And finally: I noticed another independent bug ("text-scale issue"),
i.e. most ruler's mouse operation have wrong offset if you adjust
text scale (C-x C-+).

  If anyone reading is interested to help, here's a proposed action plan:

- may you please try to play with patch1, and give advice if it could be
  acceptable for a commit 

- I guess that patch2 is unacceptable (BTW it changes the definition of
  ruler-mode-window-col, I assumed that ruler-mode.el is the only
  program calling it) but it's the best I could squeeze out of my head...
  any help or constructive criticism is welcome :)

- be aware of the "text-scale" issue but don't test for it; it seems
  a totally independent issue so I'd rather send another bug report for it

  Thanks for your attention, kindest regards.

  Andrea