Adjust .chosen-dropup class based on viewport scroll position

[JJJ]

harvesthq#2036 added screen edge detection, allowing for the drop down to drop up when necessary. Once open however, it is always either up or down, and does not react to a change in viewport scroll position.

This enables the awkward user experience depicted below:

As you can see, the drop down can be scrolled upwards beyond where it is visible. It can similarly be done dropped down by default, and will not drop up when space allows.

Steps to reproduce

1. Open a Chosen drop down
2. Scroll the page out of bounds
3. Notice it does not adapt to fit the viewport

Expected behavior

The drop down should adjust itself, either up or down.

Actual behavior

The drop down retains the state from which it was first made visible.

Environment

• Chosen Version: 2.0.0
• jQuery or Prototype Version: any
• Browser and Version: any
• OS and Version: any