Is some way to stop dragging when the pointer out of box?

[leogitpro]

Hi, I have a problem on draging event.

I have a simple dom tree:

<div id="parent" style="overflow:hidden">
    <div id="child" style="width: 100%; height:100%"></div>
</div>

The guesture event bind to div#child. the child dom only in the parent dom moving. and I don't want to set the bounds for div#child.
But I want stop the dragging when the pointer moved out of the div#parent.
The state object looks not include the pointer offset information. the offset: [x, y] not the truly the DOM offset values.
Because I don't know the div#parent width and height,
I don't know the div#child is scaled draged.
I only want to stop the dragging when the finger or pointer out of div#parent.
Who can give me a good idea.
Thanks very much!