doctests use the wrong terminal width

While running selftests, it seems that the DocTest adapter uses the wrong width for the terminal. I'm guessing it is using the raw 'terminal_width' as its cutoff point, when it should use 'terminal_width -1'. This is what I see:

$ bzr selftest
/ [1/24114 in 0s] bzrlib.doc.api.DocFileTest(C:\Users\jameinel\dev\bzr\alt_work\bzrlib\doc\api\branc
- [1/24114 in 0s] bzrlib.doc.api.DocFileTest(C:\Users\jameinel\dev\bzr\alt_work\bzrlib\doc\api\branc
\ [1/24114 in 0s] bzrlib.doc.api.DocFileTest(C:\Users\jameinel\dev\bzr\alt_work\bzrlib\doc\api\branc
| [1/24114 in 0s] bzrlib.doc.api.DocFileTest(C:\Users\jameinel\dev\bzr\alt_work\bzrlib\doc\api\branc
/ [1/24114 in 0s] bzrlib.doc.api.DocFileTest(C:\Users\jameinel\dev\bzr\alt_work\bzrlib\doc\api\branc
[87/24114 in 37s] blackbox.test_branch.TestBranchStacked.test_branch_stacked_from_smart_server

Note that the blackbox tests seem to be properly truncated (so as to not cause accidental wrap) but the DocFileTest cases all cause an artificial line-feed.