import AppKit
import SwiftUI

/// The lane strip — the board itself (03-board-ui.md § Layout — full visibility).
///
/// **Every lane is always on screen.** There is no horizontal scroll and no minimum lane width: the
/// window's width divides across the lanes' width units, a lane of n units taking n whole units of
/// that division, and resizing the window is the width control. A board with more units than the
/// window comfortably fits compresses every lane; that degenerate case is accepted, not floored
/// (the remedy is the user's — fewer units or a bigger window).
///
/// Two mechanisms change a lane's width and they are deliberately opposites (03-board-ui.md § Lane):
/// the **right-edge drag** grows or shrinks the *window* one standard width per snap so the other
/// lanes keep their exact pixels (`LaneResizeSession`), while the **stepper** — and its ⌥⌘→/⌥⌘←
/// keyboard face — re-divides the existing window width across the new unit total, compressing the
/// siblings and never touching the window (`BoardStore.setLaneWidth`).
///
/// ### The three interactions it hosts
///
/// - **Lane resize** — the right-edge grab strip (above). Deliberately *not* a drag session
///   (DRAG-REORDER.md § Adjacent interaction).
/// - **Drag & drop** — cards, lanes and trash rows travel as **system drag sessions**, which is what
///   crosses window boundaries, draws the copy badge and gives the full-size replica
///   (`DragSession`, `BoardDrops.swift`, DRAG-REORDER.md). The strip owns the drop geometry
///   registry and the strip-level drop target; the lanes own theirs.
/// - **The rubber band** — a drag from any empty surface sweeps a selection (`MarqueeSession`,
///   `MarqueeMath`); the strip owns the session and the target registry, and hands both down.
/// - **The board's fixed grammar keys** (11-command-nexus.md ▸ Fixed grammar keys) — the four
///   arrows and their ⇧/⌥ modes, Return's create/rename dispatch, ⌫'s tombstone, Escape's step
///   outward, and Select All. The chorded commands are the menu's (`BoardCommands`); everything
///   here is a plain key or a grammar modifier, which is exactly the split 04-interactions.md ▸
///   Configurable bindings draws between what remaps and what does not.
/// - **The standard Edit items the board answers as a responder** — Select All, and Cut/Copy/Paste
///   beside it (`ClipboardCommands.swift`). They are not menu items of ours: the Edit menu already
///   carries those titles, and titles are the remapping mechanism's key, so a second row sharing one
///   is ruled out (04-interactions.md ▸ Configurable bindings).
///
/// - **The trash quasi-lane** — trailing, one fixed unit, joining and leaving the width division as
///   View ▸ Show Trash toggles it (`TrashLaneView`, 03-board-ui.md § Trash).
///
/// ### The live search filter
///
/// The filter itself is a pure predicate (`SearchFilter`) and the field is the toolbar's
/// (`BoardSearchField`, installed by `BoardWindowHost`); what belongs to this file is the two places
/// the board *reads* it — the arrow grammar's order lists and jump containers, so navigation walks
/// the filtered board, and Escape's middle step. Everything else follows from `LaneView`'s and
/// `TrashLaneView`'s own narrowing, because the drop zones, the marquee and the file-drop targets
/// all read what those two rendered.
///
/// ### What is deliberately not here yet
///
/// External Finder file drops join the very drop delegates this file already attaches (see
/// `BoardDrops.swift`).
struct BoardView: View {

    let store: BoardStore

    /// How the resize session reaches the host window it grows and shrinks. Injected by
    /// `BoardWindowHost`, which owns the window controller; a closure because the window attaches
    /// after the first body evaluation.
    let window: @MainActor () -> NSWindow?

    /// The window's purge-alert host — see `TrashConfirmations` for why a menu item's confirmation
    /// has to be presented from here.
    let confirmations: TrashConfirmations

    /// Opens a card's window — ⌘↩'s second half (04-interactions.md ▸ Grammar). A closure from
    /// `BoardWindowHost` rather than an `openWindow` call here, because building a `CardWindowRef`
    /// needs the board's own window ref, which is the host's identity and not the board's.
    let openCard: (ItemID) -> Void

    /// The toolbar search field's handle (`BoardSearchPresentation`), threaded down so the strip can
    /// fill in `focusBoard` — Escape's "in an empty field it returns focus to the board" needs the
    /// strip's own `@FocusState`, which nothing outside this view can reach.
    let search: BoardSearchPresentation

    /// The app-wide quick-style recents, for the board-anchored style editor (03-board-ui.md §
    /// Styling ▸ Controls).
    @Environment(AppModel.self) private var appModel

    /// One resize at a time, per window. `@State` so it lives exactly as long as this board window's
    /// view does, which is the interaction's whole lifetime.
    @State private var resize = LaneResizeSession()

    /// Where this window's lanes draw their card grids and how tall their cards are — the measured
    /// half of a card drop's geometry, plus the strip's own frame (`LaneDropRegistry`). `@State` for
    /// the resize session's reason: one per window, living exactly as long as the window.
    @State private var laneDrops = LaneDropRegistry()

    /// One rubber band at a time, per window (`MarqueeSession`).
    @State private var marquee = MarqueeSession()

    /// Where every sweepable item is drawn, in strip coordinates. Owned here because the band is —
    /// the cards and trash rows only *register* into it (`MarqueeTargetRegistry`).
    @State private var marqueeTargets = MarqueeTargetRegistry()

    /// The name of the strip's coordinate space, which is what a drop out of the trash is resolved
    /// in: `LaneLayoutMath.laneIndex` reads an x measured from the strip's leading edge, outer margin
    /// included, and no global or lane-local space is that.
    static let stripSpace = "board-strip"

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

    /// Whether the strip holds keyboard focus, which is what makes the grammar keys arrive. Restored
    /// deliberately whenever an inline editor closes: the field that had focus is gone, and Return
    /// must go back to meaning create/rename rather than nothing at all.
    @FocusState private var isBoardFocused: Bool

