Created attachment 723939 [details] Sample HTML showing the bug The process to reproduce the bug is the following: 1) Create a canvas in a page and draw anything inside it. 2) Absolute position an element on top of the canvas (with auto overflow) and make the scrollbars appear. 3) Scroll inside the element using the scrollbar or the mouse wheel. The rendering of the scrollbars becomes buggy, it looks like something is not refreshed until you mouse the mouse over the scrollbar again.
This was fixed by Bug 828146 in Firefox20beta and later.