import SwiftUI

// MARK: - The strip's half of the header drag

/// What the strip lends a lane so its **whole title bar** can be the drag surface (03-board-ui.md §
/// Lane, "no separate grip").
///
/// The gesture lives on the header — that is where the design puts it — but the two things it needs
/// are the strip's: where this lane currently rests (the origin the translation is measured from)
/// and what a release means (a proposal computed against the live lane order, then a write). Both
/// arrive as closures rather than as values because both must be read at *gesture* time, not at
/// body-evaluation time.
@MainActor
struct LaneHeaderDrag {
    /// This lane's resting centre in strip coordinates, read the instant the drag begins and frozen
    /// for its duration (`LaneReorderSession.startCentre`).
    let startCentre: () -> CGFloat
    /// Commit the reorder at whatever the current proposal is, and end the session.
    let commit: () -> Void
}

// MARK: - LaneView

/// One lane: a title bar and a vertically scrolling masonry of cards (03-board-ui.md § Lane).
///
/// ### The title bar (this milestone's subject)
///
/// Leading SF Symbol from `icon` — lenient, an unknown name renders the `square.stack` default
/// (`ItemSymbol`) — then the title or its quiet "Untitled" placeholder, a quiet secondary
/// card-count badge, and a trailing quiet new-card button. **The whole bar is the drag surface**:
/// a click selects the lane — toggling off on a repeat, exactly as empty space does
/// (04-interactions.md § Selection, settled) — and movement past a small threshold begins a reorder
/// (`LaneReorderSession`). The one thing carved out of the drag region is the button, which sits in
/// an overlay outside the gesture so a click on it can never be read as the beginning of a drag.
///
/// ### The lane's one context menu
///
/// "The lane has one context menu (settled), invoked on the header or on lane empty space alike"
/// (03-board-ui.md § Lane), so both surfaces attach the *same* `laneMenu`. It carries Style…, the
/// quick-style recents row and the Width stepper today; Rename and Delete are m5's context-menus
/// card, and their rows go into that same builder rather than into a second menu.
///
/// ### What is still a later card's
///
/// The search-aware filtering behind the count belongs to a later milestone. The card face is real
/// (`CardFaceView`); what it still owes is the cut treatment and the sole-selected card's attachment
/// carousel.
struct LaneView: View {

    let store: BoardStore
    let lane: Lane

    /// The app-wide quick-style recents (03-board-ui.md § Styling ▸ Controls — "never board data"),
    /// read from the environment rather than threaded down the strip: the list belongs to the app,
    /// not to this board, and every context menu in the window wants it.
    @Environment(AppModel.self) private var appModel

    /// Interior masonry columns — the lane's width units, or the resize session's snapped count
    /// while this lane is being dragged. Passed in rather than read off `lane` so the live drag can
    /// override it (see `BoardView.laneSlot`).
    let columns: Int

    /// The strip's reorder session, so the header knows whether *it* is the lane in flight.
    let reorder: LaneReorderSession

    let headerDrag: LaneHeaderDrag

    /// The strip's rubber band: the lane's empty space is one of its three surfaces, and every card
    /// face registers its frame into the same registry (`MarqueeControl`).
    let marquee: MarqueeControl

    /// Opens a card's window — ⌘↩'s second half (04-interactions.md ▸ Grammar, "commits and opens
    /// the card window"). Supplied by the strip, which is supplied by the host: a lane has no
    /// business knowing about `WindowGroup` keys.
    let openCard: (ItemID) -> Void

    /// Reduce Motion, for the card transition below (10-accessibility.md). Read from the environment
    /// and handed to `Motion`, which owns what "reduced" means.
    @Environment(\.accessibilityReduceMotion) private var reduceMotion

    /// Spacing between cards, and between the interior columns.
    private let cardSpacing: CGFloat = 8

    /// The lane plate's corner radius — shared by the selection treatment and the accent band, whose
    /// top corners round to exactly this so the band reads as the lane's own edge.
    private let cornerRadius: CGFloat = 10

    /// C7 · full-column top edge (03-board-ui.md § Styling ▸ Capabilities).
    private let bandHeight: CGFloat = 5

    var body: some View {
        // `spacing: 0` and the padding moved inside: the accent band is **full-width** along the
        // lane's top edge, so it must sit outside the content inset rather than in it.
        VStack(alignment: .leading, spacing: 0) {
            accentBand
            VStack(alignment: .leading, spacing: 8) {
                header
                cardStack
            }
            .padding(6)
        }
        .background(selectionBackground)
        .overlay(selectionStroke)
    }

