lanework/Kanban/UI/Board/BoardDrops.swift

import AppKit
import SwiftUI
import UniformTypeIdentifiers

// MARK: - What each lane draws, as the drag reads it

/// Where each lane's card grid and title bar are drawn and how tall its cards are — the measured
/// half of the card masonry's drop geometry, one registry per board window.
///
/// **Deliberately not `@Observable`.** Nothing renders off it: it exists so a drop delegate and the
/// autoscroll driver can ask, at *event* time, where the grid is and what the resting row extents
/// are. Observing it would invalidate the strip on every layout pass, which is the animation
/// feedback loop this whole model exists to avoid.
///
/// ### What is measured here, and why that is animation-proof
///
/// 03-board-ui.md § Motion forbids reading *mid-flight* measurements. Two things are read here and
/// neither is one:
///
/// - **The grid's frame.** A lane's card area does not move while a card session is in flight: the
///   masonry reflows *inside* it, and the strip only reflows for a lane session, which reads none of
///   this.
/// - **Each card's height.** A card's height is content-driven — the column width is fixed by the
///   lane's unit count — so it does not animate under the reflow; only positions do. The positions
///   are never measured: they are replayed analytically from these heights through
///   `MasonryPlacement.frames(heights:)`, which is the very function `MasonryLayout` places with
///   (DRAG-REORDER.md § The card masonry).
/// - **Each lane's header frame.** The title bar sits outside the card scroll view and above it, so
///   it neither scrolls nor reflows for anything a drop can do; it is read for one rule only — "a
///   release on the lane header resolves to the topmost position" (04-interactions.md ▸ Drag and
///   drop, settled 2026-07-28), which needs an edge the scrolling masonry cannot supply.
///
/// The one input that *is* frozen at drag start is the **dragged** cards' own heights, which live on
/// `DragSession`: the pickup transition scales the replica and corrupts its last measured frame.
@MainActor
final class LaneDropRegistry {

    /// One lane's masonry, as drawn.
    struct Grid: Equatable, Sendable {
        /// The card area's frame in the window's SwiftUI global space — the space the physical
        /// cursor is converted into (`BoardDropContext.globalCursor`).
        var frame: CGRect
        /// Interior masonry columns — the lane's width units.
        var columns: Int
        /// Spacing between columns and between stacked cards.
        var spacing: CGFloat
    }

    /// The height a card with no registered measurement is assumed to have — a lane whose faces have
    /// not laid out yet. Nominal rather than zero, so the resting rows still tile.
    ///
    /// **Font-derived** (`BoardMetrics.nominalCardHeight`, 10-accessibility.md's full-relative-scaling
    /// rule), and it matters more here than at most sites: this is the stand-in the drop model tiles
    /// rows with before anything has measured itself, so a figure fixed at 13pt's 44 points would put
    /// every un-measured slot boundary in the wrong place at a large system text size — and the drop
    /// model is forbidden from reading measured frames mid-flight (03-board-ui.md § Motion), so this
    /// guess is all it has until the lane lays out.
    @MainActor
    static var nominalCardHeight: CGFloat {
        BoardMetrics.nominalCardHeight(bodyPointSize: BoardMetrics.bodyPointSize)
    }

    /// The board strip's own frame in the window's SwiftUI global space — the origin the strip
    /// coordinates `DropSlotMath.laneExtents` is written in are measured from. It lives here rather
    /// than in the view's `@State` so a delegate reads the *current* rectangle at event time.
    var stripFrame: CGRect = .zero

    private(set) var grids: [ItemID: Grid] = [:]
    private(set) var heights: [ItemID: CGFloat] = [:]

    /// Each lane's title bar, in the same global space `Grid.frame` is written in — the topmost-rule
    /// stripe (`FileDropZones.landing`). Absent for a lane that has not laid its header out yet, and
    /// the file zones then let the masonry answer alone.
    private(set) var headers: [ItemID: CGRect] = [:]

    func update(_ grid: Grid, for laneID: ItemID) { grids[laneID] = grid }
    func removeGrid(_ laneID: ItemID) { grids.removeValue(forKey: laneID) }

    func update(header frame: CGRect, for laneID: ItemID) { headers[laneID] = frame }
    func removeHeader(_ laneID: ItemID) { headers.removeValue(forKey: laneID) }

    func update(height: CGFloat, for cardID: ItemID) { heights[cardID] = height }
    func removeHeight(_ cardID: ItemID) { heights.removeValue(forKey: cardID) }
}

// MARK: - The board window's half of a drop

/// Everything a drop delegate — and the autoscroll driver — needs to answer "where would this land",
/// read at **event** time rather than captured at body-evaluation time.
///
/// The closures are the point: 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.
@MainActor
struct BoardDropContext {

    let store: BoardStore
    let session: DragSession
    let registry: LaneDropRegistry

    /// The strip's inter-lane gap, which is also its outer margin.
    let gap: CGFloat

    /// The window hosting this board — the physical cursor is converted through it.
    let window: @MainActor () -> NSWindow?

