A stationary modifier flip answers at the keystroke — the third client of the one shared retarget

Modifiers were sampled only inside resolveOperation, reached from
dropUpdated and the commit — and drop callbacks arrive only while the
mouse moves, so ⌥ pressed against a still pointer changed nothing
until the next twitch. DragSession now arms a local .flagsChanged
watch for exactly the session's lifetime (begin arms, end stops, the
watchdog guarantees end; ⇧/⌃/caps don't count, a settled hold
swallows) and publishes modifierGeneration; the three retargets record
which window's surface resolved the proposal (RetargetOrigin — weak
registry identity, never board root, so cross-board and two-windows-
one-board both answer correctly), and the hovered window replays that
same retarget with the operation re-resolved FIRST, since the index
space is a function of it. The monitor is injectable — the real one
needs a live event stream no test bundle has.

Drag-perf card d491e7d3.

Claude-Session: https://claude.ai/code/session_01CqjXB7ASoWtbyoGod68k97

Kanban/UI/Board/BoardView.swift

@@ -188,6 +188,21 @@ struct BoardView: View {
         .onChange(of: store.landedReloads) { _, _ in
             resize.handOff(against: store.snapshot.lanes)
         }
+        // **The stationary modifier flip's re-proposal** (`BoardDropContext.retargetAfterModifierFlip`).
+        // Drop callbacks arrive only while the mouse moves, so a ⌥ pressed against a still pointer
+        // reaches the drag through the session's `.flagsChanged` watch instead — and lands back on
+        // the one shared retarget here, in the board window whose surface resolved the proposal.
+        //
+        // Every open board window carries this handler and exactly one of them acts: the flip
+        // replays against a `LaneDropRegistry`, which is this window's alone.
+        //
+        // **No transaction of its own**, deliberately: a flip must produce exactly what the next
+        // mouse sample would have produced, and `dropUpdated` wraps nothing either. What moves is
+        // keyed declaratively where it is drawn — the strip on its proposal, each lane on its shadow
+        // run (03-board-ui.md § Motion, transactions keyed narrowly).
+        .onChange(of: appModel.dragSession.modifierGeneration) { _, _ in
+            dropContext.retargetAfterModifierFlip()
+        }
         .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).