bug#17619: 24.3; js-indent-line mixes display position with character position

[uu1101]

`js-indent-line' mixes up display column with character count. This
results in incorrect indentation when both differ. For example, I am
using the following snippet to ``prettify'' function keywords, resulting
in the characters 'function' being displayed as a single 'f':

-- >8 --
(font-lock-add-keywords
      'js-mode `(("\\(function *\\)("
                  (0 (progn (compose-region (match-beginning 1)
                                            (match-end 1) "\u0192")
                            nil)))))
-- >8 --

The current implementation of `js-indent-line' is the following:

-- >8 --
(defun js-indent-line ()
  "Indent the current line as JavaScript."
  (interactive)
  (save-restriction
    (widen)
    (let* ((parse-status
            (save-excursion (syntax-ppss (point-at-bol))))
           (offset (- (current-column) (current-indentation))))
      (indent-line-to (js--proper-indentation parse-status))
      (when (> offset 0) (forward-char offset)))))
-- >8 --

Please, notice how `current-column', which returns the _display_
position is used to calculate the offset, but then the result is used
with `forward-char' which expects a character offset instead.

I am using the following modification to work-around the issue, although
it is not correct either: it mixes character count with display offset
returned by `current-indentation'. It fixes the issue with my font-lock
customization.

-- >8 --
(defun js-indent-line ()
 "Indent the current line as JavaScript."
 (interactive)
 (save-restriction
   (widen)
   (let* ((parse-status (save-excursion (syntax-ppss (point-at-bol))))
          (current-char-position (save-excursion
                                   (let ((final (point)))
                                     (beginning-of-line)
                                     (- final (point)))))
          (offset (- current-char-position (current-indentation))))
     (indent-line-to (js--proper-indentation parse-status))
     (when (> offset 0) (forward-char offset)))))
-- >8 --
 
Best regards,
Urbano Ustero.