Login screen background image not displayed in web authentication window

Happens under Windows when launched from MSI insgtallation (but not from Eclipse!) and OS X from DMG installation (not tested from Eclipse), works fine under Linux.

Seems like the background: url('...') directive set on the body element is badly interpreted by the Qt embedded WebKit:

body {
  background: url('/nuxeo/img/login_bg.jpg') no-repeat center center fixed #333;
  ... 
}