    /// The inter-lane gap, and the strip's outer margin — one number, because the standard-width
    /// formula counts `units + 1` of them (03-board-ui.md § Layout; `LaneLayoutMath.standardWidth`).
    private let spacing: CGFloat = 12

    var body: some View {
        GeometryReader { _ in
            // The strip's slots: the lanes the strip should *show*, with the drag's N contiguous
            // shadows opened at the proposal. Recomputed on every render, so a foreign reload
            // mid-drag simply moves the zones (rule 1 of 04-interactions.md ▸ Drag and drop's
            // re-grounding trio) — nothing about a drag is cached across a snapshot.
            let slots = stripSlots
            ZStack(alignment: .topLeading) {
                backdrop
                laneStrip(slots)
            }
            .padding(spacing)
            .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)
            // The band, drawn **outside the padding** so its offset is a strip coordinate directly —
            // and outside every animated modifier above, because 03-board-ui.md § Motion puts the
            // marquee in the animation-free-by-construction list ("1:1 cursor following — animating
            // input echo would be lag").
            .overlay(alignment: .topLeading) { marqueeBand }
            // The space the marquee is resolved in — see `BoardView.stripSpace`. It goes on the
            // padded container so x = 0 is the strip's leading edge with the outer margin included,
            // which is the origin `LaneLayoutMath`'s arithmetic assumes. Every marquee coordinate —
            // the band's own drag samples and each registered item frame — is measured here too, so
            // nothing ever converts between spaces.
            .coordinateSpace(.named(Self.stripSpace))
            // The same rectangle in the window's *global* space, which is where the physical cursor
            // lands once converted (`BoardDropContext.globalCursor`). Written into the registry
            // rather than into `@State` so the drop delegates read it live at event time rather than
            // as of the last body evaluation.
            .onGeometryChange(for: CGRect.self) { $0.frame(in: .global) } action: { laneDrops.stripFrame = $0 }
            // **The strip's drop target** — the backdrop, the gaps, the outer margin, and the trash
            // column's footprint, which is never a landing spot of its own (04-interactions.md ▸ The
            // trash) and so simply falls through to here. It accepts *every* session type — ours and
            // external Finder file drags alike — and routes internally, because single-target
            // dispatch has no fall-through (DRAG-REORDER.md).
            .onDrop(of: boardDropTypes, delegate: StripDropDelegate(context: dropContext))
        }
        .background(boardBackground)
        // The window-level fallback, *behind* the specific targets: a release over any in-window
        // region they don't cover commits the current proposal instead of leaking the session into a
        // cancel-snapback — the drop lands where the shadows show, which is what the shadows promise.
        .background {
            Color.clear
                .onDrop(of: boardDropTypes, delegate: BoardFallbackDropDelegate(context: dropContext))
        }
        // **The committed-overlay hold's hand-off** (DRAG-REORDER.md § The committed-overlay hold):
        // the overlay stands in for an arrangement that is on disk but not yet in the snapshot, and
        // discards itself the moment a snapshot lands — because holding a moment longer would draw
        // the arrangement twice.
        .onChange(of: store.snapshotGeneration) { _, generation in
            appModel.dragSession.handOff(root: store.rootURL, generation: generation)
        }
        .trashPurgeAlert(store: store, confirmations: confirmations)
        // The board's own anchor for the Style… popover — the surface a board-targeted session hangs
        // off, since the board has no item to attach to (`styleEditorPresentation`'s `nil` anchor).
        .popover(isPresented: styleEditorPresentation(store, anchor: nil), arrowEdge: .top) {
            StyleEditorPopover(store: store, recents: appModel.styleRecents)
        }
        // The board is a focus target so the grammar keys reach it at all. The focus *ring* is off:
        // the strip is the window's content, not a control, and a rectangle around the whole board
        // would read as an error state.
        .focusable()
        .focusEffectDisabled()
        .focused($isBoardFocused)
        .onAppear {
            isBoardFocused = true
            // **Escape's second step, wired from the side that can perform it** (04 § Search): the
            // field can resign first responder on its own, but only the strip can *take* the
            // keyboard, and a window with a resigned field and an unfocused board would swallow
            // every grammar key. `@FocusState`'s setter is nonmutating, so the closure writes the
            // same storage this view reads.
            search.focusBoard = { isBoardFocused = true }
        }
        .onChange(of: store.isEditingInline) { _, editing in
            // An editor took focus and has now given it back. Without this the strip stays unfocused
            // after every rename and Return silently stops working.
            if !editing { isBoardFocused = true }
        }
        .onKeyPress(keys: [.return], phases: .down) { handleReturn($0) }
        .onKeyPress(.escape) { handleEscape() }
        .onKeyPress(keys: [.delete], phases: .down) { handleDelete($0) }
        // **The arrows** (04-interactions.md ▸ Grammar), on `.down` *and* `.repeat`: holding an
        // arrow must walk the board, and a handler registered for `.down` alone sees the first
        // press only.
        .onKeyPress(
            keys: [.upArrow, .downArrow, .leftArrow, .rightArrow],
            phases: [.down, .repeat]
        ) { handleArrow($0) }
        // **Select All** (04-interactions.md ▸ The map). Edit ▸ Select All is the standard menu
        // item and it dispatches `selectAll:` down the responder chain, so the board answers it as a
        // responder rather than growing a second menu item with the same title — which titles-are-API
        // forbids outright (04 ▸ Configurable bindings). A focused text field consumes it first, so
        // ⌘A inside an inline editor stays text selection with no guard needed here.
        .onCommand(#selector(NSText.selectAll(_:))) { store.selectAll() }
        // **Cut / Copy / Paste** (04-interactions.md ▸ Clipboard), through the same responder door
        // Select All above uses and for the same titles-are-API reason. Each handler is attached only
        // while its command applies, which is what makes AppKit's automatic enablement mirror the
        // validation exactly — see `boardClipboardCommands`.
        .boardClipboardCommands(store: store, clipboard: appModel.clipboard)
        // **Belt** for a session SwiftUI does report ending: immediate cleanup, gated on the physical
        // mouse button being up. A finished session's phase events can arrive *after* the user has
        // started the next drag, and an ungated handler would wipe the new session's state — no
        // shadow, drop dead. `DragSession`'s watchdog is the braces (see `armWatchdog`), and it is
        // what makes refusing here free.
        .onDragSessionUpdated { session in
            switch session.phase {
            case .ended, .dataTransferCompleted:
                MainActor.assumeIsolated { appModel.dragSession.endIfButtonReleased() }
            default:
                break
            }
        }
    }