    /// The strip's frame in the window's SwiftUI global space.
    let stripFrame: @MainActor () -> CGRect

    /// The strip's 1× lane width for the current drag context (`LaneLayoutMath.standardWidth`).
    let standard: @MainActor () -> CGFloat

    // MARK: The cursor

    /// The physical cursor in the window's SwiftUI global space.
    ///
    /// **`NSEvent.mouseLocation`, never `DropInfo.location`** (DRAG-REORDER.md § Animation-proof
    /// inputs): the drop callback's location is expressed in the target view's space, and that view
    /// may itself be mid-reflow. This is also what `LaneResizeSession` and `MarqueeSession` already
    /// read.
    func globalCursor() -> CGPoint? {
        guard let window = window(), let content = window.contentView else { return nil }
        let inWindow = window.convertPoint(fromScreen: NSEvent.mouseLocation)
        let inContent = content.convert(inWindow, from: nil)
        // SwiftUI's global space is top-left-origin; an unflipped `NSView` is bottom-left.
        let y = content.isFlipped ? inContent.y : content.bounds.height - inContent.y
        return CGPoint(x: inContent.x, y: y)
    }

    /// The cursor in strip coordinates — 0 at the strip's leading edge, outer margin included, which
    /// is the origin `DropSlotMath.laneExtents` assumes.
    func stripCursor() -> CGPoint? {
        guard let cursor = globalCursor() else { return nil }
        let frame = stripFrame()
        return CGPoint(x: cursor.x - frame.minX, y: cursor.y - frame.minY)
    }

    // MARK: Re-grounding

    /// **Rule 2 of the mid-drag re-grounding trio** (04-interactions.md ▸ Drag and drop): a proposal
    /// whose target lane vanished in a reload is invalidated — deleted lanes are
    /// never drop targets — the shadow withdraws, and no proposal stands until the pointer reaches a
    /// live target.
    ///
    /// Run at the top of every callback *and* again at release, against the snapshot as it is then;
    /// the store's commits enforce the same rule independently, so the gesture and the write cannot
    /// disagree.
    func revalidateProposal() {
        guard let proposal = session.proposal,
              DragLocality.isSameBoard(proposal.boardRoot, store.rootURL),
              let laneID = proposal.laneID
        else { return }
        guard !store.snapshot.lanes.contains(where: { $0.id == laneID }) else { return }
        session.propose(nil)
    }

    // MARK: Retargeting — the one shared answer

    /// Where a **lane** session would land on this board's strip.
    ///
    /// The zones are analytic — `DropSlotMath.laneExtents` over the remaining lanes' unit counts and
    /// this strip's standard width — and the cursor is the physical mouse, so neither input is a
    /// measured frame (03-board-ui.md § Motion).
    ///
    /// **The terminal slot is clamped before the trash.** The column consumes one unit while
    /// shown and is never a *position* on the strip (04-interactions.md ▸ The trash: "no move or
    /// paste ever targets the trash"), so it is absent from the slot list by construction and the end
    /// slot's uncapped reach past the last real lane lands *before* it. The delete a lane drag can
    /// propose over the column is not a slot at all — it is the column's own target answering
    /// (`retargetTrash`), and it names `.trash` rather than an index in this list.
    func retargetLanes() {
        guard session.isDraggingLanes, let cursor = stripCursor() else { return }
        let hidden = session.hiddenMembers(onBoardRooted: store.rootURL)
        let resting = store.snapshot.lanes.filter { !hidden.contains($0.id) }
        let restingUnits = resting.map { LaneLayoutMath.displayUnits(of: $0) }
        let slot = DropSlotMath.laneSlot(
            cursorX: cursor.x,
            restingUnits: restingUnits,
            draggedUnits: session.laneUnits,
            standard: standard(),
            gap: gap,
            current: session.stripProposal(onBoardRooted: store.rootURL)
        )
        guard let slot else { return }   // a dead region: hold the current proposal
        let index = min(max(0, slot), restingUnits.count)
        session.propose(DropTarget(boardRoot: store.rootURL, container: .strip, index: index))
    }

    /// Where a **card** session would land in `laneID`'s masonry.
    ///
    /// The single answer the lane's own drop delegate, the strip's fall-through, and the autoscroll
    /// driver all go through — "the lane's drop delegate and the autoscroll driver must go through
    /// one shared retarget, so they can never disagree" (DRAG-REORDER.md § Edge autoscroll).
    func retargetCards(inLane laneID: ItemID) {
        guard session.isDraggingCards, let cursor = globalCursor() else { return }
        guard let lane = store.snapshot.lanes.first(where: { $0.id == laneID }) else {
            revalidateProposal()
            return
        }
        guard let grid = registry.grids[laneID] else { return }

