Created attachment 619460 [details] screenshot after regression If the head of the last column in the threadpane contains text which cannot displayed, e.g. 'Total in Thread' narrowed to 'Tot', there is now a horizontal scrollbar in the threadpane. This has been observed on SM/x86_64, SM/NT6.1 and TB/NT5.1. See Attachment 'screenshot after regression'. The timeframe of the regression suggested a backout of Changeset d871849ac3a3 which solved the problem. Unfortunately this changeset contains 95 hidden changesets and a backout is not longer possible because of new checkins.
A new build containing the latest checkins solved the problem for me. Marking as RESOLVED WORKSFORME.
In this case, I guess the patch in bug 749658 fixed this as well, the description sounds similar to this case here.