    // MARK: - Header

    private var header: some View {
        headerContent
            // The bar is the drag surface, so it must be hit-testable across its whole width —
            // including the empty stretch between the badge and the button.
            .contentShape(Rectangle())
            .gesture(headerGesture)
            .overlay(alignment: .trailing) { newCardButton }
            .contextMenu { laneMenu }
            // The lane's half of the Style… popover. Anchored on the header because that is the
            // lane's own furniture — `styleEditorPresentation` decides whether this lane is the
            // session's presenting anchor at all.
            .popover(isPresented: styleEditorPresentation(store, anchor: lane.id), arrowEdge: .bottom) {
                StyleEditorPopover(store: store, recents: appModel.styleRecents)
            }
    }

    /// The lane's colour as C7 — "a lane's color paints a full-width band along its top edge; the
    /// surfaces themselves keep the standard chrome, so colored title text never sits on a colored
    /// fill" (03-board-ui.md § Styling ▸ Capabilities, settled in the pathfinder's treatment
    /// shootout).
    ///
    /// A value that resolves to nothing paints **no band**, and the bytes stay on disk exactly as
    /// written — the card stripe's rule, for its reason: there is no sensible default colour for
    /// "the author meant something we can't read", and a wrong colour is worse than none.
    @ViewBuilder
    private var accentBand: some View {
        if let color = Palette.color(for: lane.background) {
            UnevenRoundedRectangle(topLeadingRadius: cornerRadius, topTrailingRadius: cornerRadius)
                .fill(color)
                .frame(height: bandHeight)
                .frame(maxWidth: .infinity)
                // Decoration only: the header below it owns the lane's click and drag.
                .allowsHitTesting(false)
        }
    }

    // MARK: - The lane's one context menu

    /// Rename, Style…, the quick-style recents row, the Width control, Delete (11-command-nexus.md ▸
    /// Context menus) — the style trio and the width stepper today.
    @ViewBuilder
    private var laneMenu: some View {
        // m5-context-menus: Rename (a twin of Board ▸ Rename) and Delete (a twin of File ▸ Delete)
        // belong to the card that brings the selection model and the delete command; both are rows
        // of *this* menu when they land, not of a second one.
        StyleMenuItems(store: store, recents: appModel.styleRecents, target: styleTarget)

        Divider()

        widthControl
    }

    /// The width stepper — "the header context menu's Width control (stepper, uncapped) is the
    /// precise control … it never touches the window, it **re-divides** the existing width across the
    /// new unit total" (03-board-ui.md § Lane). A +/− pair rather than a slider or a fixed 1×/2×/3×
    /// list, because the control is uncapped in one direction and floored at one unit in the other.
    ///
    /// **Single-lane by nature**, unlike the style entries above it: the design gives the batch to
    /// the ⌥⌘→/⌥⌘← menu items and keeps the stepper on the lane whose menu is open.
    private var widthControl: some View {
        let units = LaneLayoutMath.displayUnits(of: lane)
        return Section("Width — \(units)×") {
            Button("Increase Width") {
                store.setLaneWidth(lane.id, units: units + 1)
            }
            Button("Decrease Width") {
                store.setLaneWidth(lane.id, units: units - 1)
            }
            // A one-unit lane cannot shrink (`width` is ≥ 1), and an item whose only outcome is a
            // no-op reads better disabled than dead — `LaneWidthCommands`' rule, same floor.
            .disabled(units <= 1)
        }
        .disabled(!store.acceptsBoardMutations)
    }

    /// What this lane's menu styles: the whole selection when this lane is part of it, else this lane
    /// alone — standard macOS context-menu targeting (the pathfinder's `styleSelection`, kept).
    /// Right-clicking something outside the selection acts on what was clicked.
    private var styleTarget: StyleTarget {
        guard store.selection.liveness == .live, store.selection.ids.contains(lane.id) else {
            return .items([lane.id])
        }
        return .items(store.selection.ids)
    }

    private var headerContent: some View {
        HStack(alignment: .firstTextBaseline, spacing: 6) {
            Image(systemName: ItemSymbol.name(lane.icon, fallback: ItemSymbol.lane))
                .foregroundStyle(.secondary)
                .imageScale(.medium)
            headerTitle
            countBadge
            Spacer(minLength: 0)
        }
        // Reserves the button's width so a long title truncates before it collides, and keeps the
        // button out of the gestured region.
        .padding(.trailing, 22)
        .padding(.horizontal, 4)
    }

