fix: insertion marker position when connection is resized (#7426)

core/block_svg.ts

@@ -1536,6 +1536,32 @@ export class BlockSvg
     }
   }
 
+  /**
+ * Reposition a block after its connection has been resized, to match exactly the target block position.
+ * The block to position is usually either the first block in a dragged stack
+ * or an insertion marker.
+ *
+ * @param sourceConnection The connection on the moving block's stack.
+ * @param originalOffsetInBlock The connection original offset in its block, before the resize occured
+ * @internal
+ */
+  repositionAfterConnectionResize(
+    sourceConnection: RenderedConnection,
+    originalOffsetInBlock: Coordinate,
+  ) {
+    // We only need to position the new block if it's before the existing one,
+    // otherwise its position is set by the previous block.
+    if (
+      sourceConnection.type === ConnectionType.NEXT_STATEMENT ||
+      sourceConnection.type === ConnectionType.INPUT_VALUE
+    ) {
+      const dx = originalOffsetInBlock.x - sourceConnection.getOffsetInBlock().x;
+      const dy = originalOffsetInBlock.y - sourceConnection.getOffsetInBlock().y;
+
+      this.moveBy(dx, dy);
+    }
+  }
+
   /**
    * Find all the blocks that are directly nested inside this one.
    * Includes value and statement inputs, as well as any following statement.

core/insertion_marker_manager.ts

@@ -616,8 +616,11 @@ export class InsertionMarkerManager {
     // Connect() also renders the insertion marker.
     imConn.connect(closest);
 
+    let originalOffsetInBlock = imConn.getOffsetInBlock().clone();
+    const imConnConst = imConn;
     renderManagement.finishQueuedRenders().then(() => {
       insertionMarker?.getSvgRoot().setAttribute('visibility', 'visible');
+      insertionMarker?.repositionAfterConnectionResize(imConnConst, originalOffsetInBlock);
     });
 
     this.markerConnection = imConn;