        let hidden = session.hiddenMembers(onBoardRooted: store.rootURL)
        let rendered = lane.cards.filter { !hidden.contains($0.id) }
        let heights = rendered.map { registry.heights[$0.id] ?? LaneDropRegistry.nominalCardHeight }
        let placement = MasonryPlacement(
            columnCount: grid.columns,
            columnWidth: MasonryPlacement.columnWidth(
                totalWidth: grid.frame.width, columnCount: grid.columns, spacing: grid.spacing),
            spacing: grid.spacing,
            origin: grid.frame.origin
        )
        let slot = DropSlotMath.cardSlot(
            cursor: cursor,
            placement: placement,
            heights: heights,
            // The run's footprint at the landing spot: the first dragged card's frozen height, which
            // is the trigger rect the cursor is over (the rest stack below it).
            draggedHeight: session.cardHeights.first ?? LaneDropRegistry.nominalCardHeight,
            current: session.laneProposal(onBoardRooted: store.rootURL, laneID: laneID)
        )
        guard let slot else { return }   // a dead region: hold
        session.propose(DropTarget(boardRoot: store.rootURL, container: .lane(laneID), index: slot))
    }

    /// The strip's fall-through for card sessions: which lane is under the cursor, analytically.
    ///
    /// This is both the safety net for a lane whose own drop region goes dead (DRAG-REORDER.md §
    /// Single-target dispatch) and the live handler for the strip's own regions. A cursor over a gap,
    /// the outer margin, or the trash column is over no lane at all — `LaneLayoutMath.laneIndex`
    /// answers `nil` there — and the proposal simply **holds**, which is the hysteresis contract.
    func retargetCardsFromStrip() {
        guard session.isDraggingCards, let cursor = stripCursor() else { return }
        let lanes = store.snapshot.lanes
        let index = LaneLayoutMath.laneIndex(
            atX: cursor.x,
            unitCounts: lanes.map { LaneLayoutMath.displayUnits(of: $0) },
            standard: standard(),
            gap: gap
        )
        guard let index, lanes.indices.contains(index) else { return }
        retargetCards(inLane: lanes[index].id)
    }

    /// The retarget for whichever session type is in flight, from the strip's own surfaces.
    func retargetFromStrip() {
        revalidateProposal()
        if session.isDraggingLanes {
            retargetLanes()
        } else {
            retargetCardsFromStrip()
        }
    }

    // MARK: Retargeting — the trash column

    /// Whether the trash column would take the session in flight right now — `TrashDrop.accepts`
    /// with this board's state read in (04-interactions.md ▸ The trash, settled 2026-07-28).
    ///
    /// A method rather than a property because it is not free of consequence: the operation is
    /// re-resolved against the modifiers *at this instant*, which is also what keeps the badge honest
    /// while the cursor sits over the column (`dropProposal`).
    func acceptsTrashDrop() -> Bool {
        guard let sourceRoot = session.sourceRoot else { return false }
        return TrashDrop.accepts(
            kind: session.kind,
            container: session.container,
            isWithinBoard: DragLocality.isSameBoard(sourceRoot, store.rootURL),
            operation: session.resolveOperation(destinationRoot: store.rootURL),
            isTrashShown: store.transient.isTrashVisible,
            acceptsMutations: store.acceptsBoardMutations
        )
    }

    /// Where a session over the **trash column** would land: the topmost row, or nowhere.
    ///
    /// **A refusal falls through to the strip's own answer rather than withdrawing the proposal**,
    /// which is precisely what this column did before it had a drop target of its own: a cursor over
    /// it resolves to no lane, so `retargetCardsFromStrip` holds whatever the shadows already show
    /// and `retargetLanes` clamps the terminal slot in front of the trash column. That is the
    /// hysteresis contract (DRAG-REORDER.md § Hysteresis) and it is also the honest reading of "the
    /// trash proposes nothing for you": the column declines to be a target, it does not cancel the
    /// drag the user is still holding. So a lane drag reorders across the column exactly as it always
    /// did, and an ⌥-copy released over it still lands where its shadows are.
    func retargetTrash() {
        revalidateProposal()
        guard acceptsTrashDrop() else {
            retargetFromStrip()
            return
        }
        session.propose(DropTarget(
            boardRoot: store.rootURL,
            container: .trash,
            index: TrashDrop.landingIndex
        ))
    }

    // MARK: Retargeting — external Finder file sessions

    /// Whether `info` is an **external Finder file** session rather than one of ours.
    ///
    /// Never ambiguous: our own drags arm `DragSession` synchronously at `.onDrag` time, before any
    /// drop callback can arrive, so a session that is not active but carries `.fileURL` items came
    /// from outside the app. (A board drag also carries a plain-text representation and no file URL,
    /// so the two type sets never overlap.)
    func isFileSession(_ info: DropInfo) -> Bool {
        !session.isActive && info.hasItemsConforming(to: [.fileURL])
    }

    /// Whether this board accepts a file drop at all — **the mutating-gesture rule, applied to the
    /// one gesture that arrives from outside the app**: under the read-only lock (02-architecture.md
    /// § The lock's scope) or with an inline title editor focused (04-interactions.md ▸ Grammar's
    /// focused-editor rule) a file drop refuses at the board, with no highlight and no proposal —
    /// the same refusal `.onDrag` makes by handing back an empty item provider.
    var acceptsFileDrops: Bool {
        !store.isReadOnly && !store.isEditingInline
    }

