DDHighlightCreateWithRectsInVisibleRectWithStyleAndDirection adjust the location of the button based on the visible rect, trying to keep the button visible. We're handing it the wrong visible rect, though, not taking scrolling into account. This leads to pages that scroll horizontally showing the button on the left even if there's space for it on the right, or sometimes not showing it at all. <rdar://problem/17907752>
Created attachment 236275 [details] patch
http://trac.webkit.org/changeset/172344