I tested patch1 somewhat randomly, opening 4/5 windows horizontally,
1/2 windows vertically, and did also some attempt splitting windows
in two frames. Played also a bit on the text-only Emacs (using MSYS shell).

  Everything worked fine with patch1, except for an unexpected small
and non-blocking issue (let's call it "diagonal-drag" issue) that
can be reproduced as follows:

- C-x 3 M-x ruler-mode (you should see two windows with ruler)
- click S-<mouse-1> on the ruler of the rightmost window, this will set
  a left margin, choose a width of around 10~20 chars
- drag "diagonally" a column, for example moving down-left the
  fill-column, until the cursor goes a few pixels below the ruler area
- you will see that the dragged column will "jump" innaturally to the
  left of around 10~20 chars

  Tentative diagnosys: function `ruler-mode-window-col' returns a
different column value depending if you are inside or outside the 
ruler area. The actual problem is probably inherited from `posn-col-row',
but I could not clarify any better.

  I wrote patch 2, that worked for me to solve the "diagonal-drag" issue.