    /// The title, or the rename editor when this lane is the one being renamed.
    ///
    /// A lane's **only** rename path is Board ▸ Rename (04-interactions.md ▸ Selection: "the menu
    /// item is a lane's only rename path, since Return on a lane creates a card"), so nothing in
    /// this view opens the editor — it only renders one that is already open.
    @ViewBuilder
    private var headerTitle: some View {
        if isRenaming {
            InlineTitleField(
                text: renameDraft,
                prompt: "Lane name",
                onCommit: { store.commitRename() },
                onAbandon: { store.transient.discardRename() },
                onFocusLoss: { store.commitRename() },
                // A lane has no card window; ⌘↩ still commits, which is the half of the rule that
                // applies (04 ▸ Grammar's carve-out is "commits the edit — placeholder or rename —
                // and open[s] the card window", and only a card has one to open).
                onCommitAndOpen: { store.commitRename() }
            )
            .font(.headline)
        } else {
            Text(lane.title.value ?? "Untitled")
                .font(.headline)
                .foregroundStyle(lane.title.value == nil ? .secondary : .primary)
                .lineLimit(1)
                .truncationMode(.tail)
        }
    }

    /// The card-count badge — quiet, secondary (03-board-ui.md § Lane).
    ///
    /// **It counts exactly what the body renders**, because it reads the same `renderedCards` the
    /// masonry iterates. That is deliberate rather than incidental: "The count reads the search
    /// filter like every other surface — during a search it shows the visible count, not the
    /// total", so when m5's search card narrows `renderedCards` to the filter's survivors the badge
    /// follows by construction, with no second rule to keep in step.
    private var countBadge: some View {
        Text("\(renderedCards.count)")
            .font(.caption)
            .monospacedDigit()
            .foregroundStyle(.secondary)
            .padding(.horizontal, 6)
            .padding(.vertical, 1)
            .background(Capsule().fill(.quaternary))
    }

    /// The new-card button — a **pointer twin** of File ▸ New Card whose click *names its target*:
    /// "the lane header's new-card button overrides [the ⌘N target] rule — the click names its
    /// target lane, selection notwithstanding" (11-command-nexus.md ▸ Pointer grammar, settled), so
    /// it passes this lane and no anchor rather than consulting `NewCardTarget`.
    private var newCardButton: some View {
        Button {
            store.transient.beginPlaceholder(inLane: lane.id)
        } label: {
            Image(systemName: "plus")
                .imageScale(.small)
                .foregroundStyle(.secondary)
                .contentShape(Rectangle())
        }
        .buttonStyle(.plain)
        .accessibilityLabel("New card in \(lane.title.value ?? "Untitled")")
        // Mutating, so the read-only lock disables it like every other write path
        // (02-architecture.md § The lock's scope), and the focused-editor rule closes it while an
        // inline editor is open (04 ▸ Grammar) — the pointer twin of a disabled menu item.
        .disabled(store.isReadOnly || store.isEditingInline)
    }

    /// One gesture recognising both halves of 04-interactions.md ▸ Selection's click-vs-drag split:
    /// "a plain click on the title bar selects the lane; the drag surface engages only on movement".
    ///
    /// `minimumDistance: 0` so the release is seen even when nothing moved — that release *is* the
    /// click. `.global` coordinates because the strip's own space shifts as siblings reflow under
    /// the proposal, and a translation measured against a moving frame is not a pointer delta.
    private var headerGesture: some Gesture {
        DragGesture(minimumDistance: 0, coordinateSpace: .global)
            .onChanged { value in
                // Selection stays live under the lock; reordering does not (02 § The lock's scope).
                // The focused-editor rule holds a drag off too: a reorder is a board command.
                guard !store.isReadOnly, !store.isEditingInline else { return }
                if !reorder.isDragging(lane.id) {
                    guard abs(value.translation.width) > LaneReorderSession.threshold else { return }
                    reorder.begin(laneID: lane.id, startCentre: headerDrag.startCentre())
                }
                reorder.update(translation: value.translation.width)
            }
            .onEnded { _ in
                if reorder.isDragging(lane.id) {
                    headerDrag.commit()
                } else {
                    // **The header toggles like empty space** (04-interactions.md § Selection,
                    // settled): "a click on the already-selected lane's header unselects, one
                    // lane-click behavior everywhere, so a full lane keeps a pointer path out of
                    // selection". Hence the same `togglesOnRepeat` the empty space passes — the two
                    // surfaces differ only in where they are.
                    store.click(
                        SelectionTarget(id: lane.id, kind: .lane, side: .live),
                        modifier: .current,
                        togglesOnRepeat: true
                    )
                }
            }
    }

