Comment 2 for bug 1275519

This bug is caused by line 2125 in gtk-widgets.css:

[...]
.source-list.view:selected:focus,
[...]

http://bazaar.launchpad.net/~elementary-design/egtk/4.x/view/head:/gtk-3.0/gtk-widgets.css#L2125

A quick solution would be overriding the border. Adding this code should fix the problem:

.source-list.view.badge:selected:focus {
    border-width: 0;
}

To make the badge look completely flat:

.source-list.view.badge:selected:focus {
    border-width: 0;
    box-shadow: none;
}