Diagonal scrolling in a WebView does not repaint correctly.

To replicate, load a large (or zoomed) page into a WebView container so you have both horizontal and vertical scrollbars. Then pan diagonally (easily done on a touch screen device, or alternatively on Windows, pressing the middle mouse button to active pan mode). The screen is not redrawn and ends up looking a mess.

This is quite a big deal on systems such as the Microsoft Surface, where it is almost impossible _not_ to scroll diagonally when there are both horizontal and vertical scroll bars.