This seems to be related to a lot of fixed position and scrolling problems, but the end result seems different than the others. As demonstrated in this pen the dropdown will get cut off when scrolling back up after the dropdown's parent is moved with transform.

Steps to Reproduce:

1. Visit this pen: http://codepen.io/nickhoobin/pen/QbzYdb
2. Open the dropdown
3. Move the mouse outside the dropdown, into the body
4. Scroll the body
5. The header will get transformed and move off of the page
6. While keeping the dropdown open scroll the body up until it is back at the top
7. The header will move back into place
8. The dropdown does not reposition itself
9. Scrolling the dropdown back up keeps the first option cut off
   

It might not seem like much,but if the dropdown is heavily themed and templated, a lot of information can get cut off.

Chrome seems to be the browser that is affected. Edge, Firefox, IE 11 seem to handle the transform a lot more gracefully.