In r54084 I've noticed that text selection by dragging either refuses to work or selects the incorrect text when the source view is scrolled horizontally. This doesn't happen in a build of WebKit from earlier in the week (the source view wraps lines rather than providing for scrolling). Steps to reproduce: 1) Inspect a page, then switch to the Content view of the main resource. 2) Resize the window slightly to ensure that you have a horizontal scrollbar. 3) Scroll to the right by a small amount. 4) Attempt to select some text in the source using the mouse. Result: Depending on how far you're scrolled and which text you attempt to select, you'll either end up creating a selection to the right of where you intended, or no selection at all.
<rdar://problem/7594374>
Fixed in http://trac.webkit.org/changeset/54133. https://bugs.webkit.org/show_bug.cgi?id=34400