White box when tabbing across form fields

[jake74]

Has anyone seen this before, and if so, why does it happen?
http://dablackmarket.com/stage/main.html
Click the Contact Us form and tab from one field to another. Or fill in the "Go Black" form at the bottom of the page and tab between fields. What's the deal with that white box? Where does it come from? Why is it there? How do I kill it forever?

[jake74]

I figured out that the white box is a movie clip mask that is set dynamically. I don't know why tabbing makes it show up though.

[rhamej]

movieClipName.tabEnabled=false
or
btnName.tabEnabled=false