I'm using Firefox 27 on Ubuntu 12.04.

Steps to reproduce:

• Open any pull request in the 'Diff' tab, say https://stash.atlassian.com/projects/BIZPLAT/repos/intsys-utils/pull-requests/6/diff
• Note that the first visible line number is "1"
• Scroll down until the part of the header displaying the "Clone", "Fork", etc buttons has disappeared
• Note that the file has now jumped down over a hundred lines, in my case the first line is now "123". The "Merge", "Decline", "Edit" buttons are still visible and the diff view has not reached the top of the page yet.