I am now at proof concept patch version 012.  The patch applies to the master branch as of yesterday (07/07/2018).  Multiple fake cursors and crosshairs now use the same function (mc_move_it) to obtain the relevant X/HPOS coordinates.  This version provides better compatibility with horizontal scrolling.  Crosshairs now works with X11 and Windows.

1.  Download a current version of Emacs master branch.

2.  Reset the repository to 07/07/2018:  git reset --hard 98463bed07ecc9057094928ffdc2321eaaf3ab52

3.  Apply the attached patch from the root repository directory.

4.  Build a GUI version of Emacs:  X11, NS, or Windows.

5.  To try out real fake cursors with Magnar Sveen's library (multiple-fake-cursors), install that package first.

6.  Open crosshairs.el (attached) and evaluate the buffer.

7.  To play with crosshairs, type:  M-x +-mode

8.  To see an example of crosshairs playing nicely with tab stretches, open up something like simple.el and enable +-mode.

9.  To see an example of a variety of cursor styles and widths, type:  M-x mc-test

X11 and NS are much quicker than Windows.  At some point in the future, I will work on more efficient means of calculating X and HPOS and other ways to speed this up.  Before moving on to more complicated aspects, I will continue working on compatibility with horizontal scrolling, tab stretches, and word-wrap/window-wrap/truncate.

Keith