Safari rendering glitch: shadows

Hello, I was developing a small HTML code, when I saw a glitch due to shadows and contenteditable boxes : When the contenteditable box gets a new line, it only actualise the shadow at the last line, so the shadow on the entire object looks glitchy. Thsi glitch disappear when the user click outside of the box, when the boxs content changed or when another element actualise the graphics near to it.

This glitch only works when the new line doesn't make the browser scroll.

If you want an example, see there (it's the small code I made) :

http://adri326.comli.com/live_edit/save_1/