--- a/devtools/client/responsive.html/test/browser/head.js
+++ b/devtools/client/responsive.html/test/browser/head.js
@@ -187,18 +187,18 @@ function getElRect(selector, win) {
  * Drag an element identified by 'selector' by [x,y] amount. Returns
  * the rect of the dragged element as it was before drag.
  */
 function dragElementBy(selector, x, y, win) {
   let React = win.require("devtools/client/shared/vendor/react");
   let { Simulate } = React.addons.TestUtils;
   let rect = getElRect(selector, win);
   let startPoint = {
-    clientX: rect.left + Math.floor(rect.width / 2),
-    clientY: rect.top + Math.floor(rect.height / 2),
+    clientX: Math.floor(rect.left + rect.width / 2),
+    clientY: Math.floor(rect.top + rect.height / 2),
   };
   let endPoint = [ startPoint.clientX + x, startPoint.clientY + y ];
 
   let elem = win.document.querySelector(selector);
 
   // mousedown is a React listener, need to use its testing tools to avoid races
   Simulate.mouseDown(elem, startPoint);