    // MARK: - The strip's layers

    /// The empty surface behind the lanes: a plain click clears the selection, a drag rubber-bands.
    ///
    /// `Color.clear` with a `contentShape` rather than a real fill — the board's *painted*
    /// background is `boardBackground`, outside the geometry reader, and this layer exists only to
    /// be hit. Modified clicks are deliberately no-ops: ⌘ and ⇧ on the backdrop name no target, and
    /// Finder's own desktop behaves the same way.
    private var backdrop: some View {
        Color.clear
            .contentShape(Rectangle())
            .onTapGesture {
                guard ClickModifier.current == .plain else { return }
                store.clearSelection()
            }
            .simultaneousGesture(marqueeControl.gesture(side: .live))
    }

    /// The lanes and the drag's shadows, plus the trash column when it is shown.
    @ViewBuilder
    private func laneStrip(_ slots: [StripSlot]) -> some View {
        let standard = standardWidth
        HStack(alignment: .top, spacing: spacing) {
            ForEach(slots) { slot in
                switch slot {
                case let .lane(lane):
                    laneSlot(lane, standard: standard)
                        // "Appear/disappear is scale + fade … lanes ~0.9" (03-board-ui.md § Motion).
                        // A create, a delete and a Put Back all reach the strip as a lane arriving in
                        // or leaving this `ForEach`; whether that *performs* is decided upstream, at
                        // the reload that carried it (`Motion.reloadAnimates`) — a transition with no
                        // animated transaction around it is simply an appearance.
                        .transition(Motion.laneTransition(reduced: reduceMotion))
                case let .shadow(_, units):
                    // One of the drag's N contiguous shadows, at the exact width the arriving lane
                    // will occupy — its units measured against *this* strip's standard, which is
                    // what makes the drop land precisely where the shadow shows.
                    DragShadow()
                        .frame(width: LaneLayoutMath.slotWidth(units: units, standard: standard, gap: spacing))
                        .frame(maxHeight: .infinity)
                }
            }
            if isTrashVisible {
                // Trailing, always — the quasi-lane has no position of its own to lose, which is
                // also why it never appears in the drop proposal's inputs (those are built from
                // `liveLanes`) and why the terminal slot clamps in front of it.
                TrashLaneView(
                    store: store,
                    confirmations: confirmations,
                    drops: dropContext,
                    marquee: marqueeControl
                )
                .frame(width: LaneLayoutMath.slotWidth(units: 1, standard: standard, gap: spacing))
                .frame(maxHeight: .infinity, alignment: .top)
                // It arrives and leaves like a lane, because that is what it looks like — the
                // column scales and fades while every real lane compresses to make room for its
                // unit (03-board-ui.md § Motion, § Trash's re-divide). The transaction is the
                // menu toggle's (`ShowTrashCommand`).
                .transition(Motion.laneTransition(reduced: reduceMotion))
            }
        }
        // The drag's reflow-to-make-room, keyed on the **drop proposal** and nothing else
        // (03-board-ui.md § Motion: transactions are keyed narrowly, "on the drag's drop
        // proposal … never on broad state"). Narrowed further to the *strip's* proposal: a card
        // session moving its shadow inside a lane must not re-time the whole strip. The replica's
        // own tracking is the system drag image's and touches nothing here, which is the same
        // bullet's other half — animating input echo would be lag.
        //
        // It stays on the `HStack` rather than moving out to the `ZStack`, so the marquee band
        // drawn beside it is never inside an animated transaction (03 § Motion again).
        .animation(Motion.dragReflow(reduced: reduceMotion), value: stripProposal)
        // **The search filter's reflow**, keyed on **the query** and nothing else — 03-board-ui.md
        // § Motion names it in the narrow-keys list ("on the search query (filter reflow)") — and in
        // the *content* voice rather than the structural one: "search filtering and undo/redo
        // restore, deliberately paired so a restore reads like the search filter — leavers and
        // arrivers run their transition, survivors reflow under one gentle spring". The leavers and
        // arrivers are the card and row transitions already attached inside the lanes and the trash
        // column; this is the survivors' spring around them.
        //
        // **Every way the query changes rides it**, which is the reason the key is the query rather
        // than the transaction being wrapped at each mutation: typing, the field's Escape, the
        // board's Escape, and creation's clear (`TransientBoardState.beginPlaceholder`) all land
        // here without any of them knowing about motion.
        .animation(Motion.contentReflow(reduced: reduceMotion), value: store.searchQuery)
    }

    /// The rubber band itself: a translucent accent fill with a hairline border, in strip
    /// coordinates and **never animated** (03-board-ui.md § Motion — the marquee "tracks the cursor
    /// 1:1", and an eased band visibly lags the mouse).
    ///
    /// Hit-testing off, because the band is feedback: the drag that draws it is already recognised,
    /// and a rectangle that swallowed clicks would eat the release.
    @ViewBuilder
    private var marqueeBand: some View {
        if let rect = marquee.rect {
            Rectangle()
                .fill(Color.accentColor.opacity(0.12))
                .frame(width: rect.width, height: rect.height)
                .overlay(Rectangle().strokeBorder(Color.accentColor.opacity(0.5), lineWidth: 1))
                .offset(x: rect.minX, y: rect.minY)
                .allowsHitTesting(false)
        }
    }