    /// Whether this board accepts *this* drag — the board's own state **and the payload's**
    /// (04-interactions.md ▸ Drag and drop: "Folders are refused at hover").
    ///
    /// A drag carrying nothing but folders is refused here, at every delegate's `validateDrop` and
    /// again before any retarget, which is the whole of "a drag containing only folders never
    /// engages — no highlight, no drop proposal, the standard incompatible-payload read". A mixed
    /// drag engages for its files alone: it has something to import, and the folders are named at
    /// the drop (`FinderDrop.land`).
    func acceptsFileDrop(_ info: DropInfo) -> Bool {
        acceptsFileDrops && FinderDrop.importableCount(info.itemProviders(for: [.fileURL])) > 0
    }

    /// Where a **file** session would land in `laneID` — the file mode's twin of `retargetCards`,
    /// resolved against the very same analytic masonry geometry.
    ///
    /// The three answers and the order they are asked in are `FileDropZones.landing`'s, kept there so
    /// the ruling is checkable without a window; this is the adapter that feeds it the snapshot and
    /// the registry and turns its answer into a proposal. In short: the **header** is the topmost
    /// position, a **card under the cursor** attaches, and everything else is the **create slot** the
    /// ordinary card zones produce.
    ///
    /// **Created cards land at the drop position** (04-interactions.md ▸ Drag and drop, settled
    /// 2026-07-28): "resolved through the same card-grid zones an ordinary card drag uses, shadow
    /// included — drops are positional everywhere, and append-at-bottom stays the creation *trio*'s
    /// rule, not the drop's."
    ///
    /// **The landing shadow is the create path's whole feedback** (settled, same bullet): no lane-level
    /// highlight is proposed here or drawn anywhere, because "each target gets one clear signal, and
    /// the card-attach highlight exists precisely because that target has no shadow".
    ///
    /// While a create shadow is open the *drawn* cards sit lower than their resting frames, which is
    /// exactly the tradeoff every proposal in this app makes: the answer stays a pure function of the
    /// cursor and the snapshot, so it cannot oscillate — the drawn layout never feeds back into it.
    func retargetFile(inLane laneID: ItemID, info: DropInfo) {
        guard acceptsFileDrop(info), let cursor = globalCursor(),
              let lane = store.snapshot.lanes.first(where: { $0.id == laneID }),
              let grid = registry.grids[laneID]
        else {
            session.proposeFile(nil)
            return
        }

        let rendered = lane.cards
        let heights = rendered.map { registry.heights[$0.id] ?? LaneDropRegistry.nominalCardHeight }
        let placement = MasonryPlacement(
            columnCount: grid.columns,
            columnWidth: MasonryPlacement.columnWidth(
                totalWidth: grid.frame.width, columnCount: grid.columns, spacing: grid.spacing),
            spacing: grid.spacing,
            origin: grid.frame.origin
        )
        let count = FinderDrop.shadowCount(info.itemProviders(for: [.fileURL]))

        let landing = FileDropZones.landing(
            cursor: cursor,
            headerBottom: registry.headers[laneID]?.maxY,
            placement: placement,
            heights: heights,
            nominalHeight: LaneDropRegistry.nominalCardHeight,
            current: session.fileLaneProposal(onBoardRooted: store.rootURL, laneID: laneID)?.index
        )

        switch landing {
        case .hold:
            return   // a dead region: hold whatever the create slot already was
        case let .attach(index):
            guard rendered.indices.contains(index) else { return }
            session.proposeFile(FileDropTarget(
                boardRoot: store.rootURL,
                landing: .attach(cardID: rendered[index].id),
                fileCount: count
            ))
        case let .create(index):
            session.proposeFile(FileDropTarget(
                boardRoot: store.rootURL,
                landing: .create(laneID: laneID, index: index),
                fileCount: count
            ))
        }
    }

    /// The strip's fall-through for file sessions: which lane is under the cursor, analytically.
    ///
    /// Both a safety net for a lane whose own drop region goes dead (DRAG-REORDER.md § Single-target
    /// dispatch) and the live handler for the strip's own surfaces — and unlike a card session, those
    /// surfaces genuinely clear the proposal rather than holding it. A cursor over a gap, the outer
    /// margin, or **the trash column** is over no lane at all (`LaneLayoutMath.laneIndex` answers
    /// `nil` there, since the trash column is absent from the lane list by construction), so the
    /// highlight withdraws and a release refuses: "Finder file drops (attachment import) on
    /// trash cards are inert" and the trash column is never a file-drop target (04-interactions.md ▸ The
    /// trash).
    func retargetFileFromStrip(_ info: DropInfo) {
        guard acceptsFileDrop(info), let cursor = stripCursor() else {
            session.proposeFile(nil)
            return
        }
        let lanes = store.snapshot.lanes
        let index = LaneLayoutMath.laneIndex(
            atX: cursor.x,
            unitCounts: lanes.map { LaneLayoutMath.displayUnits(of: $0) },
            standard: standard(),
            gap: gap
        )
        guard let index, lanes.indices.contains(index) else {
            session.proposeFile(nil)
            return
        }
        retargetFile(inLane: lanes[index].id, info: info)
    }