    // MARK: - Body

    /// The card stack. Its empty space is a click target in its own right (04 ▸ Selection): one
    /// click selects the lane or, when it is already the selection, clears it; a double click
    /// creates a card at the bottom with its title editor focused.
    ///
    /// **"Selection scrolls into view"** (04-interactions.md ▸ Grammar): the reader watches the
    /// navigation head — the cursor the arrows move, not the whole selection — and scrolls only when
    /// the head names a card *this* lane renders, so exactly one lane responds to any one press.
    /// Deliberately unwrapped by `withAnimation`: 03-board-ui.md § Motion has selection follow
    /// "whatever transaction is active rather than easing on its own".
    private var cardStack: some View {
        ScrollViewReader { proxy in
            scrollableCards
                .onChange(of: store.transient.selectionHead) { _, head in
                    guard let head, let card = renderedCards.first(where: { $0.id == head }) else { return }
                    proxy.scrollTo(LaneSlot.identity(of: card.id))
                }
        }
    }

    private var scrollableCards: some View {
        ScrollView(.vertical) {
            // Cards stay standard width whatever the lane spans: at a slot width of
            // `units × standard + (units - 1) × gap`, `MasonryLayout` divides back into exactly
            // `units` columns of `standard` (03-board-ui.md § Layout — full visibility).
            MasonryLayout(columns: columns, spacing: cardSpacing) {
                ForEach(slots) { slot in
                    Group {
                        switch slot {
                        case let .card(card):
                            CardFaceView(
                                store: store,
                                card: card,
                                registry: marquee.registry,
                                openCard: openCard
                            )
                        case .placeholder:
                            NewCardStubView(store: store, openCard: openCard)
                        }
                    }
                    // "Appear/disappear is scale + fade (cards scale from ~0.8 …)"
                    // (03-board-ui.md § Motion), which is how a create, a delete, a Put Back and
                    // (m5) a search filter's leavers all reach the masonry. The placeholder wears it
                    // too: it is the card, one round trip early. Whether any of it *performs* is
                    // decided upstream — at the reload for the real cards (`Motion.reloadAnimates`),
                    // at the gesture for the placeholder, which touches no disk.
                    .transition(Motion.cardTransition(reduced: reduceMotion))
                    // The scroll target. `ForEach` already carries this identity, but `scrollTo`
                    // resolves against an explicit `.id`, and it goes outermost so the transition
                    // above stays inside the identified view rather than around it.
                    .id(slot.id)
                }
            }
            .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)
            .contentShape(Rectangle())
            // Order matters: the two-tap recogniser must be attached first so a double click is not
            // consumed as two singles.
            .onTapGesture(count: 2) {
                guard !store.isReadOnly, !store.isEditingInline else { return }
                store.transient.beginPlaceholder(inLane: lane.id)
            }
            // "Single click selects the lane (click again to unselect)" — the toggle the header
            // shares (04-interactions.md § Selection), and the modifier grammar on top of it.
            .onTapGesture {
                store.click(
                    SelectionTarget(id: lane.id, kind: .lane, side: .live),
                    modifier: .current,
                    togglesOnRepeat: true
                )
            }
            // The rubber band's first surface — "click-drag rubber-bands across lanes". Simultaneous
            // so the taps above stay instant; the band's own begin guard is what keeps a drag that
            // started on a card face out of it (`MarqueeControl`).
            .simultaneousGesture(marquee.gesture(side: .live))
            // The same menu the header carries — "one menu, invoked on the header or lane empty
            // space alike" (03-board-ui.md § Lane, settled).
            .contextMenu { laneMenu }
        }
    }

    /// What the masonry lays out: the rendered cards, plus the new-card placeholder when this lane
    /// is the one being created into.
    ///
    /// The overlay is inserted **at the position the card will actually take** — after its anchor
    /// for ⌘N's "immediately after it", at the bottom otherwise — by asking the very function the
    /// commit uses to compute the rank (`BoardStore.insertionIndex`). One answer, so the pseudo-card
    /// cannot appear anywhere but where the real card lands.
    private var slots: [LaneSlot] {
        var result = renderedCards.map(LaneSlot.card)
        guard let placeholder = store.transient.newCardPlaceholder, placeholder.laneID == lane.id else {
            return result
        }
        let position = BoardStore.insertionIndex(after: placeholder.anchorCardID, among: renderedCards)
        result.insert(.placeholder, at: position ?? result.count)
        return result
    }

    /// **Tombstoned cards render nowhere**, and neither do the cards of a tombstoned lane — the
    /// ancestor walk is absolute (01-storage-format.md § Deletion, 02-architecture.md's effective
    /// liveness). The lane half of that rule is `BoardView`'s, which never builds a `LaneView` for a
    /// tombstoned lane at all.
    ///
    /// This is also the collection m5's search filter narrows, which is what keeps the count badge
    /// honest for free — see `countBadge`.
    private var renderedCards: [Card] {
        lane.cards.filter { !$0.isDeleted }
    }

