emacs-left-margin-test-codes.el

;; handle left click on left window margin & return text in position
(global-set-key (kbd "<left-margin> <mouse-1>") (lambda (p) (interactive "e")
                                                  (message "%S" (posn-string (car (cdr p))))))
;; unset key bind
(global-unset-key (kbd "<left-margin> <mouse-1>"))

;; make overlay on window start pos & insert propertized text
;; mouse-face doesn't work but help echo works..... weird....
(let ((str (propertize "hello" 'face '(:foreground "cyan") 'help-echo "mouse-1: say hello" 'mouse-face 'highlight)))
  (setq  ov (make-overlay (window-start) (window-start)))
  (overlay-put ov 'before-string (propertize " " 'display `((margin left-margin) ,str))))

;; remove overlay
(delete-overlay ov)