On some pages that are scrollable, scroll bars are displayed at the top left portion of a page after moving the mouse. See screenshot. I haven't come up with a test case for this. But I can reproduce it easily at http://www.mathies.com/mozilla/. I think this is the root cause for a number of a failing mochitests.

For consideration as a Defect or Change Story.

I have no clue why this was happening! If you were using precise input before getting in the page, all work fine. If you open the page using imprecise then switched to precise the mysterious scrollbars would show up. My guess is that it has to do with registering/unregistering cursor.css plus setting the overflow on browser.