    /// What `dropUpdated` answers for a file session: always `.copy` — importing a file leaves the
    /// original where it was, which is what the badge should say — and `.cancel` when nothing under
    /// the cursor will take it.
    func fileDropProposal() -> DropProposal {
        session.fileTarget != nil ? DropProposal(operation: .copy) : DropProposal(operation: .cancel)
    }

    /// Commits a file drop: the files land where the highlight or the shadows showed.
    ///
    /// **The target is captured and the hover state cleared before the load starts.** A provider's
    /// file URL loads asynchronously — it is never synchronous for a Finder drag — and a fast second
    /// drag arriving in the meantime must not find a stale target sitting there. The store call then
    /// happens back on the main actor, one `performWrite` bracket per gesture, whatever the file
    /// count (`BoardStore.importAttachments` / `createCards`).
    ///
    /// **The resolved URLs are the authority on what is a folder** (04-interactions.md ▸ Drag and
    /// drop): the hover read is a declared-type guess and the drop is a filesystem fact, so
    /// `FinderDrop.land` re-partitions here and writes only the files — see its own note on why the
    /// two reads cannot be one.
    func commitFileDrop(_ info: DropInfo) -> Bool {
        guard acceptsFileDrops,
              let target = session.fileTarget,
              DragLocality.isSameBoard(target.boardRoot, store.rootURL)
        else {
            session.proposeFile(nil)
            return false
        }
        let providers = info.itemProviders(for: [.fileURL])
        session.proposeFile(nil)
        guard !providers.isEmpty else { return false }

        let store = self.store
        Task { @MainActor in
            var urls: [URL] = []
            for provider in providers {
                if let url = await FileDropLoading.url(from: provider) { urls.append(url) }
            }
            guard !urls.isEmpty else { return }

            // The sandbox's half: a Finder drag hands the app an extension for what it dropped, and
            // the copy is the read that needs it. `start…` answers false for a URL that carries no
            // scope of its own — an ordinary in-container path — so only the ones that opened are
            // closed again.
            let scoped = urls.filter { $0.startAccessingSecurityScopedResource() }
            defer { for url in scoped { url.stopAccessingSecurityScopedResource() } }

            // Inside the scope: the directory read is itself a read of the dropped item, and the
            // extension the drag handed over is what makes it answer honestly.
            FinderDrop.land(urls, landing: target.landing, into: store)
        }
        return true
    }

    // MARK: The drop proposal the badge tracks

    /// What `dropUpdated` answers: the effective operation while the shadows are on *this* board,
    /// `.cancel` otherwise.
    ///
    /// The operation is re-resolved here rather than at pickup, which is what makes the badge track
    /// live as the cursor crosses a board boundary (04-interactions.md ▸ Drag and drop).
    func dropProposal() -> DropProposal {
        guard let proposal = session.proposal,
              DragLocality.isSameBoard(proposal.boardRoot, store.rootURL)
        else { return DropProposal(operation: .cancel) }
        let operation = session.resolveOperation(destinationRoot: store.rootURL)
        return DropProposal(operation: operation == .copy ? .copy : .move)
    }

    // MARK: The commit

    /// Commits the current proposal — **the drop always lands exactly where the shadows show**
    /// (DRAG-REORDER.md § The pieces).
    ///
    /// The three re-grounding rules are applied here, against the snapshot as it is *now* rather than
    /// against whatever the last render believed:
    ///
    /// 1. the geometry was re-derived on every sample and the proposal is what it produced;
    /// 2. a proposal naming a vanished lane is invalidated, and **release with no valid
    ///    proposal cancels** — items return, nothing is written;
    /// 3. an emptied drag cancels itself, and a partly emptied one drops the survivors.
    ///
    /// The commit is the **destination** store's, one `performWrite` bracket per gesture whatever the
    /// set's size (DRAG-REORDER.md § The drop commits).
    ///
    /// **One of the containers is not a destination but a verb.** A proposal naming the trash commits
    /// a delete — the same write ⌫ performs, through the same `BoardWriter.deleteCardToTrash` /
    /// `deleteLaneToTrash` in the same bracket (`BoardStore.deleteByDrag`, `deleteLanesByDrag`), so an
    /// item deleted by drop is indistinguishable on disk from one deleted by keystroke
    /// (04-interactions.md ▸ The trash, settled 2026-07-28; lanes extended 2026-07-29).
    ///
    /// The write is the first half; the second is the **committed-overlay hold**
    /// (`DragSession.commit`). The write is still in flight when this returns, so the session flips
    /// from proposing to committed and keeps drawing the arrangement it was showing — the shadows at
    /// their landing slots, the originals lifted out — until this store's echo reload lands.
    func commitDrop() -> Bool {
        guard session.isActive, let kind = session.kind, let sourceRoot = session.sourceRoot else {
            return false
        }
        revalidateProposal()
        guard let target = session.proposal,
              DragLocality.isSameBoard(target.boardRoot, store.rootURL)
        else {
            cancelDrop()
            return false
        }
        let survivors = session.survivors
        guard !survivors.isEmpty else {
            cancelDrop()
            return false
        }
        let ids = survivors.map { session.members[$0] }
        let folders = survivors.map { session.folders[$0] }
        let within = DragLocality.isSameBoard(sourceRoot, store.rootURL)
        let operation = session.resolveOperation(destinationRoot: store.rootURL)