    /// What the strip lends its empty surfaces and its sweepable items — the band's session, the
    /// registry, and the store it selects into (`MarqueeControl`).
    private var marqueeControl: MarqueeControl {
        MarqueeControl(session: marquee, registry: marqueeTargets, store: store)
    }

    // MARK: - Styling

    /// The board's `background`, painting "the board window's content background (the surface behind
    /// and between lanes)" (03-board-ui.md § Styling ▸ Capabilities).
    ///
    /// Unlike the lane band and the card stripe this one is a **fill**, because at board level that
    /// is what the design asks for — and it is why the board is the level 10-accessibility.md binds
    /// its ≥ 4.5:1 rule to: text does sit on it. That runtime contrast computation (a hex background's
    /// text colour, recomputed against the composited backdrop on appearance change) is not this
    /// card's — what ships here is the palette path, whose twelve pairs are AA-verified at design
    /// time.
    ///
    /// A value that resolves to nothing paints nothing, so the window keeps the standard background:
    /// the same lenient degrade as the other two levels, and the bytes stay as written.
    @ViewBuilder
    private var boardBackground: some View {
        if let color = Palette.color(for: store.snapshot.background) {
            color
        }
    }

    // MARK: - Lanes

    /// One lane's strip slot, plus its trailing grab strip.
    ///
    /// Normally a plain `LaneView` sized to its unit count's slot width (a wide lane swallows the
    /// interior gaps it spans). While THIS lane is being resized the slot holds two layers, kept
    /// structurally stable so the `LaneView` never loses identity — its scroll position, its
    /// masonry cache — across the drag:
    ///
    ///   • a shadow at the SNAPPED slot width, full strip height, behind the lane — the resting
    ///     footprint the siblings and the window are already aligned to;
    ///   • the live `LaneView` in front at `liveWidth`, which tracks the cursor and so overflows
    ///     (drawing over the right neighbour, hence the slot's `zIndex(1)`) or underfills the shadow
    ///     between ticks.
    ///
    /// The outer frame is always the snapped slot width, so the `HStack` lays the other lanes out
    /// off the tidy snapped layout regardless of the live overflow.
    ///
    /// A lane being **dragged** is simply absent from the strip: it is lifted out of the resting
    /// layout at pickup and stays out until release, whatever the effective operation is
    /// (DRAG-REORDER.md § Resting-layout zones), while the system drag session carries its replica.
    @ViewBuilder
    private func laneSlot(_ lane: Lane, standard: CGFloat) -> some View {
        let resizing = resize.isResizing(lane.id)
        let units = resizing ? resize.units : LaneLayoutMath.displayUnits(of: lane)
        let slotWidth = LaneLayoutMath.slotWidth(units: units, standard: standard, gap: spacing)
        ZStack(alignment: .topLeading) {
            if resizing {
                DragShadow(dashed: false)
                    .frame(width: slotWidth)
                    .frame(maxHeight: .infinity)
            }
            // Interior columns follow the SNAPPED unit count while this lane is being resized — a
            // column count is integral, so it tracks k (which ticks and animates), not the live
            // continuous width and not the not-yet-committed `lane.width`. The live width still
            // narrows and widens the columns continuously, so the cards reflow under the cursor
            // between ticks (free via `MasonryLayout`).
            LaneView(
                store: store,
                lane: lane,
                columns: units,
                slotWidth: slotWidth,
                drops: dropContext,
                marquee: marqueeControl,
                openCard: openCard
            )
            .frame(width: resizing ? resize.liveWidth : slotWidth, alignment: .leading)
        }
        .frame(width: slotWidth, alignment: .topLeading)
        .zIndex(resizing ? 1 : 0)
        .overlay(alignment: .trailing) {
            LaneResizeHandle(
                store: store,
                session: resize,
                laneID: lane.id,
                committedUnits: LaneLayoutMath.displayUnits(of: lane),
                standard: standard,
                gap: spacing,
                window: window
            )
            // The read-only lock disables every mutating gesture, not just the menu items
            // (02-architecture.md § The lock's scope). It matters more here than elsewhere: a drag
            // resizes the *window* on the way, so a refused commit would leave the window grown
            // around a lane that snapped back — and the lock's row is already saying why nothing
            // can be written. The focused-editor rule closes it too, like every board command, and
            // so does a drag session in flight — the edge drag "refuses to start while a card/lane
            // session is in flight" (DRAG-REORDER.md § Adjacent interaction): two gestures mutating
            // one strip layout is not a state this view has a meaning for.
            .disabled(store.isReadOnly || store.isEditingInline || appModel.dragSession.isActive)
        }
    }

    /// The lanes the strip lays out, in snapshot order. **Tombstoned lanes render nowhere here** —
    /// 03-board-ui.md § Trash collapses each into a single restorable entry in the trash quasi-lane
    /// (a later card), and a lane that is not on the board consumes none of the window's width.
    private var liveLanes: [Lane] {
        store.snapshot.lanes.filter { !$0.isDeleted }
    }

    // MARK: - Trash

    /// Whether the trash quasi-lane is on screen — transient, board-scoped, hidden on every open
    /// (03-board-ui.md § Trash ▸ Visibility). Read in two places (the unit total and the slot), so it
    /// gets a name rather than being spelled twice.
    private var isTrashVisible: Bool {
        store.transient.isTrashVisible
    }

    /// The trash's rows as the column is showing them — the shown trash "participates in the filter
    /// like any lane" (03-board-ui.md § Trash), and the arrows walk what is on screen
    /// (`TrashLaneView.entries` applies the identical predicate to the identical rows).
    ///
    /// Read by the three keyboard destinations that reach into the column — the arrow origin's
    /// order list, ⌥↑/⌥↓'s container, and ⌥→'s jump — so none of them can walk onto a row the
    /// filter took away.
    private var trashEntries: [TrashEntry] {
        let filter = store.searchFilter
        return TrashModel.entries(of: store.snapshot).filter { filter.matches($0) }
    }

