Add drag & drop to relocate placed pieces

InputMapper tracks a mouse-down over a placed piece and promotes it to
drag mode once the cursor travels past an 8px threshold. Legal drop
cells (those where the piece's start→end vector still fits a legal
placement) are highlighted in green. Releasing on a legal cell emits a
RelocateRequested signal; Main feeds it to MovePieceCommand, which is
already undoable via the existing history stack.

Escape or releasing on an invalid cell cancels. The harness gains a
relocate() helper so UI tests can script drag-and-drop moves without
synthesizing motion events.

Scripts/Automation/CommandDispatcher.cs

@@ -41,6 +41,7 @@ internal class CommandDispatcher
             "set_speed" => SetSpeed(args),
             "load_mission" => LoadMission(args),
             "back_to_menu" => BackToMenu(),
+            "relocate" => Relocate(args),
             "quick_save" => QuickSave(),
             "quick_load" => QuickLoad(),
             "undo" => Undo(),
@@ -235,6 +236,23 @@ internal class CommandDispatcher
         };
     }
 
+    private JsonNode? Relocate(JsonObject args)
+    {
+        var pieceId = args["pieceId"]!.GetValue<int>();
+        var newStart = ParseCoords(args["newStart"]);
+        var newEnd = ParseCoords(args["newEnd"]);
+        _facade.Input.EmitSignal(InputMapper.SignalName.RelocateRequested,
+            pieceId, newStart.Col, newStart.Row, newEnd.Col, newEnd.Row);
+        var sim = _facade.Sim();
+        if (sim == null) return new JsonObject { ["relocated"] = false };
+        var piece = sim.GetSnapshot().Pieces.FirstOrDefault(p => p.Id == pieceId);
+        return new JsonObject
+        {
+            ["relocated"] = piece != null && piece.StartCell == newStart && piece.EndCell == newEnd,
+            ["pieceId"] = pieceId
+        };
+    }
+
     private JsonNode? Undo()
     {
         var sim = _facade.Sim();

Scripts/Input/InputMapper.cs

@@ -16,9 +16,13 @@ public partial class InputMapper : Node
     public delegate void CellClickedEventHandler(int col, int row);
     [Signal]
     public delegate void CancelledEventHandler();
+    [Signal]
+    public delegate void RelocateRequestedEventHandler(int pieceId, int newStartCol, int newStartRow, int newEndCol, int newEndRow);
 
     public enum PlacementPhase { None, SelectingStart, SelectingEnd }
 
+    private const float DragThreshold = 8f;
+
     private BoardView _boardView = null!;
     private PieceKind? _selectedKind;
     private Coords? _selectedStart;
@@ -26,6 +30,11 @@ public partial class InputMapper : Node
     private BoardSnapshot? _snapshot;
     private Coords? _hoverCoords;
 
+    // Drag & drop of a placed piece
+    private int? _dragPieceId;
+    private Vector2 _dragMouseStart;
+    private bool _dragging;
+
     public PlacementPhase CurrentPhase => _phase;
 
     public void Initialize(BoardView boardView)
@@ -72,22 +81,140 @@ public partial class InputMapper : Node
 
     public override void _UnhandledInput(InputEvent @event)
     {
-        if (@event is InputEventMouseButton mouseEvent && mouseEvent.Pressed)
-        {
-            if (mouseEvent.ButtonIndex == MouseButton.Left)
+        if (@event is InputEventMouseButton mouseEvent && mouseEvent.ButtonIndex == MouseButton.Left)
         {
             var localPos = _boardView.GetLocalMousePosition();
-                GD.Print($"[InputMapper] LEFT CLICK — localPos={localPos}, phase={_phase}");
-                HandleLeftClick();
+            if (mouseEvent.Pressed)
+                HandleLeftPress(localPos);
+            else
+                HandleLeftRelease(localPos);
         }
+
+        if (@event is InputEventMouseMotion && _dragPieceId != null)
+        {
+            var localPos = _boardView.GetLocalMousePosition();
+            UpdateDrag(localPos);
         }
 
         if (@event is InputEventKey keyEvent && keyEvent.Pressed && keyEvent.Keycode == Key.Escape)
         {
+            CancelDrag();
             Cancel();
         }
     }
 