        switch kind {
        case .lanes:
            if target.isTrash {
                // **The pointer's delete gesture, at the lane level** (04-interactions.md ▸ The
                // trash, lanes extended 2026-07-29: "a lane drag over the shown trash proposes the
                // delete alongside its strip slots"): release moves the lane's folder — subtree
                // intact — into `.trash/`, exactly the ⌫ delete (`BoardStore.deleteLanesByDrag`).
                //
                // The gate is re-asked here rather than trusted from the hover, the card branch's
                // rule for its reason: the modifiers can change after the proposal stood, and no
                // callback reports it. A refusal cancels — the lane returns, nothing is written.
                guard TrashDrop.accepts(
                    kind: kind,
                    container: session.container,
                    isWithinBoard: within,
                    operation: operation,
                    isTrashShown: store.transient.isTrashVisible,
                    acceptsMutations: store.acceptsBoardMutations
                ) else {
                    cancelDrop()
                    return false
                }
                store.deleteLanesByDrag(laneIDs: ids)
                break
            }
            // **A lane drag never targets a masonry** — a lane session proposes lane slots and the
            // trash column, and nothing else (04-interactions.md ▸ The trash). True by construction,
            // since `retargetLanes` and `retargetTrash` are the only things that propose for one;
            // written down because a commit that trusted the container implicitly would be the one
            // place the invariant could break silently.
            guard target.container == .strip else {
                cancelDrop()
                return false
            }
            if within {
                // **The trash side is the restore, and it is a different write** (04 ▸ The trash:
                // "a trashed lane row [dropped] onto its own board's strip — is an ordinary move to
                // the drop position"): the folder comes *out* of `.trash/`, where `moveLanes`'
                // rank arithmetic — a permutation of the strip's own lanes — has nothing to say
                // about it. `restoreLanes` is that move, with the same one-bracket, one-step shape.
                if session.container == .trash {
                    store.restoreLanes(Set(ids), toIndex: target.index)
                } else {
                    store.moveLanes(Set(ids), toIndex: target.index)
                }
            } else {
                // Cross-board, from either container: the ordinary arrival, copy by default and
                // move under ⌘ — "Dropped on *another* board it follows the copy default … ⌘-drag
                // forces the true cross-board restore-move" (04 ▸ The trash).
                store.receiveLanes(folders, operation: operation, at: target.index)
            }

        case .cards:
            if target.isTrash {
                // **The pointer's delete gesture** (04-interactions.md ▸ The trash, settled
                // 2026-07-28): "release moves the dragged card(s) into `.trash/`" — exactly the ⌫
                // delete.
                //
                // The gate is re-asked here rather than trusted from the hover, because the one input
                // that can change between them arrives through no callback at all: ⌥ pressed after
                // the proposal stood would otherwise delete an original the copy grammar had just
                // promised to leave alone. A refusal cancels — items return, nothing is written.
                guard TrashDrop.accepts(
                    kind: kind,
                    container: session.container,
                    isWithinBoard: within,
                    operation: operation,
                    isTrashShown: store.transient.isTrashVisible,
                    acceptsMutations: store.acceptsBoardMutations
                ) else {
                    cancelDrop()
                    return false
                }
                store.deleteByDrag(cardIDs: ids)
                break
            }
            guard let laneID = target.laneID else {
                cancelDrop()
                return false
            }
            // **The trash side needs no branch of its own any more** (04-interactions.md ▸ The
            // trash, resettled 2026-07-28: "Drag-to-restore follows the locality model: dropping a
            // trash card into one of its own board's lanes is an ordinary move to the drop
            // position"). `moveCards`/`copyCards` resolve their members in either container, so a
            // restore *is* the within-board move and a cross-board restore *is* the ordinary
            // arrival — which is exactly what retiring the restore-specific machinery bought.
            if within {
                if operation == .copy {
                    store.copyCards(Set(ids), toLane: laneID, at: target.index)
                } else {
                    store.moveCards(Set(ids), toLane: laneID, at: target.index)
                }
            } else {
                store.receiveCards(folders, operation: operation, toLane: laneID, at: target.index)
            }
        }

        // The committed-overlay hold: keep drawing the arrangement until this store's next snapshot.
        session.commit(into: store)
        return true
    }

    /// Release with nothing valid to write: the items return and nothing is written.
    func cancelDrop() {
        withAnimation(Motion.dragReflow(reduced: Motion.prefersReducedMotion)) { session.end() }
    }
}

// The Finder-drag payload rules this file used to hold — `FileDropLoading` and `FinderDrop` — moved
// to `Kanban/UI/FinderDrop.swift` verbatim when the card window's whole-window attachment drop
// (05-card-window.md ▸ Attachments) became their second caller. Everything below still calls them by
// the same names; only their file changed.

