Correct me if wrong, but I think it used to work like this. It is the default way of operating in many Windows applications at least.
You have a piece of text, you hold shift and click (with the left mouse button) to where you want to extend the selection. You now have a selection. Now you should be able to shift-click elsewhere to extend. That works. Now to reduce the selection shift-click inside the already selected text, this does NOT reduce the selection area as it does in many other applications.