    // MARK: - Selection

    private var isSelected: Bool {
        store.selection.liveness == .live && store.selection.ids.contains(lane.id)
    }

    /// The selection treatment: a subtle whole-lane accent wash and stroke. Deliberately quiet —
    /// 03-board-ui.md gives lane *colour* to the top-edge accent band, so selection must not read as
    /// a fill that would compete with it once that lands.
    private var selectionBackground: some View {
        RoundedRectangle(cornerRadius: 10)
            .fill(isSelected ? AnyShapeStyle(Color.accentColor.opacity(0.08)) : AnyShapeStyle(.clear))
    }

    private var selectionStroke: some View {
        RoundedRectangle(cornerRadius: 10)
            .strokeBorder(isSelected ? AnyShapeStyle(Color.accentColor) : AnyShapeStyle(.clear), lineWidth: 1.5)
    }

    // MARK: - Rename plumbing

    private var isRenaming: Bool {
        store.transient.renameEditor?.targetID == lane.id
    }

    /// The draft, as a binding onto transient state rather than as `@State`: the editor's text lives
    /// in `TransientBoardState` because a reload has rules about it (the vanish discard), and a
    /// second copy in the view would be the one the commit did not read.
    private var renameDraft: Binding<String> {
        Binding(
            get: { store.transient.renameEditor?.draftTitle ?? "" },
            set: { store.transient.updateRenameDraft($0) }
        )
    }
}

// MARK: - Lane slots

/// What a lane's masonry lays out — its cards, plus at most one pseudo-card.
///
/// The placeholder is not a `Card` and never will be: it has no disk presence and no UUID until its
/// title commits (02-architecture.md § Layering, the one named exception to the one-way flow).
/// Modelling it as a sibling case rather than as a fake `Card` is what keeps that true — nothing can
/// accidentally hand it to code expecting an item that exists.
private enum LaneSlot: Identifiable {
    case card(Card)
    case placeholder

    var id: String {
        switch self {
        case let .card(card): Self.identity(of: card.id)
        // Constant, because there is only ever one placeholder in one lane at a time and it must
        // keep its identity — and therefore its keyboard focus — while the user types.
        case .placeholder: "placeholder"
        }
    }

    /// A card slot's id, spelled once so the scroll-into-view call and the slot itself cannot
    /// disagree about what `scrollTo` is looking for.
    static func identity(of card: ItemID) -> String { "card:\(card.rawValue)" }
}

// MARK: - Card face

/// The card face: a rounded plate carrying a leading SF Symbol, the title (or its quiet "Untitled"
/// placeholder), a quiet trailing attachments indicator, and a left-edge colour accent stripe
/// (03-board-ui.md § Card face, § Styling ▸ Capabilities).
///
/// ### Title-only, deliberately
///
/// **No body excerpt** — settled, "the face stays title-only … the old 'iterate on the card face
/// later' item is closed with no growth". The only face chip in scope is attachments, "a quiet
/// indicator when the card has files — the title dominates", which is why the paperclip is a
/// secondary-tinted caption and not a count pill: the eye should land on the title.
///
/// ### Two lenient fields, two different fallbacks
///
/// `icon` and `iconColor` are hand-written-only on cards (`iconColor` is **schema yes, control
/// no** — the app never offers a picker for it, but honours what an author writes). Both degrade
/// rather than fail: an unknown symbol name draws the level default (`ItemSymbol`), and a colour
/// value that resolves to nothing draws the standard secondary tint. `background` degrades a third
/// way — to **no stripe at all** — because there is no sensible default colour for "the author
/// meant something we can't read", and a wrong colour is worse than none. In every case the bytes
/// on disk are untouched (`Palette`, 01-storage-format.md § Frontmatter).
///
/// ### Room for the carousel
///
/// The face is a top-aligned `VStack` and its two decorations — the accent stripe and the selection
/// stroke — are shapes in overlays, so both stretch to whatever height the content takes. That is
/// what lets m5's carousel expand *inside* this card without any of it being re-derived: the
/// masonry already isolates column heights, so a taller card pushes only the cards below it in its
/// own column.
private struct CardFaceView: View {

    let store: BoardStore
    let card: Card

    /// Where the rubber band looks up what it is sweeping. The face registers its own drawn frame
    /// here and takes it out again when it leaves — see `View.marqueeTarget`.
    let registry: MarqueeTargetRegistry

    let openCard: (ItemID) -> Void

    /// The app-wide quick-style recents — see `LaneView`'s own note.
    @Environment(AppModel.self) private var appModel

