Re: Calculating screen relative X when horizontal scrolling.

[Keith David Bershatsky]

In addition to the explanation in the previous post about temporarily hijacking 
w->phys_cursor.x and setting it with my own relative x before calling 
draw_window_cursor, the following screenshots may help to explain (by way of 
visual example) the relative x that I am seeking when horizontal scrolling:

(scroll-left 9):  https://lawlist.com/images/scroll_left_09.png

(scroll-left 10):  https://lawlist.com/images/scroll_left_10.png

(scroll-left 11):  https://lawlist.com/images/scroll_left_11.png

In the (scroll-left 9) screenshot, the following code renders the _correct_ 
relative x for the entire current line.

relative_x = it->current_x - ((window_hscroll_limited (it->w, it->f) * 
FRAME_COLUMN_WIDTH (it->f))

However, that same code does not work when I evaluate (scroll-left 10) or 
(scroll-left 11).  The problem appears to have something to do with the varying 
pixel widths of the different characters.

The same problem arises when temporarily or semi-permanently scrolling _only_ 
the current line; i.e., when either of the following tests are _true_:

/* EXAMPLE:  (scroll-left 5); and, then press the left arrow key one time.*/
bool hscl_temp_p = (!it->w->suspend_auto_hscroll
                    && auto_hscroll_mode_p
                    && it->w->hscroll > 0
                    && it->w->min_hscroll == 0);

/* EXAMPLE:  C-u C-x < and do something that causes the current line to be
             horizontally scrolled differently. */
bool hscl_perm_p = (!it->w->suspend_auto_hscroll
                    && auto_hscroll_mode_p
                    && it->w->hscroll > 0
                    && it->w->min_hscroll > 0
                    && it->w->min_hscroll != it->w->hscroll);