+    private void HandleLeftPress(Vector2 localPos)
+    {
+        // In placement mode, ignore drag behavior — click advances placement
+        if (_phase != PlacementPhase.None) return;
+
+        var coords = _boardView.PixelToCoords(localPos);
+        if (coords == null || _snapshot == null) return;
+
+        var piece = _snapshot.Pieces.FirstOrDefault(
+            p => p.StartCell == coords.Value || p.EndCell == coords.Value);
+        if (piece != null)
+        {
+            _dragPieceId = piece.Id;
+            _dragMouseStart = localPos;
+            _dragging = false;
+        }
+    }
+
+    private void UpdateDrag(Vector2 localPos)
+    {
+        if (_dragPieceId == null) return;
+
+        if (!_dragging && (localPos - _dragMouseStart).Length() > DragThreshold)
+        {
+            _dragging = true;
+            HighlightLegalDropsFor(_dragPieceId.Value);
+        }
+    }
+
+    private void HandleLeftRelease(Vector2 localPos)
+    {
+        if (_dragging && _dragPieceId != null)
+        {
+            var dropCoords = _boardView.PixelToCoords(localPos);
+            TryRelocate(_dragPieceId.Value, dropCoords);
+            CancelDrag();
+            return;
+        }
+
+        // Normal click flow
+        CancelDrag();
+        HandleLeftClick();
+    }
+
+    private void CancelDrag()
+    {
+        _dragPieceId = null;
+        _dragging = false;
+        _boardView.ClearHighlights();
+    }
+
+    private void HighlightLegalDropsFor(int pieceId)
+    {
+        var legal = ComputeLegalDrops(pieceId);
+        _boardView.ClearHighlights();
+        _boardView.HighlightCells(legal, new Color("#44FF88AA"));
+    }
+
+    private List<Coords> ComputeLegalDrops(int pieceId)
+    {
+        var result = new List<Coords>();
+        if (_snapshot == null) return result;
+
+        var piece = _snapshot.Pieces.FirstOrDefault(p => p.Id == pieceId);
+        if (piece == null) return result;
+
+        var dc = piece.EndCell.Col - piece.StartCell.Col;
+        var dr = piece.EndCell.Row - piece.StartCell.Row;
+
+        var boardState = GetBoardStateForValidation();
+        if (boardState == null) return result;
+
+        for (int c = 0; c < _snapshot.Width; c++)
+        {
+            for (int r = 0; r < _snapshot.Height; r++)
+            {
+                var newStart = new Coords(c, r);
+                var newEnd = new Coords(c + dc, r + dr);
+                if (!newEnd.IsOnBoard(_snapshot.Width, _snapshot.Height)) continue;
+                if (_snapshot.Grid[newStart.Col, newStart.Row] == CellType.Wall) continue;
+                if (_snapshot.Grid[newEnd.Col, newEnd.Row] == CellType.Wall) continue;
+                if (!MoveValidator.IsLegalPlacement(piece.Kind, newStart, newEnd, boardState)) continue;
+                result.Add(newStart);
+            }
+        }
+        return result;
+    }
+
+    private void TryRelocate(int pieceId, Coords? dropCoords)
+    {
+        if (_snapshot == null || dropCoords == null) return;
+        var piece = _snapshot.Pieces.FirstOrDefault(p => p.Id == pieceId);
+        if (piece == null) return;
+
+        var dc = piece.EndCell.Col - piece.StartCell.Col;
+        var dr = piece.EndCell.Row - piece.StartCell.Row;
+
+        var newStart = dropCoords.Value;
+        var newEnd = new Coords(newStart.Col + dc, newStart.Row + dr);
+
+        if (!newEnd.IsOnBoard(_snapshot.Width, _snapshot.Height)) return;
+        if (_snapshot.Grid[newStart.Col, newStart.Row] == CellType.Wall) return;
+        if (_snapshot.Grid[newEnd.Col, newEnd.Row] == CellType.Wall) return;
+
+        var boardState = GetBoardStateForValidation();
+        if (boardState == null) return;
+        if (!MoveValidator.IsLegalPlacement(piece.Kind, newStart, newEnd, boardState)) return;
+
+        EmitSignal(SignalName.RelocateRequested, pieceId,
+            newStart.Col, newStart.Row, newEnd.Col, newEnd.Row);
+    }
+
     private void HandleLeftClick()
     {
         var localPos = _boardView.GetLocalMousePosition();

Scripts/Main.cs

@@ -422,6 +422,7 @@ public partial class Main : Node2D
 		_pieceStockPanel.PieceSelected += OnPieceKindSelected;
 		_inputMapper.PlacementRequested += OnPlacementRequested;
 		_inputMapper.Cancelled += OnPlacementCancelled;
+		_inputMapper.RelocateRequested += OnRelocateRequested;
 		_controlBar.PlayPressed += OnPlay;
 		_controlBar.PausePressed += OnPause;
 		_controlBar.StepPressed += OnStep;
@@ -616,6 +617,53 @@ public partial class Main : Node2D
 		_pieceStockPanel.ClearSelection();
 	}
 
