Description:
Apparently, mode-line width does not respect window-divider. I could not
find an offending commit, so I am not sure for how long this issue
exists. I am currently debugging the Emacs internals to find the root
cause.

Steps:
1) emacs -Q (master 690f7ac86ad9a9d714b1107d05c5e856a43bb18d)

2) Eval:

(progn
  (set-face-attribute 'mode-line nil
                      :background "yellow"
                      :box '(:line-width 4 :color "orange"))

  (set-face-attribute 'mode-line-inactive nil
                      :background "yellow"
		      :box '(:line-width 4 :color "orange"))

  (setopt window-divider-default-places t
          window-divider-default-bottom-width 12
          window-divider-default-right-width 12)

  (set-face-attribute 'window-divider nil :foreground "green")
  (set-face-attribute 'window-divider-last-pixel nil :foreground "green")
  (set-face-attribute 'window-divider-first-pixel nil :foreground "green")

  (window-divider-mode 1))

3) Open some windows with C-x 3

Expected: mode-line width is properly displayed considering the
window-divider width (orange borders in mode-line are properly
displayed).

Actual: mode-line width display does not properly consider the
window-divider width (missing orange right border in mode-line).

See red circle in the attached image.