How to record the line number pixel width for each window.

[Keith David Bershatsky]

I am working on feature requests 22873 (multiple fake cursors) and 17684 
(crosshairs and visible fill column, both of which able to vertically intersect 
characters at any screen X coordinate).  The latest patch is:

    VERSION: 022.005 [11/17/2019]

    https://debbugs.gnu.org/cgi/bugreport.cgi?bug=22873#176

Features 17684/22873 have been purposefully designed to obviate the need to 
call start_display; i.e., there is no need to move IT in order to obtain any 
values used in the implementation of 17684/22873.

I do not want to use the built-in function line_number_display_width, which 
fires up start_display and moves IT to figure out the line number pixel width 
(among other values).  Instead, I would like to devise a reliable method 
whereby the display engine records the value (line number pixel width) for each 
window (wherever features 17684/22873 are active), such that those values are 
reliably accessible when Emacs calls update_window (located in dispnew.c).

I have been using the snippet below to record the value of the line number 
pixel width for each window.  This value, however, is unreliable when dealing 
with a window containing folded/hidden text, such as an org-mode buffer.  When 
dealing with folded/hidden text (e.g., org-mode), the recorded value 
sporadically changes while moving the cursor (e.g. pressing the arrow keys) 
even though the contents of the window do not change.

How can I perfect the recorded value of the line number pixel width, and also 
avoid expressly firing up a new instance of start_display and moving IT for the 
sole purpose of determining the value for each window during update_window (in 
dispnew.c)?

Thanks,

Keith

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diff --git a/src/xdisp.c b/src/xdisp.c
index 2467b33..dd901da 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -22720,6 +22720,28 @@ maybe_produce_line_number (struct it *it)
        }
     }
 
+
+/* *************************************************************************** 
*/
+/* MULTIPLE-CURSORS */
+
+  struct buffer *b = XBUFFER (it->w->contents);
+  struct buffer *old_buffer = NULL;
+  /* Needed so that buffer-local values can be determined; e.g., when switching
+  to the minibuffer. */
+  if (b != current_buffer)
+    {
+      old_buffer = current_buffer;
+      set_buffer_internal (b);
+    }
+  it->w->mc.lnum_pixel_width = (!NILP (Vdisplay_line_numbers))
+                               ? tem_it.current_x
+                               : 0;
+  if (old_buffer)
+    set_buffer_internal (old_buffer);
+
+/* *************************************************************************** 
*/
+
+
   /* Record the width in pixels we need for the line number display.  */
   it->lnum_pixel_width = tem_it.current_x;
   /* Copy the produced glyphs into IT's glyph_row.  */