Right click on anything, and click Inspect Element. The web inspector pops up, and everything except the selected element will be grey. If you change tabs, the grey stays, and the same area of the screen is still left not-grey. If you resize the window, the grey stays the same size, even if it extends beyond the window. Closing either the Inspector or the tab where the element is (which in turn closes the inspector) makes the grey go away.
*** This bug has been marked as a duplicate of 14254 ***