feat(client): pointer-event drag controller for the phantom layer

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/client/src/lib/stores/phantom-drag.svelte.ts

@@ -0,0 +1,82 @@
+import type { PieceType, Square } from '@blind-chess/shared';
+import { phantoms } from './phantoms.svelte.js';
+import { game } from './game.svelte.js';
+
+export type DragSource =
+  | { kind: 'palette'; type: PieceType }
+  | { kind: 'board'; from: Square; type: PieceType };
+
+/**
+ * Pointer-event drag controller for the phantom layer. A drag past THRESHOLD
+ * px places/moves/removes a phantom; a sub-threshold press is a tap and is
+ * left for the board's normal click handler. Real moves are unaffected.
+ */
+function makeDrag() {
+  const state = $state<{
+    active: DragSource | null;
+    x: number;
+    y: number;
+    moved: boolean;
+  }>({ active: null, x: 0, y: 0, moved: false });
+
+  let startX = 0;
+  let startY = 0;
+  // Set only when a board-phantom drag ends back on its origin square — the
+  // browser then fires a spurious `click` on that square's button which must
+  // be swallowed so it doesn't trigger a real move.
+  let suppressClickOn: Square | null = null;
+  const THRESHOLD = 6;
+
+  function onMove(e: PointerEvent) {
+    if (!state.active) return;
+    state.x = e.clientX;
+    state.y = e.clientY;
+    if (!state.moved && Math.hypot(e.clientX - startX, e.clientY - startY) > THRESHOLD) {
+      state.moved = true;
+    }
+  }
+
+  function onUp(e: PointerEvent) {
+    window.removeEventListener('pointermove', onMove);
+    window.removeEventListener('pointerup', onUp);
+    const src = state.active;
+    const wasDrag = state.moved;
+    state.active = null;
+    state.moved = false;
+    if (!src || !wasDrag) return; // a tap — the board click handler deals with it
+
+    const el = document.elementFromPoint(e.clientX, e.clientY);
+    const sqEl = el?.closest('[data-square]') as HTMLElement | null;
+    const target = sqEl?.dataset.square as Square | undefined;
+
+    if (src.kind === 'board') {
+      if (target === src.from) { suppressClickOn = src.from; return; }
+      if (!target) { phantoms.remove(src.from); return; } // dropped off the board
+      if (game.state.view?.pieces[target]) return;        // your own real piece — reject
+      phantoms.move(src.from, target);
+      return;
+    }
+    // palette → board
+    if (target && !game.state.view?.pieces[target]) phantoms.place(target, src.type);
+  }
+
+  function start(src: DragSource, e: PointerEvent) {
+    suppressClickOn = null;
+    state.active = src;
+    state.x = startX = e.clientX;
+    state.y = startY = e.clientY;
+    state.moved = false;
+    window.addEventListener('pointermove', onMove);
+    window.addEventListener('pointerup', onUp);
+  }
+
+  /** The board calls this first in its square-click handler. */
+  function shouldSuppressClick(sq: Square): boolean {
+    if (suppressClickOn === sq) { suppressClickOn = null; return true; }
+    return false;
+  }
+
+  return { state, start, shouldSuppressClick };
+}
+
+export const phantomDrag = makeDrag();