    /// The plate's corner radius — shared with the accent stripe, which rounds its left corners to
    /// exactly this so the stripe reads as part of the card's edge rather than a bar laid over it.
    private let cornerRadius: CGFloat = 8

    /// K1 · left edge stripe (03-board-ui.md § Styling ▸ Capabilities, settled in the pathfinder's
    /// treatment shootout).
    private let stripeWidth: CGFloat = 4

    var body: some View {
        VStack(alignment: .leading, spacing: 6) {
            titleRow
            // m5-carousel: the sole selected card's paged attachment carousel expands here — below
            // the title, inside this same plate, keyed on the selection transaction
            // (03-board-ui.md § Card face). It needs `card.attachments` (already loaded) and this
            // view's `isSelected`; nothing above it changes.
        }
        .frame(maxWidth: .infinity, alignment: .leading)
        .padding(10)
        // Constant, whether or not a stripe paints: every card's text sits on the same grid, so
        // colouring a card never shifts its title relative to its uncoloured neighbours.
        .padding(.leading, stripeWidth)
        .background(RoundedRectangle(cornerRadius: cornerRadius).fill(.background.secondary))
        .overlay(alignment: .leading) { accentStripe }
        .overlay(
            RoundedRectangle(cornerRadius: cornerRadius)
                .strokeBorder(isSelected ? AnyShapeStyle(Color.accentColor) : AnyShapeStyle(.clear), lineWidth: 1.5)
        )
        .contentShape(Rectangle())
        // **Clicking never edits** (04-interactions.md ▸ Selection, a pivot from the pathfinder's
        // two-stage Finder rename): one click selects and that is all it does — no timer, no
        // slow-second-click rename, no accidental edit on a hesitant click. Rename is Return or
        // Board ▸ Rename. The modifier grammar — plain replaces, ⌘ toggles, ⇧ ranges — is
        // `SelectionGrammar`'s, reached through the store's one funnel.
        .onTapGesture {
            store.click(SelectionTarget(id: card.id, kind: .card, side: .live), modifier: .current)
        }
        // "A fast double-click opens the card window (⌘↩'s pointer twin)" (04 ▸ Selection).
        //
        // `simultaneousGesture` rather than a second `onTapGesture(count: 2)`, deliberately: a
        // second tap recogniser on the same view makes the single click *wait* to see whether a
        // second one arrives, and selection must stay instant. Simultaneous means the first click
        // of the pair selects and the second opens — Finder's own behaviour.
        //
        // **Plain only.** ⌘ and ⇧ double-clicks are selection gestures that happened twice; opening
        // a window out from under a range the user is still building would be a surprise.
        .simultaneousGesture(TapGesture(count: 2).onEnded {
            guard ClickModifier.current == .plain else { return }
            openCard(card.id)
        })
        .marqueeTarget(card.id, kind: .card, side: .live, in: registry)
        .contextMenu { cardMenu }
        .popover(isPresented: styleEditorPresentation(store, anchor: card.id), arrowEdge: .bottom) {
            StyleEditorPopover(store: store, recents: appModel.styleRecents)
        }
    }

    // MARK: - Context menu

    /// Open, Rename, Style…, the quick-style recents row, Delete (11-command-nexus.md ▸ Context
    /// menus) — the style pair today.
    @ViewBuilder
    private var cardMenu: some View {
        // m5-context-menus: Open (a twin of Board ▸ Open Card, always the clicked card alone —
        // a card window is tied to one card), Rename, and Delete land with the selection-model and
        // delete cards, as rows of this same menu.
        StyleMenuItems(store: store, recents: appModel.styleRecents, target: styleTarget)
    }

    /// What this card's menu styles: the whole selection when this card is part of it, else this card
    /// alone. Standard macOS — right-clicking outside the selection acts on what was clicked.
    private var styleTarget: StyleTarget {
        guard store.selection.liveness == .live, store.selection.ids.contains(card.id) else {
            return .items([card.id])
        }
        return .items(store.selection.ids)
    }

    // MARK: - Title row

    private var titleRow: some View {
        HStack(alignment: .firstTextBaseline, spacing: 6) {
            Image(systemName: ItemSymbol.name(card.icon, fallback: ItemSymbol.card))
                .foregroundStyle(iconTint)
                .imageScale(.medium)
            titleOrEditor
                // The title takes the row's width so the indicator sits hard against the trailing
                // edge — and so the rename field fills the same span the title occupied.
                .frame(maxWidth: .infinity, alignment: .leading)
            attachmentsIndicator
        }
    }

