How wide can a sliding door be open, before it becomes a security risk?
If its open its going to be easier to break in whether its 1cm or 4inches. 

A security gate always did wonders for us in South Africa, pretty sure in many other countries too.



TIP: When you install the security gate, make sure that it cannot be screwed out from its sockets, form the OUTSIDE. Or tampered with in any other way from the outside.