This is due to a problem in Explorer / IE and how they try to avoid excessive repaints on the toolbar when resizing.

If you look very carefully you will see similar issues with Windows XP style.  Resize the window small and move another window over it to ensure it has fully repainted.  Then expand it really wide.  Now look near the horizontal lines and you will see it has not repainted them correctly.

If you have the address bar on the right then it is even more obvious.  Move another window up over the bottom half of the toolbar and then away again and you should see a visible difference between the repainted area and the non repainted area.

It is less obvious with Windows XP style simply because the toolbar background graphic colours are all very similar.