diff --git a/lib/binding_web/test/tree-test.js b/lib/binding_web/test/tree-test.js
index 29993eb6..a79da588 100644
--- a/lib/binding_web/test/tree-test.js
+++ b/lib/binding_web/test/tree-test.js
@@ -396,14 +396,16 @@ function spliceInput(input, startIndex, lengthRemoved, newText) {
   ];
 }
 
+// Gets the extent of the text in terms of zero-based row and column numbers.
 function getExtent(text) {
   let row = 0;
-  let index;
-  for (index = 0; index !== -1; index = text.indexOf('\n', index)) {
-    index++;
+  let index = -1;
+  let lastIndex = 0;
+  while ((index = text.indexOf('\n', index + 1)) !== -1) {
     row++;
+    lastIndex = index + 1;
   }
-  return {row, column: text.length - index};
+  return {row, column: text.length - lastIndex};
 }
 
 function assertCursorState(cursor, params) {