feat: Experiment: unconstrained movement using Dragger

Create an experimental implementation of unconstrained dragging
using the existing Dragger class.  Implement this on a new
Mover subclass DragMover, to keep this experimental code (that
will probably be thrown away) separate from the main Mover
implementation.

Known bugs:

* Blocks lurch around somewhat wildly while being moved.
* Moving top-level blocks works reasonably well, but attempting
  to move a non-top-level block will result in it being deleted
  unless the move is aborted.

Author: cpcallen

src/actions/drag_mover.ts

@@ -0,0 +1,179 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import * as Constants from '../constants';
+import {
+  ASTNode,
+  Connection,
+  ShortcutRegistry,
+  common,
+  registry,
+  utils,
+} from 'blockly';
+import type {Block, BlockSvg, IDragger, WorkspaceSvg} from 'blockly';
+import {Mover, MoveInfo} from './mover';
+import {Navigation} from '../navigation';
+
+/**
+ * An experimental implementation of Mover that uses a dragger to
+ * perform unconstraind movment.
+ */
+export class DragMover extends Mover {
+  /**
+   * The distance to move an item, in workspace coordinates, when
+   * making an unconstrained move.
+   */
+  UNCONSTRAINED_MOVE_DISTANCE = 20;
+
+  /**
+   * Map of moves in progress.
+   *
+   * An entry for a given workspace in this map means that the this
+   * Mover is moving a block on that workspace, and will disable
+   * normal cursor movement until the move is complete.
+   */
+  protected declare moves: Map<WorkspaceSvg, DragMoveInfo>;
+
+  /**
+   * Start moving the currently-focused item on workspace, if
+   * possible.
+   *
+   * Should only be called if canMove has returned true.
+   *
+   * @param workspace The workspace we might be moving on.
+   * @returns True iff a move has successfully begun.
+   */
+  override startMove(workspace: WorkspaceSvg) {
+    const cursor = workspace?.getCursor();
+    const curNode = cursor?.getCurNode();
+    const block = curNode?.getSourceBlock() as BlockSvg | null;
+    if (!cursor || !block) throw new Error('precondition failure');
+
+    // Select and focus block.
+    common.setSelected(block);
+    cursor.setCurNode(ASTNode.createBlockNode(block)!);
+    // Begin dragging block.
+    const DraggerClass = registry.getClassFromOptions(
+      registry.Type.BLOCK_DRAGGER,
+      workspace.options,
+      true,
+    );
+    if (!DraggerClass) throw new Error('no Dragger registered');
+    const dragger = new DraggerClass(block, workspace);
+    // Record that a move is in progress and start dragging.
+    this.moves.set(workspace, new DragMoveInfo(block, dragger));
+    dragger.onDragStart(new PointerEvent('pointerdown'));
+    return true;
+  }
+
+  /**
+   * Finish moving the currently-focused item on workspace.
+   *
+   * Should only be called if isMoving has returned true.
+   *
+   * @param workspace The workspace on which we are moving.
+   * @returns True iff move successfully finished.
+   */
+  override finishMove(workspace: WorkspaceSvg) {
+    if (!workspace) return false;
+    const info = this.moves.get(workspace);
+    if (!info) throw new Error('no move info for workspace');
+
+    info.dragger.onDragEnd(
+      new PointerEvent('pointerup'),
+      new utils.Coordinate(0, 0),
+    );
+
+    this.moves.delete(workspace);
+    return true;
+  }
+
+  /**
+   * Abort moving the currently-focused item on workspace.
+   *
+   * Should only be called if isMoving has returned true.
+   *
+   * @param workspace The workspace on which we are moving.
+   * @returns True iff move successfully aborted.
+   */
+  override abortMove(workspace: WorkspaceSvg) {
+    if (!workspace) return false;
+    const info = this.moves.get(workspace);
+    if (!info) throw new Error('no move info for workspace');
+
+    // Monkey patch dragger to trigger call to draggable.revertDrag.
+    (info.dragger as any).shouldReturnToStart = () => true;
+    info.dragger.onDragEnd(
+      new PointerEvent('pointerup'),
+      new utils.Coordinate(0, 0),
+    );
+
+    this.moves.delete(workspace);
+    return true;
+  }
+
+  /**
+   * Action to move the item being moved in the given direction,
+   * constrained to valid attachment points (if any).
+   *
+   * @param workspace The workspace to move on.
+   * @returns True iff this action applies and has been performed.
+   */
+  override moveConstrained(
+    workspace: WorkspaceSvg,
+    /* ... */
+  ) {
+    // Not yet implemented.  Absorb keystroke to avoid moving cursor.
+    alert(`Constrained movement not implemented.
+
+Use alt+arrow (option+arrow on macOS) for unconstrained move.
+Use enter to complete the move, or escape to abort.`);
+    return true;
+  }
+
+  /**
+   * Action to move the item being moved in the given direction,
+   * without constraint.
+   *
+   * @param workspace The workspace to move on.
+   * @param xDirection -1 to move left. 1 to move right.
+   * @param yDirection -1 to move up. 1 to move down.
+   * @returns True iff this action applies and has been performed.
+   */
+  override moveUnconstrained(
+    workspace: WorkspaceSvg,
+    xDirection: number,
+    yDirection: number,
+  ): boolean {
+    if (!workspace) return false;
+    const info = this.moves.get(workspace);
+    if (!info) throw new Error('no move info for workspace');
+
+    info.totalDelta.x +=
+      xDirection * this.UNCONSTRAINED_MOVE_DISTANCE * workspace.scale;
+    info.totalDelta.y +=
+      yDirection * this.UNCONSTRAINED_MOVE_DISTANCE * workspace.scale;
+
+    info.dragger.onDrag(new PointerEvent('pointermove'), info.totalDelta);
+    return true;
+  }
+}
+
+/**
+ * Information about the currently in-progress move for a given
+ * Workspace.
+ */
+class DragMoveInfo extends MoveInfo {
+  /** Total distance moved, in screen pixels */
+  totalDelta = new utils.Coordinate(0, 0);
+
+  constructor(
+    public readonly block: Block,
+    public readonly dragger: IDragger,
+  ) {
+    super(block);
+  }
+}

src/navigation_controller.ts

@@ -34,7 +34,7 @@ import {ExitAction} from './actions/exit';
 import {EnterAction} from './actions/enter';
 import {DisconnectAction} from './actions/disconnect';
 import {ActionMenu} from './actions/action_menu';
-import {Mover} from './actions/mover';
+import {DragMover as Mover} from './actions/drag_mover';
 
 const KeyCodes = BlocklyUtils.KeyCodes;
 const createSerializedKey = ShortcutRegistry.registry.createSerializedKey.bind(