RetinaJS and LESS : Background image doesn't show on iOS

I am trying to make a background image into a retina image using LESS CSS and RetinaJs:

in my index.html file :

<html>
    <head>
        <meta http-equiv="content-type" content="text/html; charset=UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=0, minimum-scale=1.0, maximum-scale=1.0">
        <meta name="apple-mobile-web-app-capable" content="yes">
        <meta name="apple-mobile-web-app-status-bar-style" content="black">
        [...]
        <link type="text/css" rel="stylesheet/less" href="resources/css/retina.less">
        <script type="text/javascript" src="resources/js/less-1.3.0.minjs" ></script>
        [...]
    </head>
    <body>
    [...]
        <script type="text/javascript" src="resources/js/retina.js"></script>
    </body>
</html>

in my retina.less file:

.at2x(@path, @w: auto, @h: auto) {
  background-image: url("@{path}");
  @at2x_path: ~`"@{path}".split('.').slice(0, "@{path}".split('.').length - 1).join(".") + "@2x" + "." + "@{path}".split('.')["@{path}".split('.').length - 1]`;

  @media all and (-webkit-min-device-pixel-ratio : 1.5) {
    background-image: url("@{at2x_path}");
    background-size: @w @h;
  }  
}


.topMenu {
    .at2x('../../resources/img/topMenuTitle.png');
}

I have both topMenuTitle.png (320px x 40px) and [email protected] (640px x 80px) in the same folder.

When test this code:

In Firefox i have the normal Background
In the XCode iPhone simulator I also have the normal Background
In the iPhone device, I don't have any background at all.
I'm using GWT if that matters.

Any suggestions ? Thanks.

Solution

A newer version of retina.less fixes the problem :

You'll need to replace

  background-image: url(@path);

with

   background-image: url("@{path}");

or else this will not compile on iOS