> Why
 > doesn't "the problem" happen in the trunk build with toolkit scroll
 > bars?  Is this perhaps because Martin's window changes add an extra
 > pixel or two, leaving enough space?

I don't think so.  For example here on Windows XP, emacs -Q with current
trunk, evaluating

(progn
   (custom-set-faces
    '(mode-line ((t (:background "#000040" :foreground "wheat" :box (:line-width 2 :color "#000040") :weight bold :family "Verdana"))))
    '(mode-line-inactive ((t (:inherit mode-line :background "grey48" :foreground "wheat" :box (:line-width 2 :color "#000040"))))))
   (setq calendar-week-start-day 1)
   (calendar))

gives screenshot calendar-1.png while evaluating

(progn
   (custom-set-faces
    '(mode-line ((t (:background "#000040" :foreground "wheat" :box (:line-width 2 :color "#000040") :weight bold :family "Verdana"))))
    '(mode-line-inactive ((t (:inherit mode-line :background "grey48" :foreground "wheat" :box (:line-width 2 :color "#000040"))))))
   (setq calendar-week-start-day 1)
   (sit-for 0)
   (calendar))

gives the expected screenshot calendar-2.png.

martin