Re: Add a function that returns pixel distance between points?

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 02 Feb 2021 18:29:15 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: emacs-devel@gnu.org
> 
> > From: Yuan Fu <casouri@gmail.com>
> > Date: Mon, 1 Feb 2021 18:00:18 -0500
> > Cc: emacs-devel <emacs-devel@gnu.org>
> > 
> > This code inserts a mini table
> > 
> > | looooong |
> > | world |
> > 
> > And tries to aligns it to
> > 
> > | looooong |
> > | world    |
> > 
> > You will notice there are extra white space on the right of the cells, and the table actually looks like this:
> > 
> > | loooooong    |
> > | world        |
> > 
> > That’s because window-text-pixel-size returned a width that’s larger than the true with of the cells. If you enlarge the line-prefix, the extra space grows.
> 
> Hmm... this sounds like some bug in window-text-pixel-size.  Let me
> take a closer look.

It turned out the function wasn't designed to handle text that spans
less than a single screen line.

Does the patch below fix your problems?

--- src/xdisp.c~0	2020-04-15 14:22:25.000000000 +0300
+++ src/xdisp.c	2021-02-03 16:20:03.222230600 +0200
@@ -10428,8 +10428,22 @@ include the height of both, if present, 
      same directionality.  */
   it.bidi_p = false;
 
+  /* Start at the beginning of the line containing FROM.  Otherwise
+     IT.current_x will be incorrect.  */
+  reseat_at_previous_visible_line_start (&it);
+  it.current_x = it.hpos = 0;
+  if (IT_CHARPOS (it) != start)
+    move_it_to (&it, start, -1, -1, -1, MOVE_TO_POS);
+
+  /* Now move to TO.  */
+  int start_x = it.current_x;
   int move_op = MOVE_TO_POS | MOVE_TO_Y;
   int to_x = -1;
+  it.current_y = 0;
+  /* If FROM is on a newline, pretend that we start at the beginning
+     of the next line.  */
+  if (FETCH_BYTE (start) == '\n')
+    it.current_x = 0;
   if (!NILP (x_limit))
     {
       it.last_visible_x = max_x;
@@ -10472,6 +10486,11 @@ include the height of both, if present, 
         x = max_x;
     }
 
+  /* If text spans more than one screen line, we don't need to adjust
+     the x-span for start_x.  */
+  if (it.current_y > 0)
+    start_x = 0;
+
   /* Subtract height of header-line which was counted automatically by
      start_display.  */
   y = it.current_y + it.max_ascent + it.max_descent
@@ -10500,7 +10519,7 @@ include the height of both, if present, 
   if (old_b)
     set_buffer_internal (old_b);
 
-  return Fcons (make_fixnum (x), make_fixnum (y));
+  return Fcons (make_fixnum (x - start_x), make_fixnum (y));
 }
 \f
 /***********************************************************************