'Create Issue Board' dropdown menu positioned incorrectly

Summary

When creating a new issue board, the milestone dropdown menu is in the wrong position.

Steps to reproduce

Go to https://gitlab.com/gitlab-org/gitlab-ce and select Issues from the menu, then Boards from the sub-menu

Click the Board selection dropdown on the far left to 'Create a new board'

Click the 'milestone' dropdown to select a milestone for your new board

What is the current bug behavior?

The dropdown is positioned at the bottom of the screen and remains fixed even when scrolling the page.

What is the expected correct behavior?

The dropdown should appear directly below the dropdown selector

Relevant logs and/or screenshots

Seen on iOS Chrome Version 58.0.3029.110 (64-bit)

cc: @iamphill