Opencart - remove scrolling when adding products and keep the notification.

edit the common.js and make sure to use a copy in your custom theme.

header.tpl
change from:
<script src="catalog/view/javascript/common.js" type="text/javascript"></script>
to:
<script src="catalog/view/theme/[yourtheme]/javascript/common.js" type="text/javascript"></script>

in catalog/view/theme/[yourtheme]/javascript/common.js
comment ( // ) all the occurences of the following line 
//$('html, body').animate({ scrollTop: 0 }, 'slow');


in the css do the following:
/*add to the theme - stylesheet.css this part*/
.alert{
    position:fixed;
    width: 100%;
    top:0;
    left:0;
    z-index:10000;
}