Confirming with Build 2002061707, WinXP. When the blue layer is placed inside the visible area (i.e. by setting style.left to 0) it is painted with the right background color while it is painted with a white background with the code given in the bug report. When only part of the blue layer is created on screen (i.e. by subtracting 10 from the variable left in the given code) this part (10px in this case) has a blue background while the part off screen has a white background.
The two "off-screen" divs are very narrow, since "right" basically defaults to "do not go outside the containing block". As a result, the text overflows the divs (this is trivial to verify by setting overflow:hidden on them). The actual divs are correctly colored blue. Marking invalid, since behavior is correct.