fix: do not create extra shadow blocks when mirroring events

core/events/events_block_create.ts

@@ -138,6 +138,7 @@ export class BlockCreate extends BlockBase {
           'the constructor, or call fromJson',
       );
     }
+    if (allShadowBlocks(workspace, this.ids)) return;
     if (forward) {
       blocks.append(this.json, workspace);
     } else {
@@ -154,6 +155,26 @@ export class BlockCreate extends BlockBase {
     }
   }
 }
+/**
+ * Returns true if all blocks in the list are shadow blocks. If so, that means
+ * the top-level block being created is a shadow block. This only happens when a
+ * block that was covering up a shadow block is removed. We don't need to create
+ * an additional block in that case because the original block still has its
+ * shadow block.
+ *
+ * @param workspace Workspace to check for blocks
+ * @param ids A list of block ids that were created in this event
+ * @returns True if all block ids in the list are shadow blocks
+ */
+const allShadowBlocks = function (
+  workspace: Workspace,
+  ids: string[],
+): boolean {
+  const shadows = ids
+    .map((id) => workspace.getBlockById(id))
+    .filter((block) => block && block.isShadow());
+  return shadows.length === ids.length;
+};
 
 export interface BlockCreateJson extends BlockBaseJson {
   xml: string;

tests/mocha/event_block_create_test.js

@@ -56,6 +56,42 @@ suite('Block Create Event', function () {
     chai.assert.equal(event.xml.tagName, 'shadow');
   });
 
+  test('Does not create extra shadow blocks', function () {
+    const shadowId = 'shadow_block';
+    const blockJson = {
+      'type': 'math_arithmetic',
+      'id': 'parent_with_shadow',
+      'fields': {'OP': 'ADD'},
+      'inputs': {
+        'A': {
+          'shadow': {
+            'type': 'math_number',
+            'id': shadowId,
+            'fields': {'NUM': 1},
+          },
+        },
+      },
+    };
+
+    // If there is a shadow block on the workspace and then we get
+    // a block create event with the same ID as the shadow block,
+    // this represents a block that had been covering a shadow block
+    // being removed.
+    Blockly.serialization.blocks.append(blockJson, this.workspace);
+    const shadowBlock = this.workspace.getBlockById(shadowId);
+    const blocksBefore = this.workspace.getAllBlocks();
+
+    const event = new Blockly.Events.BlockCreate(shadowBlock);
+    event.run(true);
+
+    const blocksAfter = this.workspace.getAllBlocks();
+    chai.assert.deepEqual(
+      blocksAfter,
+      blocksBefore,
+      'No new blocks should be created from an event that only creates shadow blocks',
+    );
+  });
+
   suite('Serialization', function () {
     test('events round-trip through JSON', function () {
       const block = this.workspace.newBlock('row_block', 'block_id');