bug#30226: Fixing it->pixel_width / it->current_x when tabs and line numbers.

[Eli Zaretskii]

> Date:  Sun, 28 Jan 2018 11:52:48 -0800
> From:  Keith David Bershatsky <esq@lawlist.com>
> Cc:  30226@debbugs.gnu.org
> 
> > Bottom line: to get accurate values of pixel width, you need to
> > subtract it->current_x value at some position from current_x at the
> > next glyph position.  This is the only reliable way to obtain accurate
> > pixel width values when using the move_it_* functions.
> 
> Thank you, Eli, for the detailed explanation regarding what is happening 
> underneath the Emacs hood.  The "bottom line" method does not seem to be 
> working in this example to calculate the tab STRETCH because it.current_x of 
> the character that _follows_ the tab STRETCH is wrong when w->hscroll >= 2.  
> In the revised bug-30226 function below:  When w->hscroll >= 2, we take the 
> it.current_x of the letter "H" in "Hello-world" (which is ostensibly 84); 
> and, we subtract (it.first_visible_x + it.lnum_pixel_width).  This gives us a 
> purported tab STRETCH of 42; however, it should really be 35.

No, the method does work, you just used the results of the call to
move_it_in_display_line_to not as I intended.

> 
> A.  This is the first (1st) time we call the revised bug-30226 [w->hscroll == 
> 1]:
> 
> 1.  NOTHING
>     it.c (0)
>     w->hscroll (1)
>     it.current_x (0)
>     it.pixel_width (0)
> 
> 2.  TAB CHARACTER
>     it.c (187)
>     w->hscroll (1)
>     it.current_x (0)
>     it.pixel_width (7)
> 
> 3.  TAB STRETCH
>     it.c (9)
>     w->hscroll (1)
>     it.current_x (7)
>     it.pixel_width (49)
> 
> "Bottom Line" method -- pixel-width of tab STRETCH:  0
> 
> 4.  TAB STRETCH
>     it.c (9)
>     w->hscroll (1)
>     it.current_x (35)
>     it.pixel_width (42)
> 
> "Bottom Line" method -- pixel-width of tab STRETCH:  42
> 
> 5.  TEXT
>     it.c (72)
>     w->hscroll (1)
>     it.current_x (77)
>     it.pixel_width (7)

Observer how here, 77 - 35 (the previous value of current_x) gives you
42, which is what you want.

> B.  This is the second (2nd) time we call the revised bug-30226 [w->hscroll 
> == 2]:
> 
> 1.  NOTHING
>     it.c (0)
>     w->hscroll (2)
>     it.current_x (0)
>     it.pixel_width (0)
> 
> 2.  TAB CHARACTER
>     it.c (187)
>     w->hscroll (2)
>     it.current_x (0)
>     it.pixel_width (7)
> 
> 3.  TAB STRETCH
>     it.c (9)
>     w->hscroll (2)
>     it.current_x (7)
>     it.pixel_width (49)
> 
> "Bottom Line" method -- pixel-width of tab STRETCH:  42
> 
> 4.  TEXT
>     it.c (72)
>     w->hscroll (2)
>     it.current_x (84)
>     it.pixel_width (7)

And here, 84 - 7 gives you 77.  But since 84 is greater than
first_visible_x + lnum_pixel_width = 14 + 28 = 42, you must subtract
42 from the result, which gives 77 - 42 = 35, as you want.  All the
following results for w->hscroll > 2 will work the same as this one.

>        fprintf (stderr, "\n\"Bottom Line\" method -- pixel-width of tab 
> STRETCH:  %d\n",
>                 it.current_x - (it.first_visible_x + it.lnum_pixel_width));

This is not how I meant for you to calculate the width.  You need to
subtract consecutive values of current_x, and then correct the result
as explained above.