How to prevent font-lock from being lazy?

Question

Related to this question.

Here's my current font lock setup for python-mode:

(require 'org)
(defconst lpy-font-lock-keywords
  '(("^#\\(\\* .*\\)$" 1 'org-level-1 prepend)
    ("^#\\(\\*\\* .*\\)$" 1 'org-level-2 prepend)
    ("^#\\(\\*\\*\\* .*\\)$" 1 'org-level-3 prepend)
    ("^#\\(\\*\\*\\*\\* .*\\)$" 1 'org-level-4 prepend)
    ("^#\\(\\*\\*\\*\\*\\* .*\\)$" 1 'org-level-5 prepend)
    (lpy-outline-comment-highlight 1 'default prepend)
    ("`\\([^\n']+\\)'" 1 font-lock-constant-face prepend)))

(font-lock-add-keywords 'python-mode lpy-font-lock-keywords)
(defun lpy-outline-comment-highlight (limit)
  (when (re-search-forward "^#\\(?:[^*]\\|$\\)" limit t)
    (let* ((pt (point))
           (success (save-excursion
                      (and (re-search-backward "^#\\*" nil t)
                           (null (re-search-forward "^[^#]" pt t))))))
      (when success
        (set-match-data (list (line-beginning-position) (line-end-position)
                              (point) (line-end-position)))
        (end-of-line)
        t))))

The code appears to do what I want (have comment bodies, but not starters, in default face for comments that belong to outlines), but only for small buffers, or places that I modify.

For larger buffers (larger than one screen), font-lock will be lazy and not apply the required faces. However, if I modify the text around that region, it will be highlighted correctly.

In the image above, font-lock is being lazy in the "Multi-dimensional indexing outline". So I moved a point there and entered a space - the highlight became correct.

How do I force font-lock to highlight every line on the screen?

Answer 1

[ Indeed, your rules are "single line", so it's not among the problems described in (elisp)Font Lock Multiline. It really doesn't have anything to do with "laziness". ]

The problem is quite simple: in your lpy-outline-comment-highlight function, if you find a match but you decide that success nil, then you return nil, which will tell font-lock to stop looking any further.

IOW you need to replace your when with a loop.

Solution:

(defun lpy-outline-comment-highlight (limit)
  (catch 'done
    (while (re-search-forward "^#\\(?:[^*\n]\\)" limit t)
      (let* ((pt (point))
             (success (save-excursion
                        (and (re-search-backward "^#\\*" nil t)
                             (null (re-search-forward "^[^#]" pt t))))))
        (when success
          (set-match-data (list (line-beginning-position) (line-end-position)
                                (point) (line-end-position)))
          (end-of-line)
          (throw 'done t))))))

Answer 2

You can use font-lock-extend-region-functions. It contains a list of functions that extend the region that is rehighlighted. You can add your own function to it that extends the region to include the full comment, forcing your keywords to be applied correctly.