    /// The title, or the rename editor when this card is the rename target. Unchanged from the
    /// stub this face replaces: the four exits and their store calls are 04-interactions.md ▸
    /// Grammar's, stated once in `InlineTitleField`.
    @ViewBuilder
    private var titleOrEditor: some View {
        if isRenaming {
            InlineTitleField(
                text: draft,
                prompt: "Card title",
                onCommit: { store.commitRename() },
                onAbandon: { store.transient.discardRename() },
                // **Click-away commits** — a rename's rule, and the deliberate opposite of the
                // placeholder's (04-interactions.md ▸ Grammar: "focus loss = commit, matching
                // the card window's title field").
                onFocusLoss: { store.commitRename() },
                onCommitAndOpen: {
                    let id = card.id
                    store.commitRename()
                    openCard(id)
                }
            )
            .font(.body)
        } else {
            Text(card.title.value ?? "Untitled")
                .font(.body)
                .foregroundStyle(card.title.value == nil ? .secondary : .primary)
                .lineLimit(4)
        }
    }

    /// `iconColor`'s tint, or the standard secondary one. Deliberately not `AnyShapeStyle(.primary)`
    /// on the fallback path: an uncoloured card icon is chrome, and chrome is secondary — the tint
    /// exists to make a *hand-coloured* icon stand out from its neighbours.
    private var iconTint: AnyShapeStyle {
        if let color = Palette.color(for: card.iconColor) {
            AnyShapeStyle(color)
        } else {
            AnyShapeStyle(.secondary)
        }
    }

    /// The one face chip in scope — shown only when the card actually has files, and quiet enough
    /// that the title still dominates (03-board-ui.md § Card face). The count goes to the
    /// accessibility label rather than onto the face: it is useful to know, not to look at.
    @ViewBuilder
    private var attachmentsIndicator: some View {
        if !card.attachments.isEmpty {
            Image(systemName: "paperclip")
                .font(.caption)
                .foregroundStyle(.secondary)
                .accessibilityLabel("\(card.attachments.count) attachments")
        }
    }

    /// K1 · left edge stripe, painted with the resolved `background` — "a card's [colour paints] a
    /// stripe along its left edge; the surfaces themselves keep the standard chrome, so coloured
    /// title text never sits on a coloured fill" (03-board-ui.md § Styling ▸ Capabilities).
    ///
    /// A value that resolves to nothing — a typo'd palette name, a malformed hex, a sequence where
    /// a scalar belongs — draws **no stripe**, and the value stays on disk exactly as written.
    /// A `Shape` rather than a sized rectangle so it takes the plate's full height whatever the
    /// content does, m5's carousel expansion included.
    @ViewBuilder
    private var accentStripe: some View {
        if let color = Palette.color(for: card.background) {
            UnevenRoundedRectangle(topLeadingRadius: cornerRadius, bottomLeadingRadius: cornerRadius)
                .fill(color)
                .frame(width: stripeWidth)
                // Decoration only: the whole plate is one click target for selection.
                .allowsHitTesting(false)
        }
    }

    // MARK: - Selection and rename plumbing

    private var isSelected: Bool {
        store.selection.liveness == .live && store.selection.ids.contains(card.id)
    }

    private var isRenaming: Bool {
        store.transient.renameEditor?.targetID == card.id
    }

    private var draft: Binding<String> {
        Binding(
            get: { store.transient.renameEditor?.draftTitle ?? "" },
            set: { store.transient.updateRenameDraft($0) }
        )
    }
}

// MARK: - The new-card placeholder

/// The card being created, drawn as a pseudo-card in the masonry flow at standard card width —
/// 02-architecture.md § Layering's one named exception to the one-way flow, finally rendered.
///
/// Two faces, one per phase:
///
/// - **`.editing`** — a focused text field. Return commits, Escape abandons, and **click-away
///   discards**: the placeholder's rule, "the deliberate exception because nothing exists on disk
///   yet" (04-interactions.md ▸ Grammar).
/// - **`.awaitingArrival`** — the committed title as plain text, deliberately *not* an editor. The
///   Writer's create has run and the overlay is only covering the gap until the watcher round-trips
///   the real card; leaving a live field there would invite edits that have nowhere to go, and its
///   focus loss would fire the discard rule against a card that is already on its way.
private struct NewCardStubView: View {

