Mouse down inside timeline window on the labels bar and try to drag the window.
The problem is in the following part of http://trac.webkit.org/changeset/108502: .timeline-overview-window-rulers { top: 0; bottom: 0; @@ -115,7 +131,7 @@ opacity: 0.2; border-right: 1px solid black; border-left: 1px solid black; - z-index: 150; + z-index: 250; }
Created attachment 128673 [details] Patch
Committed r108754: <http://trac.webkit.org/changeset/108754>