huge-first-line-json.el

(defun my:count-chars-of-first-line ()
  (interactive)
  (save-excursion
    (save-restriction
      (goto-char (point-min))
      (let (beg end)
        (beginning-of-line)
        (setq beg (point))
        (end-of-line)
        (setq end (point))
        (when (interactive-p)
          (message "chars=%s in first line beg=%s end=%s" (- end beg) beg end))
        (- end beg)))))

(defun my:open-with-low-cost-mode--if-huge-first-line ()
  (interactive)
  "先頭行が長過ぎる場合に、論理行での移動を止める"
  (let ((threashold 2000))
    (when (>= (my:count-chars-of-first-line) threashold)
      (fundamental-mode)
      (message "huge first line, so setq-local line-mode-visual nil")
      (setq-local line-move-visual nil) ;; C-nでの移動は論理行ではなく物理行にする
      )))


(with-eval-after-load 'js
  (add-hook 'js-json-mode-hook 'my:open-with-low-cost-mode--if-huge-first-line))