    let store: BoardStore
    let openCard: (ItemID) -> Void

    var body: some View {
        Group {
            if isEditing {
                InlineTitleField(
                    text: draft,
                    prompt: "Card title",
                    onCommit: { commit() },
                    onAbandon: { store.transient.discardPlaceholder() },
                    onFocusLoss: { store.transient.discardPlaceholder() },
                    onCommitAndOpen: {
                        // The one board command that stays enabled mid-edit: commit, then open
                        // (04 ▸ Grammar's carve-out). A commit that discarded — empty title, a
                        // vanished lane, a failed create — hands back no id and opens nothing.
                        if let id = commit() { openCard(id) }
                    }
                )
                .font(.body)
            } else {
                Text(store.transient.newCardPlaceholder?.draftTitle ?? "")
                    .font(.body)
                    .foregroundStyle(.secondary)
                    .lineLimit(4)
            }
        }
        .frame(maxWidth: .infinity, alignment: .leading)
        .padding(10)
        .background(RoundedRectangle(cornerRadius: 8).fill(.background.secondary))
        .overlay(
            RoundedRectangle(cornerRadius: 8)
                .strokeBorder(Color.accentColor.opacity(0.6), lineWidth: 1.5)
        )
    }

    private var isEditing: Bool {
        store.transient.newCardPlaceholder?.phase == .editing
    }

    /// Commits, then **re-selects the lane** — "Return commits and re-selects the lane (next Return
    /// = next card)" (04-interactions.md ▸ Grammar). The lane rather than the new card is what makes
    /// a run of Return-type-Return file a stack of cards without the user's hands leaving the
    /// keyboard.
    ///
    /// The lane is read before the commit, because every discard path clears the overlay that holds
    /// it — and re-checked after, because one of those paths is *the lane vanished*, and selecting
    /// something that renders nowhere would break the homogeneous-by-liveness invariant until the
    /// next reload swept it away.
    @discardableResult
    private func commit() -> ItemID? {
        let lane = store.transient.newCardPlaceholder?.laneID
        let id = store.commitPlaceholder()
        if let lane, store.snapshot.lanes.contains(where: { $0.id == lane && !$0.isDeleted }) {
            store.select([lane], liveness: .live)
        }
        return id
    }

    private var draft: Binding<String> {
        Binding(
            get: { store.transient.newCardPlaceholder?.draftTitle ?? "" },
            set: { store.transient.updateDraft($0) }
        )
    }
}

// MARK: - The inline title field

/// The one text field all three inline editors wear — the new-card placeholder, a card rename, and
/// a lane rename — so the grammar around it is written once (04-interactions.md ▸ Grammar).
///
/// The four exits, and who differs on them:
///
/// | Exit | Placeholder | Rename |
/// |---|---|---|
/// | Return | commits | commits |
/// | Escape | discards | abandons |
/// | Click-away | **discards** | **commits** |
/// | ⌘↩ | commits + opens | commits + opens |
///
/// Only the click-away row differs, which is why it is a caller-supplied closure rather than a
/// branch in here: this view knows *that* focus left, never what that should mean.
///
/// **Every handler must be idempotent**, because the exits overlap by construction: Return commits
/// and then the field disappears, which also fires the focus-loss handler an instant later. The
/// store's `commitRename`/`commitPlaceholder` and the transient state's `discard…` all no-op against
/// an editor that is already closed, so the overlap costs nothing.
private struct InlineTitleField: View {

    @Binding var text: String
    let prompt: String
    let onCommit: () -> Void
    let onAbandon: () -> Void
    let onFocusLoss: () -> Void
    let onCommitAndOpen: () -> Void

    @FocusState private var isFocused: Bool

    var body: some View {
        TextField(prompt, text: $text)
            .textFieldStyle(.plain)
            .lineLimit(1)
            .focused($isFocused)
            // The editor is born focused: every entry point to it is a deliberate "edit this now"
            // (Return, ⌘N, the header button, a double click, Board ▸ Rename), and one that landed
            // unfocused would need a second click to do anything.
            .onAppear { isFocused = true }
            .onSubmit(onCommit)
            // ⌘↩ before the field sees the Return: the one board command enabled mid-edit
            // (04 ▸ Grammar). Anything without the modifier is passed straight through, so plain
            // Return still reaches `onSubmit`.
            .onKeyPress(keys: [.return], phases: .down) { press in
                guard press.modifiers.contains(.command) else { return .ignored }
                onCommitAndOpen()
                return .handled
            }
            // Escape reaches a focused text field as AppKit's cancel operation on some paths and as
            // a plain key press on others; both are wired to the same idempotent abandon rather than
            // guessing which one this control will get.
            .onKeyPress(.escape) {
                onAbandon()
                return .handled
            }
            .onExitCommand(perform: onAbandon)
            .onChange(of: isFocused) { _, focused in
                guard !focused else { return }
                onFocusLoss()
            }
    }
}