+	private void OnRelocateRequested(int pieceId, int newStartCol, int newStartRow, int newEndCol, int newEndRow)
+	{
+		if (_sim == null) return;
+		var newStart = new Coords(newStartCol, newStartRow);
+		var newEnd = new Coords(newEndCol, newEndRow);
+		var events = _sim.ProcessCommand(new MovePieceCommand(pieceId, newStart, newEnd));
+		foreach (var evt in events)
+		{
+			switch (evt)
+			{
+				case PieceMovedByPlayerEvent moved:
+					UpdatePieceVisualPosition(moved);
+					_detailPanel.Hide();
+					break;
+				case CommandRejectedEvent rejected:
+					GD.Print($"Move rejected: {rejected.Reason}");
+					break;
+			}
+		}
+		_inputMapper.SetSnapshot(_sim.GetSnapshot());
+	}
+
+	private void UpdatePieceVisualPosition(PieceMovedByPlayerEvent moved)
+	{
+		// Refresh from snapshot — the cleanest path is a full rebuild of this piece's visuals
+		if (_sim == null) return;
+		_eventAnimator.UnregisterPiece(moved.PieceId);
+
+		var snap = _sim.GetSnapshot();
+		var ps = snap.Pieces.FirstOrDefault(p => p.Id == moved.PieceId);
+		if (ps == null) return;
+
+		var pieceView = new PieceView();
+		pieceView.Setup(ps.Id, ps.Kind, ps.StartCell, ps.EndCell, _boardView);
+		_boardView.AddChild(pieceView);
+
+		var color = PieceView.GetPieceColor(ps.Kind);
+		var trajectView = new TrajectView();
+		trajectView.Setup(ps.Id,
+			_boardView.CoordsToPixel(ps.StartCell),
+			_boardView.CoordsToPixel(ps.EndCell),
+			color);
+		_boardView.AddChild(trajectView);
+
+		_eventAnimator.RegisterPiece(ps.Id, pieceView, trajectView);
+	}
+
 	private void OnRemoveRequested(int pieceId)
 	{
 		if (_sim == null) return;

docs/PLAN.md

@@ -12,12 +12,6 @@ et l'extension de la campagne.
 Le moteur expose deja les commandes et events requis ; cote Godot il manque
 les surfaces d'interaction et d'animation.
 
-### 1.1 Undo (Ctrl+Z)
-Annule le dernier placement ou retrait. L'architecture event-sourcing rend
-l'implementation naturelle : conserver un historique de commandes cote
-presentation, rejouer l'etat sans la derniere. Essentiel pour l'iteration
-rapide sur un reseau en temps reel.
-
 ### 1.2 Drag & drop des pieces placees
 `MovePieceCommand` + `PieceMovedByPlayerEvent` existent cote engine.
 Cote Godot : permettre de glisser une piece placee pour deplacer son point de

tools/automation/harness.py

@@ -277,6 +277,18 @@ class Harness:
     def undo(self) -> dict[str, Any]:
         return self.send("undo")
 
+    def relocate(
+        self,
+        piece_id: int,
+        new_start: tuple[int, int],
+        new_end: tuple[int, int],
+    ) -> dict[str, Any]:
+        return self.send("relocate", {
+            "pieceId": piece_id,
+            "newStart": list(new_start),
+            "newEnd": list(new_end),
+        })
+
     def quit(self) -> dict[str, Any]:
         return self.send("quit", timeout=5.0)
 

tools/automation/test_relocate.py

@@ -0,0 +1,35 @@
+"""End-to-end smoke test for piece relocation (drag & drop via IPC)."""
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[2]))
+
+from tools.automation.harness import Harness
+
+
+def main():
+    with Harness.launch(run_name="relocate") as h:
+        h.load_mission("campaign_01", 0)
+        h.place("Pawn", (0, 0), (0, 1))
+        h.screenshot("01_placed")
+
+        s = h.state()
+        pid = s['pieces'][0]['id']
+        assert s['pieces'][0]['start'] == [0, 0]
+        assert s['pieces'][0]['end'] == [0, 1]
+
+        # Relocate pawn to (1,0)→(1,1) — vector preserved
+        r = h.relocate(pid, (1, 0), (1, 1))
+        print(f"[relocate] {r}")
+        assert r['relocated'], r
+        h.screenshot("02_relocated")
+
+        s = h.state()
+        assert s['pieces'][0]['start'] == [1, 0]
+        assert s['pieces'][0]['end'] == [1, 1]
+
+        print("OK — relocation works")
+
+
+if __name__ == "__main__":
+    main()