    // MARK: - The drag

    /// What each of this window's drop targets — and its lanes' autoscroll drivers — is handed.
    ///
    /// Every geometric input is a **closure**, read at event time (`BoardDropContext`): a captured
    /// snapshot of the strip's frame or its standard width goes stale the moment the layout animates,
    /// and two delegates holding different snapshots would flap the proposal between them.
    private var dropContext: BoardDropContext {
        BoardDropContext(
            store: store,
            session: appModel.dragSession,
            registry: laneDrops,
            gap: spacing,
            window: window,
            stripFrame: { laneDrops.stripFrame },
            standard: { standardWidth }
        )
    }

    /// The strip's 1× lane width.
    ///
    /// During a resize session the standard is **frozen** at its drag-start value: the window is
    /// animating mid-resize, so deriving the standard from the live width would feed that animation
    /// back into every lane and pulse the whole strip. The window is sized on each tick so this frozen
    /// value equals what the formula yields once the session ends — the handoff is seamless (see
    /// `LaneResizeSession`).
    ///
    /// Otherwise it is the ordinary division, over three contributions:
    ///
    /// - the **live lanes**' units. A lane in flight is still a lane on the board, so a within-board
    ///   drag does *not* re-divide the strip: "the shadow occupies its units" (DRAG-REORDER.md § The
    ///   lane strip's resting layout is arithmetic). Recomputing at pickup and again at release is
    ///   exactly the motion-feeds-back-into-logic failure the model exists to avoid.
    /// - the **trash's** one fixed unit, *only while shown* — the whole of "Show/Hide Trash is a
    ///   re-divide trigger" (03-board-ui.md § Trash): the window is never touched, the existing width
    ///   simply divides across one more unit and every lane compresses.
    /// - a **cross-board lane arrival**'s units while its shadow hovers here, by the same rule read
    ///   from the destination's side: the shadow occupies its units, and the strip has to make room
    ///   for them or the shadow would be drawn at a width the lane will not have.
    private var standardWidth: CGFloat {
        if resize.isActive { return resize.standard }
        var units = LaneLayoutMath.totalUnits(of: liveLanes, trashUnits: isTrashVisible ? 1 : 0)
        units += arrivingLaneUnits
        return LaneLayoutMath.standardWidth(
            stripWidth: laneDrops.stripFrame.width,
            totalUnits: units,
            gap: spacing
        )
    }

    /// The units a cross-board lane run would add to this strip while its shadow is proposed here;
    /// zero for a within-board drag, whose lanes are already counted.
    private var arrivingLaneUnits: Int {
        let session = appModel.dragSession
        guard session.isDraggingLanes,
              session.stripProposal(onBoardRooted: store.rootURL) != nil,
              let source = session.sourceRoot,
              !DragLocality.isSameBoard(source, store.rootURL)
        else { return 0 }
        return session.laneUnits.reduce(0, +)
    }

    /// The strip's current lane-drop proposal — the reflow's narrow animation key, and where the
    /// shadow run opens.
    private var stripProposal: Int? {
        appModel.dragSession.stripProposal(onBoardRooted: store.rootURL)
    }

    /// One position in the strip: a lane, or one of the drag's N contiguous shadows.
    private enum StripSlot: Identifiable {
        case lane(Lane)
        case shadow(index: Int, units: Int)

        var id: String {
            switch self {
            case let .lane(lane): "lane:\(lane.id.rawValue)"
            // Constant per position, so a shadow run keeps its identity as the proposal slides and
            // the run animates as a move rather than blinking out and back in.
            case let .shadow(index, _): "shadow:\(index)"
            }
        }
    }

    /// What the strip lays out: the resting lanes — the dragged run lifted out, whatever the
    /// effective operation is — with the shadows opened at the proposal.
    private var stripSlots: [StripSlot] {
        let session = appModel.dragSession
        let hidden = session.hiddenMembers(onBoardRooted: store.rootURL)
        var slots = liveLanes.filter { !hidden.contains($0.id) }.map(StripSlot.lane)
        guard let index = stripProposal else { return slots }
        let shadows = session.laneUnits.enumerated().map { StripSlot.shadow(index: $0.offset, units: $0.element) }
        slots.insert(contentsOf: shadows, at: min(max(0, index), slots.count))
        return slots
    }

    // MARK: - Grammar keys

    /// **Return**, narrowly (04-interactions.md ▸ Grammar): a sole selected live card begins an
    /// inline rename, a sole selected live lane begins a new-card placeholder at its bottom, and
    /// everything else is ignored — a multi-card selection is explicitly inert, and a lane's rename
    /// path is Board ▸ Rename precisely because Return on a lane creates.
    ///
    /// Inert while an inline editor is open: "all grammar keys inert while a title editor is
    /// focused". The field consumes Return itself, so this guard is belt over braces — but the belt
    /// matters, because a stray Return reaching here mid-edit would open a *second* editor.
    private func handleReturn(_ press: KeyPress) -> KeyPress.Result {
        // **Plain Return only**, the delete handler's rule for its reason. ⌘↩ belongs to Board ▸
        // Open Card and AppKit routes it to the menu first — but only while that item is *enabled*,
        // and a disabled one lets the chord fall through to here. ⌥↩ and ⇧↩ are nobody's key
        // equivalent at all. Neither may open a rename or a placeholder.
        guard press.modifiers.intersection([.command, .option, .control, .shift]).isEmpty else {
            return .ignored
        }
        guard !store.isEditingInline, !store.isReadOnly else { return .ignored }
        let selection = store.selection
        guard selection.liveness == .live,
              selection.ids.count == 1,
              let id = selection.ids.first,
              let target = BoardStore.liveItem(id, in: store.snapshot)
        else { return .ignored }

        if target.cardID == nil {
            store.transient.beginPlaceholder(inLane: target.laneID)
        } else {
            store.transient.beginRename(of: id, currentTitle: target.title)
        }
        return .handled
    }

