fix: improve inventory item drag-and-drop behavior

- Use `snapBack` instead of immediate coordinate reset when a drag fails
- Adjust grid snapping calculation to use center-relative coordinates
- Validate item placement by simulating removal from the grid first

packages/sts-like-viewer/src/gameobjects/InventoryItemContainer.ts

@@ -127,8 +127,8 @@ export class InventoryItemContainer extends Phaser.GameObjects.Container {
       } else if (event.type === DragDropEventType.UP) {
         this.setAlpha(1);
         if (!this.handleDragEnd()) {
-          this.x = startX;
-          this.y = startY;
+          const t = this.item.peek()?.transform;
+          t && this.snapBack(t);
         }
         startX = startY = 0;
       }
@@ -146,8 +146,8 @@ export class InventoryItemContainer extends Phaser.GameObjects.Container {
 
     const x = this.x - this.gridOffsetX;
     const y = this.y - this.gridOffsetY;
-    const targetX = Math.round((x - cellSize / 2) / cellSize);
-    const targetY = Math.round((y - cellSize / 2) / cellSize);
+    const targetX = Math.round(x / cellSize);
+    const targetY = Math.round(y / cellSize);
 
     const clampedX = Math.max(0, Math.min(targetX, 10 - shapeWidth));
     const clampedY = Math.max(0, Math.min(targetY, 10 - shapeHeight));

packages/sts-like-viewer/src/state/inventory.ts

@@ -1,3 +1,4 @@
+import { create } from "mutative";
 import { mutableSignal } from "boardgame-core";
 import {
   createGridInventory,
@@ -47,7 +48,11 @@ export function moveItem(
     ...item.transform,
     offset: { x: newX, y: newY },
   };
-  const validation = validatePlacement(inventory, item.shape, newTransform);
+
+  const removed = create(inventory, (inv) => {
+    removeItemFromGrid(inv, itemId);
+  });
+  const validation = validatePlacement(removed, item.shape, newTransform);
   if (!validation.valid) return false;
 
   inventorySignal.produce((inv) => {