Expand context region interactively

expand-context-region.el

(defun expand-context-region ()
  (interactive)
  (let* ((buffer (current-buffer))
         (saved-location (point))
         (origin (save-excursion
                   (forward-line 0)
                   (point))))
    (goto-char (line-beginning-position))
    (set-mark (save-excursion
                (forward-line 1)
                (point)))
    (activate-mark)
    (minibuffer-with-setup-hook
        (lambda () (add-hook
                    'after-change-functions
                    (lambda (_beg _end _len)
                      (when-let* ((content (minibuffer-contents))
                                  (context-line-count (string-to-number content)))
                        (with-current-buffer buffer
                          (with-selected-window (get-buffer-window buffer)
                            (deactivate-mark)
                            (goto-char origin)
                            (goto-char (save-excursion
                                         (goto-char origin)
                                         (forward-line (- context-line-count))
                                         (point)))
                            (set-mark (save-excursion
                                        (goto-char origin)
                                        (forward-line (1+ context-line-count))
                                        (point)))
                            (activate-mark)))))nil t))
      (condition-case nil
          (read-number "Select context lines: " 1)
        (quit
         (deactivate-mark)
         (goto-char saved-location))))))