    /// **Plain ⌫ tombstones the live selection** — "a plain-key synonym of File ▸ Delete, kept
    /// grammar so no second 'Delete' title exists" (11-command-nexus.md; 04-interactions.md ▸ The
    /// map).
    ///
    /// Deliberately **live-only**: the nexus scopes this key to a live selection, and ⌫'s trash-side
    /// role belongs to the ⌘⌫ twins, not to the bare key. A tombstoned selection is therefore inert
    /// here — Put Back is a chord.
    ///
    /// Inert while an inline editor is open, like every grammar key: the field owns ⌫ as backspace,
    /// and a stray one reaching the board mid-edit would delete the item being renamed.
    private func handleDelete(_ press: KeyPress) -> KeyPress.Result {
        // **Plain ⌫, spelled out.** The modified chords belong to the menu — ⌘⌫ (Delete / Put Back),
        // ⌥⌘⌫ (Delete Immediately), ⇧⌘⌫ (Empty Trash…) — and AppKit routes a key equivalent to the
        // menu before the view sees it. But ⌥⌫ and ⌃⌫ are nobody's key equivalent, and a fall-through
        // that tombstoned the selection on a mistyped text-editing chord would be exactly the kind of
        // accident 04-interactions.md's fixed grammar is careful to avoid.
        guard press.modifiers.intersection([.command, .option, .control, .shift]).isEmpty else {
            return .ignored
        }
        guard !store.isEditingInline, !store.isReadOnly else { return .ignored }
        let selection = store.selection
        guard selection.liveness == .live, !selection.isEmpty else { return .ignored }
        store.deleteSelection()
        return .handled
    }

    /// **Escape steps outward one layer per press** (04 ▸ Grammar): abandon an open editor, else
    /// clear the search, else clear the selection.
    ///
    /// **The search takes Escape before its clear-selection meaning** (04 § Search, settled): "with
    /// *board* focus and an active search, one press clears the search and the full board returns —
    /// search takes Escape before its clear-selection meaning, which applies only when no search is
    /// active." So a board-focused Escape under a query returns the board and *keeps* the selection;
    /// a second press then deselects. One press, one layer, all the way out.
    ///
    /// This is the third step of a staircase whose first two are the field's own — a non-empty field
    /// clears its query and keeps the keyboard, an empty one hands the keyboard back here — and the
    /// two halves never both fire, because exactly one of the field and the strip holds focus (see
    /// `BoardSearchField`).
    ///
    /// The editors handle Escape themselves while they hold focus; that branch is the outer net for
    /// the case where focus has drifted off the field with an editor still open, and it abandons
    /// both kinds because at most one can be open at a time.
    private func handleEscape() -> KeyPress.Result {
        if store.isEditingInline {
            store.transient.discardPlaceholder()
            store.transient.discardRename()
            return .handled
        }
        if !store.searchQuery.isEmpty {
            store.clearSearch()
            return .handled
        }
        guard !store.selection.isEmpty else { return .ignored }
        store.clearSelection()
        return .handled
    }

    // MARK: - The arrows

    /// What modifier an arrow carried, reduced to the three meanings the grammar gives it — the
    /// keyboard's `ClickModifier`.
    private enum ArrowMode {
        /// Plain: spatial navigation, replacing the selection.
        case step
        /// ⇧: extend the range from the anchor.
        case extend
        /// ⌥: jump to an end (04-interactions.md ▸ Grammar's "⌥-arrows jump").
        case jump
    }

    /// **The arrow grammar's one door** (04-interactions.md ▸ Grammar; 11-command-nexus.md ▸ Fixed
    /// grammar keys).
    ///
    /// The handlers below are deliberately thin over pure functions — `NavigationMath` for the
    /// geometry, `SelectionGrammar` for the order lists and the ranges — so what is written here is
    /// dispatch and nothing else.
    ///
    /// **⌘- and ⌥⌘-arrows never mean anything here.** They are menu key equivalents (Move Left/Right,
    /// Move Up/Down, the lane width pair) and AppKit routes them to the menu before any view sees
    /// them — but only while the item is *enabled*, so a disabled Move Right does deliver ⌘→ here.
    /// Rejecting every combination but plain, ⇧ and ⌥ is what keeps a disabled command from silently
    /// becoming a navigation gesture, and a mistyped text chord from moving the selection.
    private func handleArrow(_ press: KeyPress) -> KeyPress.Result {
        // "All grammar keys inert while a title editor is focused" — and the field owns the arrows
        // as caret movement, so this guard is load-bearing rather than belt over braces.
        guard !store.isEditingInline else { return .ignored }
        guard let direction = Self.direction(of: press.key) else { return .ignored }

        // Only the four meaningful flags are read: an arrow event also carries `.function` and
        // `.numericPad` on macOS, and testing the whole set for emptiness would reject every press.
        let modifiers = press.modifiers.intersection([.command, .control, .option, .shift])
        let mode: ArrowMode
        if modifiers.isEmpty {
            mode = .step
        } else if modifiers == .shift {
            mode = .extend
        } else if modifiers == .option {
            mode = .jump
        } else {
            return .ignored
        }

        guard let origin = arrowOrigin() else { return seed(direction, mode) }
        return origin.isLaneDomain
            ? laneArrow(direction, mode, from: origin.head)
            : cardArrow(direction, mode, from: origin.head, on: origin.side)
    }

    private static func direction(of key: KeyEquivalent) -> NavigationMath.Direction? {
        switch key.character {
        case KeyEquivalent.upArrow.character: .up
        case KeyEquivalent.downArrow.character: .down
        case KeyEquivalent.leftArrow.character: .left
        case KeyEquivalent.rightArrow.character: .right
        default: nil
        }
    }

