Clicking a <button> (which has a user agent rule of -webkit-user-select: ignore) clears the selection in contenteditable areas. This makes it impossible to create a editing toolbar. Note that non-editable selections aren't cleared in the document.
Created attachment 14797 [details] Testcase
This is a regression I believe from when this first went in to open source...
I'll take this one.
<rdar://problem/5240265>
Fixed in r24334 and r24336