
+            // of it instead.
+            .frame(maxWidth: .infinity, maxHeight: embedded ? nil : .infinity)
         }
         // **Dirty-buffer-wins, applied on every snapshot** (05 ▸ Write rules): the session takes
         // disk's word for what the file says, and takes it into the editor only when the buffer has

KanbanTests/CardCommentsLayoutTests.swift

@@ -58,6 +58,17 @@ struct CommentsMountTests {
         #expect(CommentsMount.beside.widensWindow)
         #expect(!CommentsMount.stacked.widensWindow)
     }
+
+    @Test("The continuous document arrangement is Preview-only, and stacked-only")
+    func continuousDocumentIsStackedPreviewOnly() {
+        // Title, body and the thread share one scroll only when the mount is stacked *and* the body
+        // is showing Preview — Edit needs its own stable scroll, and beside never had a split to
+        // fold away in the first place.
+        #expect(CommentsMount.stacked.showsContinuousDocument(mode: .preview))
+        #expect(!CommentsMount.stacked.showsContinuousDocument(mode: .edit))
+        #expect(!CommentsMount.beside.showsContinuousDocument(mode: .preview))
+        #expect(!CommentsMount.beside.showsContinuousDocument(mode: .edit))
+    }
 }
 
 // MARK: - The window's minimum