    /// Where the next arrow steps from, and on which of the board's two levels — `nil` when the
    /// selection names nothing to step from, which is the seed rule's cue.
    ///
    /// The head is `TransientBoardState.selectionHead` when it is still in the order list, and
    /// otherwise the selection's **last member in that list** — the same "last in flatten order"
    /// anchor the ⌘N target rule and paste already share. That fallback is what makes a marquee, a
    /// Select All and a foreign reload leave the arrows somewhere sensible without any of them
    /// having to name a cursor.
    ///
    /// The **trash's list is both kinds interleaved** (`TrashModel.entries`), because "arrows walk
    /// every trash entry in its sorted order — card and lane entries alike" (04 ▸ The trash). The
    /// per-kind lists are the *range*'s business, not the walk's.
    ///
    /// Both lists are the **filtered** board (04 § Search: "arrow nav … read[s] it"), so the
    /// fallback lands on the last *visible* member rather than on a card the query hid.
    private func arrowOrigin() -> (head: ItemID, side: Liveness, isLaneDomain: Bool)? {
        let selection = store.selection
        guard !selection.isEmpty else { return nil }

        let isLaneDomain: Bool
        let list: [ItemID]
        switch selection.liveness {
        case .live:
            guard let kind = SelectionGrammar.kind(of: selection, in: store.snapshot) else { return nil }
            isLaneDomain = kind == .lane
            list = SelectionGrammar.order(of: kind, on: .live, in: store.snapshot, filter: store.searchFilter)
        case .trashed:
            isLaneDomain = false
            list = trashEntries.map(\.id)
        }

        if let head = store.transient.selectionHead, list.contains(head) {
            return (head, selection.liveness, isLaneDomain)
        }
        guard let last = list.last(where: { selection.ids.contains($0) }) else { return nil }
        return (last, selection.liveness, isLaneDomain)
    }

    /// **An empty selection seeds at the first lane's first card** (04-interactions.md ▸ Grammar) —
    /// a deterministic origin, so an arrow from nothing always means the same thing.
    ///
    /// ⌥←/⌥→ are the exception, and the design states it: "the ⌥-jumps behave as specified
    /// regardless". Those two name an *absolute* destination and need no origin, so they run
    /// unchanged. ⌥↑/⌥↓ are relative to "the current lane", which an empty selection has none of, so
    /// they seed like a plain arrow — which is exactly what makes "two ⌥↑ presses from nothing reach
    /// the lane domain" true: the first seeds, the second escalates.
    private func seed(_ direction: NavigationMath.Direction, _ mode: ArrowMode) -> KeyPress.Result {
        if mode == .jump, direction == .left || direction == .right {
            return jumpToEndLane(direction)
        }
        guard let first = Self.firstCard(scanning: liveLanes, filter: store.searchFilter) else { return .handled }
        replaceSelection(with: first, on: .live)
        return .handled
    }

    // MARK: Card domain

    private func cardArrow(
        _ direction: NavigationMath.Direction,
        _ mode: ArrowMode,
        from head: ItemID,
        on side: Liveness
    ) -> KeyPress.Result {
        switch mode {
        case .step: step(direction, from: head)
        case .extend: extend(direction, from: head)
        case .jump:
            switch direction {
            case .left, .right: jumpToEndLane(direction)
            case .up, .down: jumpWithinContainer(direction, from: head, on: side)
            }
        }
    }

    /// **Nearest card in the direction, across interior grid columns and lanes** — and across the
    /// live/trash boundary too, since "plain arrows still walk across" (04 ▸ The trash).
    ///
    /// Every registered target is a candidate, which is also how the hidden trash stays invisible:
    /// a column that is not drawn registers nothing.
    private func step(_ direction: NavigationMath.Direction, from head: ItemID) -> KeyPress.Result {
        guard let origin = marqueeTargets.targets[head],
              let nextID = NavigationMath.nearest(
                  from: origin.frame,
                  direction: direction,
                  among: marqueeTargets.all
              ),
              let next = marqueeTargets.targets[nextID]
        else { return .handled }
        replaceSelection(with: next.id, on: next.side)
        return .handled
    }

    /// **⇧-arrow extends, and stops at both boundaries** (04 ▸ The trash, settled): "a ⇧-arrow whose
    /// next step would cross from live cards into the trash (or back), or from card entries onto a
    /// lane entry within it, is simply inert".
    ///
    /// The *step* that would cross is what goes inert — the crossing item is never stepped over in
    /// search of a legal one, because that would silently drop the held range for a longer reach
    /// than the user asked for. So the nearest neighbour is computed **unrestricted** and then
    /// tested: a different side or a different kind means this press does nothing at all.
    private func extend(_ direction: NavigationMath.Direction, from head: ItemID) -> KeyPress.Result {
        guard let origin = marqueeTargets.targets[head],
              let nextID = NavigationMath.nearest(
                  from: origin.frame,
                  direction: direction,
                  among: marqueeTargets.all
              ),
              let next = marqueeTargets.targets[nextID],
              next.side == origin.side,
              next.kind == origin.kind
        else { return .handled }

        // An extension with no anchor makes one of where it started — the keyboard's equivalent of
        // a ⇧-click after a marquee, which the grammar degrades to a plain click for the same
        // reason: a range needs an origin, and the only honest one is the cursor's own position.
        let anchor = store.transient.selectionAnchor ?? head
        guard let ids = SelectionGrammar.range(
            from: anchor,
            to: next.id,
            kind: next.kind,
            on: next.side,
            in: store.snapshot,
            // The span is the *filtered* board's, so a range under a search collects exactly the
            // rows between the two endpoints that are on screen (04 § Search: "ranges … read it").
            filter: store.searchFilter
        ) else { return .handled }
        store.select(ids, liveness: next.side, anchor: anchor, head: next.id)
        return .handled
    }

