How to calculate X when dealing with truncate-lines non-nil.

[Keith David Bershatsky]

In developing my own feature requests to draw crosshairs (#17684) using 
multiple fake cursors (#22873), I have reached a roadblock in understanding why

move_it_to (&it, PT, it.last_visible_x, it.last_visible_y - 1, -1, MOVE_TO_POS 
| MOVE_TO_X | MOVE_TO_Y);

is giving me an it.current_x of non-existent coordinates to the imaginary right 
of the visible window when truncate-lines is non-nil.

For example, my screen is 1920 x 1080.  When PT is somewhere to the right of 
the initial visible window, I'm getting X coordinates to a tune of 8,000 and so 
forth.

How can I obtain the correct X, Y, HPOS, VPOS when dealing with a non-nil 
truncate-lines situation?

[Everything works as expected when truncate-lines is nil.]

Thanks,

Keith