Clashes with mouse down events creating the element

It seems like one of the main use cases for this library is creating modals / popups / etc when a button is pressed, but if you happen to use the `onMouseDown` event handler to trigger that behaviour, `onClickOutside` will immediately trigger its own handler.

I've created a very simple sandbox to demonstrate this: https://codesandbox.io/s/musing-johnson-h8qhf

The expected behaviour is that clicking any of those buttons should display the extra `div`, and, once it is displayed, clicking anywhere outside the div should hide it. However, the first button triggers the extra `div` to be added and then immediately removed.