diff --git a/modules/parser.js b/modules/parser.js
index f73496153..42b1131cd 100644
--- a/modules/parser.js
+++ b/modules/parser.js
@@ -754,13 +754,13 @@ function getComments( rootNode ) {
 			match = timestamps[ nextTimestamp ][ 1 ];
 			range = {
 				startContainer: startNode.parentNode,
-				startOffset: Array.prototype.indexOf.call( startNode.parentNode.childNodes, startNode ),
+				startOffset: utils.childIndexOf( startNode ),
 				endContainer: node,
 				endOffset: match.index + match[ 0 ].length
 			};
 			sigRange = {
 				startContainer: firstSigNode.parentNode,
-				startOffset: Array.prototype.indexOf.call( firstSigNode.parentNode.childNodes, firstSigNode ),
+				startOffset: utils.childIndexOf( firstSigNode ),
 				endContainer: node,
 				endOffset: match.index + match[ 0 ].length
 			};
diff --git a/modules/utils.js b/modules/utils.js
index 31135b943..2963886a4 100644
--- a/modules/utils.js
+++ b/modules/utils.js
@@ -15,6 +15,20 @@ function getNativeRange( comment ) {
 	return nativeRange;
 }
 
+/**
+ * Get the index of a node in its parentNode's childNode list
+ *
+ * @param {Node} child
+ * @return {number} Index in parentNode's childNode list
+ */
+function childIndexOf( child ) {
+	var i = 0;
+	while ( ( child = child.previousSibling ) ) {
+		i++;
+	}
+	return i;
+}
+
 /**
  * Find closest ancestor element using one of the given tag names.
  *
@@ -48,6 +62,7 @@ function htmlTrim( str ) {
 
 module.exports = {
 	getNativeRange: getNativeRange,
+	childIndexOf: childIndexOf,
 	closestElement: closestElement,
 	htmlTrim: htmlTrim
 };
diff --git a/tests/qunit/testUtils.js b/tests/qunit/testUtils.js
index 31d496005..ea1147418 100644
--- a/tests/qunit/testUtils.js
+++ b/tests/qunit/testUtils.js
@@ -1,3 +1,6 @@
+var
+	utils = require( 'ext.discussionTools.init' ).utils;
+
 module.exports = {};
 
 /* eslint-disable qunit/no-commented-tests */
@@ -46,18 +49,6 @@ module.exports.overrideMwConfig = function ( config ) {
 	);
 };
 
-/**
- * Get the index of a node in its parentNode's childNode list
- *
- * @copyright 2011-2019 VisualEditor Team and others; see http://ve.mit-license.org
- *
- * @param {Node} node The node
- * @return {number} Index in parentNode's childNode list
- */
-function parentIndex( node ) {
-	return Array.prototype.indexOf.call( node.parentNode.childNodes, node );
-}
-
 /**
  * Get the offset path from ancestor to offset in descendant
  *
@@ -76,7 +67,7 @@ function getOffsetPath( ancestor, node, nodeOffset ) {
 			console.log( node, 'is not a descendant of', ancestor );
 			throw new Error( 'Not a descendant' );
 		}
-		path.unshift( parentIndex( node ) );
+		path.unshift( utils.childIndexOf( node ) );
 		node = node.parentNode;
 	}
 	return path;