When the browser height is smaller, various modal dialogues' (for example, the "New Card" button in greenhopper) buttons are below the browser's viewport and the browser is unable to scroll to see them.

The attached screenshot displays the problem – note the lack of scrollbar on the right. My screen resolution on my laptop, from which the screenshot is taken, is 1280x800.

The forum thread at the following link also describes the problem:
http://forums.atlassian.com/thread.jspa?threadID=49969&tstart=0