The pickup seeds its own slot — the lift and the shadow become one transaction

"At drag start it replaces the item's original space" is now true from the
very first frame: begin takes the run's own resting slot as a seed, set in
the same transaction that lifts the run out. Without it the vacated gap
closed un-animated and sprang back open at the first dropUpdated — a
shuffle carrying no information. The seed bypasses propose deliberately (a
pickup is not a new landing spot, so no alignment tick), and the first real
sample's re-propose of the same slot is the early-out's ordinary silence. A
⌥-pickup seeds nothing — a copy's resting layout keeps the originals in
place, so there is no vacated space to hold. Card pickups compute the seed
through the same LaneView.rendered rules the retargets count in; lane
pickups seed their strip slot the same way. DRAG-REORDER.md's own-slot
paragraph records the rule, and SeededPickupTests pins all four corners.

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

Kanban/UI/Board/DragSession.swift

@@ -611,17 +611,19 @@ final class DragSession {
     ///   - members: the dragged cards in flatten order (`SelectionGrammar.boardCards`, or the
     ///     trash's own order for a trash-card drag).
     ///   - heights: their measured heights, captured **before** the pickup transition starts.
+    ///   - seed: the run's own resting slot, proposed from the first frame — see `begin`.
     func beginCards(
         _ members: [ItemID],
         folders: [URL],
         heights: [CGFloat],
         container: ItemContainer,
         source: BoardStore,
-        mixesKinds: Bool = false
+        mixesKinds: Bool = false,
+        seed: DropTarget? = nil
     ) {
         begin(
             kind: .cards, members: members, folders: folders,
-            container: container, source: source, mixesKinds: mixesKinds
+            container: container, source: source, mixesKinds: mixesKinds, seed: seed
         )
         cardHeights = heights
         laneUnits = []
@@ -637,23 +639,37 @@ final class DragSession {
         units: [Int],
         container: ItemContainer = .board,
         source: BoardStore,
-        mixesKinds: Bool = false
+        mixesKinds: Bool = false,
+        seed: DropTarget? = nil
     ) {
         begin(
             kind: .lanes, members: members, folders: folders,
-            container: container, source: source, mixesKinds: mixesKinds
+            container: container, source: source, mixesKinds: mixesKinds, seed: seed
         )
         laneUnits = units
         cardHeights = []
     }
 
+    /// `seed` is the pickup's own-slot proposal — "at drag start it replaces the item's original
+    /// space" (DRAG-REORDER.md § The pieces), made true from the very first frame rather than from
+    /// the first `dropUpdated`. Without it the lift-out and the shadow's arrival land in different
+    /// transactions: the masonry closes the vacated gap (un-animated — `shadowRun` hasn't moved) and
+    /// then springs it back open when the first hover sample proposes the own slot, a shuffle for no
+    /// information. Seeded, the lift and the shadow are one transaction with identical geometry —
+    /// the run's frozen sizes at the run's own resting position — so nothing on the board moves at
+    /// pickup, which is what the own-slot fixed point always promised.
+    ///
+    /// Set directly rather than through `propose`, because a pickup is not a *new landing spot*:
+    /// the alignment tick stays reserved for genuine retargets. The first real `dropUpdated`
+    /// re-proposes the same slot and `propose`'s early-out makes it a silent no-op.
     private func begin(
         kind: DragKind,
         members: [ItemID],
         folders: [URL],
         container: ItemContainer,
         source: BoardStore,
-        mixesKinds: Bool
+        mixesKinds: Bool,
+        seed: DropTarget?
     ) {
         // A new drag's first `sinceLastMs` must not be the gap since the previous drag's last sample.
         #if DEBUG
@@ -669,7 +685,7 @@ final class DragSession {
         self.mixesKinds = mixesKinds
         self.sourceStore = source
         self.sourceRoot = source.rootKey
-        self.proposal = nil
+        self.proposal = seed
         self.operation = .move
         // The reload-resolved drag set: vanished members leave it silently, which is what
         // `survivors` reads and what "an emptied drag cancels itself" is stated in terms of.