// MARK: - Drop delegates

// **Single-target dispatch** (DRAG-REORDER.md, the constraint of the same name): SwiftUI/macOS
// delivers a drag session to the *deepest* drop region under the cursor with no fall-through, not
// even when that target's declared content types don't match the session's payload. So every
// delegate below accepts **all three** session types — cards, lanes, and external Finder file drags
// — and routes internally; a region whose topmost target understood only some of them would be a
// dead zone for the rest — no hover callbacks, and a release there would snap back instead of
// committing.
//
// **The file routing is resolved inside these delegates, not by a drop target of the card's own.**
// A per-face `onDrop` would be the deepest region under the cursor and would therefore have to
// re-implement card and lane routing too, just to avoid becoming a dead zone for them — a second
// copy of the dispatch, able to disagree with this one. Instead a card under the cursor is found by
// hit-testing the *analytic* masonry frames the card zones are already built from
// (`BoardDropContext.retargetFile`), which adds no drop region at all and cannot drift from the
// geometry the shadows use.
//
// **A file session is validated by its payload, not only by its type** (04-interactions.md ▸ Drag
// and drop: "Folders are refused at hover"). Every `validateDrop` below asks
// `BoardDropContext.acceptsFileDrop`, which answers false for a drag carrying nothing but folders —
// so the system reads the board as an incompatible target for it: no `dropEntered`, no highlight, no
// shadows, and a refusal cursor at the drop. A *mixed* drag validates, proposes for its files alone,
// and names the folders it left behind when it lands (`FinderDrop`).

/// The board's own drag types. Spelled once so no target can accidentally accept fewer.
let boardDragTypes: [UTType] = [.laneworkCards, .laneworkLanes]

/// Every type a board surface's `onDrop` declares: our own two, plus the external Finder file drag
/// (04-interactions.md ▸ Drag and drop). **A target that left `.fileURL` off would be a dead zone
/// that strands a file session** — no hover callbacks, and a refused release with nothing to explain
/// why (DRAG-REORDER.md § Single-target dispatch).
let boardDropTypes: [UTType] = boardDragTypes + [.fileURL]

/// One lane's drop target, attached to the whole lane body.
///
/// **Card sessions** resolve against this lane's masonry zones. **Lane sessions** are forwarded to
/// the strip's logic (cursor converted to strip space by the shared context), so lane reordering
/// keeps working while the cursor crosses lane bodies. **File sessions** resolve against those same
/// masonry zones — onto a card they become attachments, onto the grid one card per file at the drop
/// position, and onto the **header** the topmost position, since the target is the whole lane body
/// and a dead stripe across its top would be the one place a file drop refused for no reason
/// (04-interactions.md ▸ Drag and drop, settled 2026-07-28).
struct LaneDropDelegate: DropDelegate {

    let context: BoardDropContext
    let laneID: ItemID

    func validateDrop(info: DropInfo) -> Bool {
        if context.isFileSession(info) { return context.acceptsFileDrop(info) }
        return context.session.isActive && info.hasItemsConforming(to: boardDragTypes)
    }

    func dropEntered(info: DropInfo) { retarget(info) }

    func dropUpdated(info: DropInfo) -> DropProposal? {
        retarget(info)
        return context.isFileSession(info) ? context.fileDropProposal() : context.dropProposal()
    }

    /// **Only file sessions have an exit hook.** A card or lane proposal is meant to *hold* while the
    /// cursor leaves for ambiguous territory — that is the hysteresis contract (DRAG-REORDER.md §
    /// Hysteresis) — while a file proposal is only ever "what is under the cursor now", so leaving
    /// the lane body clears the highlight.
    func dropExited(info: DropInfo) {
        guard context.isFileSession(info) else { return }
        context.session.proposeFile(nil)
    }

    func performDrop(info: DropInfo) -> Bool {
        context.isFileSession(info) ? context.commitFileDrop(info) : context.commitDrop()
    }

    private func retarget(_ info: DropInfo) {
        if context.isFileSession(info) {
            context.retargetFile(inLane: laneID, info: info)
            return
        }
        context.revalidateProposal()
        if context.session.isDraggingLanes {
            context.retargetLanes()
        } else {
            context.retargetCards(inLane: laneID)
        }
    }
}

/// The strip's drop target — the backdrop, the gaps and the outer margin.
///
/// The trash column used to fall through to here too, being no landing spot of its own; it has its
/// own target now that a live card drag can *delete* into it (`TrashDropDelegate`), and that target
/// hands every session it declines straight back to this logic, so the behaviour over the column is
/// unchanged for everything but the one gesture that is new.
///
/// Lane sessions retarget against the strip's analytic zones; card sessions retarget through the
/// same shared function the lane delegates use, resolving the lane under the cursor analytically —
/// the safety net for a lane whose own drop region goes dead.
/// File sessions are the strip's twin safety net, and here its own surfaces are a *live* handler for
/// them rather than only a fallback: a file drag has no proposal to hold, so hovering a gap, the
/// outer margin, or the trash column clears the target outright.
struct StripDropDelegate: DropDelegate {

    let context: BoardDropContext

    func validateDrop(info: DropInfo) -> Bool {
        if context.isFileSession(info) { return context.acceptsFileDrop(info) }
        return context.session.isActive && info.hasItemsConforming(to: boardDragTypes)
    }

    func dropEntered(info: DropInfo) { retarget(info) }

    func dropUpdated(info: DropInfo) -> DropProposal? {
        retarget(info)
        return context.isFileSession(info) ? context.fileDropProposal() : context.dropProposal()
    }

