`jit-lock-fontify-now' contains the line
(setq next (line-beginning-position 2))
`font-lock-default-fontify-region' contains the line
(setq end (line-beginning-position 2))
In practice this means that when jit-lock is turned on, every time you
type a character jit-lock refontifies the current _and_ the following
line. Chunks fontified stealthily always overlap by one line.
One might argue that `font-lock-after-change-function' contains
(progn (goto-char end) (forward-line 1) (point)))))))
but there the adjustment might be useful - albeit undocumented - since
it allows to correct the syntactic context for the following line.
jit-lock mode has `jit-lock-context-fontify' handle syntactic context
and unconditionally refontifying the second line is plain overkill.
In `font-lock-default-fontify-region' I'd therefore replace the lines:
(when font-lock-multiline
(setq end (or (text-property-any end (point-max)
'font-lock-multiline nil)
(point-max))))
(goto-char end)
(setq end (line-beginning-position 2))
;; Now do the fontification.
either by
(cond
(font-lock-multiline
(setq end (or (text-property-any end (point-max)
'font-lock-multiline nil)
(point-max)))
(goto-char end) (setq end (line-beginning-position 2)))
;; don't modify end in jit-lock-mode
((and (boundp 'jit-lock-mode) jit-lock-mode))
(t (goto-char end) (setq end (line-beginning-position 2))))
;; Now do the fontification.
or - but this might disturb people accustomed to the current behavior of
plain font-lock -
(when font-lock-multiline
(setq end (or (text-property-any end (point-max)
'font-lock-multiline nil)
(point-max)))
(goto-char end)
(setq end (line-beginning-position 2)))
;; Now do the fontification.
Finally one could introduce a variable `font-lock-lines-after'
(defcustom font-lock-lines-after 0
"*Number of lines after the changed text to include in refontification."
:type 'integer
:group 'font-lock
:version "22.1")
and use
(goto-char end)
(setq end (line-beginning-position (1+ font-lock-lines-after)))
(when font-lock-multiline
(setq end (or (text-property-any end (point-max)
'font-lock-multiline nil)
(point-max)))
(goto-char end)
(setq end (line-beginning-position 2)))
;; Now do the fontification.
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