From 9d8ba7ad7c17c65d5e2c0ea7e480f4d7320d365a Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Fri, 20 Mar 2026 22:26:50 +0000
Subject: [PATCH] fix: group MODE field change and disconnect events in
 if_clause for correct undo
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When the user changes an if_clause block's MODE dropdown (e.g. IF → ELSE)
while a following block needs to be disconnected, validateAndDisconnectInvalidChain_
was firing BLOCK_MOVE events before Blockly fired the BLOCK_CHANGE for the field.
With no active event group, these landed in separate undo entries, requiring two
Ctrl+Z presses to undo a single user action.

Fix: the dropdown validator now opens an event group when none is already active,
and the onchange handler closes it once the corresponding BLOCK_CHANGE fires. This
ensures the disconnect and the field change are always a single undo entry.

https://claude.ai/code/session_013VxihhypLXT63EyPWiBk6j
---
 blocks/control.js | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/blocks/control.js b/blocks/control.js
index 29a88e1f..d952c0fe 100644
--- a/blocks/control.js
+++ b/blocks/control.js
@@ -461,6 +461,10 @@ export function defineControlBlocks() {
       // True while Blockly is constructing/rehydrating the block graph.
       this._isRestoring = true;
 
+      // Tracks a group we opened in the MODE validator so we can close it
+      // once Blockly fires the corresponding BLOCK_CHANGE event.
+      this._modeChangeGroupId = null;
+
       this.appendDummyInput("HEAD").appendField(
         new Blockly.FieldDropdown(
           [
@@ -469,6 +473,14 @@ export function defineControlBlocks() {
             ["%{BKY_CONTROLS_IF_MSG_ELSE}", MODE.ELSE],
           ],
           (newValue) => {
+            // If there's no active event group, open one so that any
+            // BLOCK_MOVE events fired by validateAndDisconnectInvalidChain_
+            // land in the same undo entry as the BLOCK_CHANGE that Blockly
+            // fires for this field after the validator returns.
+            if (!Blockly.Events.getGroup() && !this._modeChangeGroupId) {
+              Blockly.Events.setGroup(true);
+              this._modeChangeGroupId = Blockly.Events.getGroup();
+            }
             this.updateShape_(newValue);
             return newValue;
           },
@@ -585,6 +597,22 @@ export function defineControlBlocks() {
         return;
       }
 
+      // Close the event group we opened in the MODE validator once Blockly
+      // has fired the corresponding BLOCK_CHANGE, so that the field change
+      // and any prior disconnect events are a single undo entry.
+      if (
+        this._modeChangeGroupId &&
+        event.type === Blockly.Events.BLOCK_CHANGE &&
+        event.element === "field" &&
+        event.name === "MODE" &&
+        event.blockId === this.id
+      ) {
+        if (Blockly.Events.getGroup() === this._modeChangeGroupId) {
+          Blockly.Events.setGroup(false);
+        }
+        this._modeChangeGroupId = null;
+      }
+
       const isRelevantEvent =
         event.type === Blockly.Events.BLOCK_MOVE ||
         event.type === Blockly.Events.BLOCK_CHANGE ||