    /// See `LaneDropDelegate.dropExited` — the file mode's clear, and nothing for our own sessions.
    func dropExited(info: DropInfo) {
        guard context.isFileSession(info) else { return }
        context.session.proposeFile(nil)
    }

    func performDrop(info: DropInfo) -> Bool {
        context.isFileSession(info) ? context.commitFileDrop(info) : context.commitDrop()
    }

    private func retarget(_ info: DropInfo) {
        if context.isFileSession(info) {
            context.retargetFileFromStrip(info)
        } else {
            context.retargetFromStrip()
        }
    }
}

/// The **trash column's** drop target — the pointer's delete gesture (04-interactions.md ▸ The
/// trash, settled 2026-07-28: "dropping a live card on the shown trash deletes it").
///
/// It exists only while the column does. Hidden, the trash renders nothing, so there is no region
/// here to enter and "the trash stays undroppable-into while hidden, like every gesture" needs no
/// code — `TrashDrop.accepts` restates it anyway, because an invariant that is only true by
/// construction is worth being able to point at.
///
/// Like every other delegate it accepts **all three** session types, because single-target dispatch
/// gives the deepest region the session whether it wants it or not (see the note above), and it
/// routes them three ways:
///
/// - **card and lane sessions alike** through `retargetTrash`, which proposes the topmost row for the
///   ones the trash takes — a live item from this board, unmodified, either kind (lanes extended
///   2026-07-29) — and falls through to the strip's own answer for the rest, so a *trashed* row being
///   dragged out keeps reordering against the strip across the column exactly as it did when the
///   column was a hole in the strip's target;
/// - **Finder file sessions** by clearing the highlight outright: "Finder file drops (attachment
///   import) on trash cards are inert" (▸ The trash), and the column has nothing else to offer
///   them — no lane, no card, nothing to attach to.
struct TrashDropDelegate: DropDelegate {

    let context: BoardDropContext

    func validateDrop(info: DropInfo) -> Bool {
        if context.isFileSession(info) { return context.acceptsFileDrop(info) }
        return context.session.isActive && info.hasItemsConforming(to: boardDragTypes)
    }

    func dropEntered(info: DropInfo) { retarget(info) }

    func dropUpdated(info: DropInfo) -> DropProposal? {
        retarget(info)
        return context.isFileSession(info) ? context.fileDropProposal() : context.dropProposal()
    }

    /// See `LaneDropDelegate.dropExited` — the file mode's clear, and nothing for our own sessions,
    /// whose proposals are meant to hold across ambiguous territory.
    func dropExited(info: DropInfo) {
        guard context.isFileSession(info) else { return }
        context.session.proposeFile(nil)
    }

    /// A release on the column commits whatever stands — the delete when the trash is the
    /// proposal, and otherwise the proposal the column declined to displace, which is the same
    /// "the drop lands where the shadows show" promise as anywhere else.
    func performDrop(info: DropInfo) -> Bool {
        context.isFileSession(info) ? context.commitFileDrop(info) : context.commitDrop()
    }

    private func retarget(_ info: DropInfo) {
        if context.isFileSession(info) {
            context.session.proposeFile(nil)
            return
        }
        context.retargetTrash()
    }
}

/// The window-level fallback, behind every specific target: a release over any in-window region they
/// do not cover (the banner strip, the window's edges) commits the current proposal rather than
/// leaking the session into a cancel-snapback. It retargets nothing — the drop lands where the
/// shadows already show, which is what the shadows promise.
/// A file session released over uncovered window chrome commits whatever the file target currently
/// names, exactly as this commits the current card/lane proposal — and with no target standing it
/// simply refuses, which is the honest answer for a release over nothing.
struct BoardFallbackDropDelegate: DropDelegate {

    let context: BoardDropContext

    func validateDrop(info: DropInfo) -> Bool {
        if context.isFileSession(info) { return context.acceptsFileDrop(info) }
        return context.session.isActive && info.hasItemsConforming(to: boardDragTypes)
    }

    func dropUpdated(info: DropInfo) -> DropProposal? {
        context.isFileSession(info) ? context.fileDropProposal() : context.dropProposal()
    }

    /// The drag left the window's last covering region — with nothing below this one to take over,
    /// a standing file highlight would be stale.
    func dropExited(info: DropInfo) {
        guard context.isFileSession(info) else { return }
        context.session.proposeFile(nil)
    }

    func performDrop(info: DropInfo) -> Bool {
        context.isFileSession(info) ? context.commitFileDrop(info) : context.commitDrop()
    }
}