Created attachment 249050 [details]
Open the attached test case.
Click on the button 'Red' and then click on the button 'Lime'.
Result: The area of the lime rectangle is only repainted and since the lime rectangle is smaller than the red rectangle, the bottom of the red rectangle is not cleared.
Expected: Changing the source of an iframe to an SVG with no intrinsic size should repaint its old rectangle.
Note: The dirt is cleared is the window is refreshed (resized for example).
The bug is not reproducible if the css "float" property is set to "none".