    /// **⌥↑/⌥↓ jump to the current container's first/last card** — the lane's, or the trash
    /// quasi-lane's when that is where the cursor is.
    ///
    /// **⌥↑ escalates into the lane domain** (04 ▸ Grammar, settled — "the keyboard's one entry to
    /// lane selection"): with the lane's first card already the sole selection, the next ⌥↑ selects
    /// the *lane* itself. The trash deliberately never escalates: it "is never selectable as a lane",
    /// so a second ⌥↑ there is simply inert.
    private func jumpWithinContainer(
        _ direction: NavigationMath.Direction,
        from head: ItemID,
        on side: Liveness
    ) -> KeyPress.Result {
        let container: [ItemID]
        var lane: ItemID?
        switch side {
        case .trashed:
            container = trashEntries.map(\.id)
        case .live:
            guard let home = store.snapshot.lanes.first(where: { lane in
                !lane.isDeleted && lane.cards.contains { $0.id == head && !$0.isDeleted }
            }) else { return .handled }
            lane = home.id
            // The container is what the lane is *showing*: a jump to "the lane's first card" under
            // a search means its first surviving card, not one the filter animated out.
            let filter = store.searchFilter
            container = home.cards.filter { !$0.isDeleted && filter.matches($0) }.map(\.id)
        }

        guard let target = direction == .up ? container.first : container.last else { return .handled }
        if direction == .up, target == head, let lane, store.selection.ids == [head] {
            replaceSelection(with: lane, on: .live)
            return .handled
        }
        replaceSelection(with: target, on: side)
        return .handled
    }

    /// **⌥←/⌥→ to the first/last lane** (04 ▸ Grammar) — landing, in the card domain, on that lane's
    /// first card, since ⌥↑ is the one keyboard entry to lane selection.
    ///
    /// **⌥→ reaches the shown trash** first (04 ▸ The trash: "the shown trash is the last container
    /// for card navigation, and ⌥→ jumps to it"); an empty or hidden column is not a destination, so
    /// the jump falls through to the last lane. Empty lanes are scanned past in both directions —
    /// a jump that landed nowhere because the end lane happens to be empty would be a dead key.
    private func jumpToEndLane(_ direction: NavigationMath.Direction) -> KeyPress.Result {
        if direction == .right, isTrashVisible, let first = trashEntries.first {
            replaceSelection(with: first.id, on: .trashed)
            return .handled
        }
        let lanes = liveLanes
        let filter = store.searchFilter
        // A lane the search emptied is scanned past exactly as an empty one is — the jump lands on
        // the first lane that is *showing* a card, which is what the user can see.
        let target = direction == .right
            ? Self.firstCard(scanning: lanes.reversed(), filter: filter)
            : Self.firstCard(scanning: lanes, filter: filter)
        guard let target else { return .handled }
        replaceSelection(with: target, on: .live)
        return .handled
    }

    // MARK: Lane domain

    /// The arrows with a **lane** selected (04-interactions.md ▸ Grammar, ▸ The map).
    ///
    /// - ←/→ move the lane selection one lane, inert at the ends — **and the trash is never reached**
    ///   ("with a lane selected, ←/→ and ⌥→ stop at the last real lane"), which falls out for free
    ///   from walking the live lane order and nothing else.
    /// - ⇧←/⇧→ extend that selection from the anchor, the same range a ⇧-click would give.
    /// - ↓ descends back into the lane's cards at the first card, ⌥↓ at the last; an empty lane has
    ///   nothing to descend into.
    /// - ↑ and ⌥↑ are inert: the lane domain is the top of the hierarchy.
    /// - ⌥←/⌥→ jump to the first/last lane, staying in the lane domain.
    private func laneArrow(
        _ direction: NavigationMath.Direction,
        _ mode: ArrowMode,
        from head: ItemID
    ) -> KeyPress.Result {
        let lanes = SelectionGrammar.liveLanes(in: store.snapshot)
        guard let index = lanes.firstIndex(of: head) else { return .handled }

        switch (direction, mode) {
        case (.left, .step), (.right, .step), (.left, .extend), (.right, .extend):
            let next = index + (direction == .left ? -1 : 1)
            guard lanes.indices.contains(next) else { return .handled }
            if mode == .step {
                replaceSelection(with: lanes[next], on: .live)
            } else {
                let anchor = store.transient.selectionAnchor ?? head
                guard let ids = SelectionGrammar.range(
                    from: anchor,
                    to: lanes[next],
                    kind: .lane,
                    on: .live,
                    in: store.snapshot
                ) else { return .handled }
                store.select(ids, liveness: .live, anchor: anchor, head: lanes[next])
            }

        case (.left, .jump), (.right, .jump):
            guard let target = direction == .left ? lanes.first : lanes.last else { return .handled }
            replaceSelection(with: target, on: .live)

        case (.down, .step), (.down, .jump):
            guard let lane = store.snapshot.lanes.first(where: { $0.id == head && !$0.isDeleted }) else {
                return .handled
            }
            let cards = lane.cards.filter { !$0.isDeleted }
            guard let target = mode == .jump ? cards.last : cards.first else { return .handled }
            replaceSelection(with: target.id, on: .live)

        case (.up, _), (.down, .extend):
            // Nothing above the lane domain, and no vertical range within it.
            break
        }
        return .handled
    }

    // MARK: Shared

    /// A jump's and a plain step's shared landing: one item, both cursors on it.
    private func replaceSelection(with id: ItemID, on side: Liveness) {
        store.select([id], liveness: side, anchor: id, head: id)
    }

    /// The first rendered card of the first lane that has one — the scan every "first/last lane"
    /// destination shares, run over the lane order forwards or reversed.
    ///
    /// "Rendered" includes the search filter, so a lane whose cards the query all hid is scanned
    /// past like an empty one — `liveCards(in:filter:)`'s membership, one lane at a time.
    private static func firstCard(
        scanning lanes: some Sequence<Lane>,
        filter: SearchFilter = .inactive
    ) -> ItemID? {
        for lane in lanes {
            if let card = lane.cards.first(where: { !$0.isDeleted && filter.matches($0) }) { return card.id }
        }
        return nil
    }
}