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Subject: bug#77452: 31.0.50; Wide characters in left margin on ttys
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in emacs -Q -nw, eval=20

  (defun foo ()
    (interactive)
    (with-current-buffer (get-buffer-create "*foo*")
      (setq left-margin-width 2)
      (erase-buffer)
      (insert "abc")
      (insert (propertize "def" 'display '((margin left-margin) "1=F0=9F=92=
=A1")))))

and M-x foo.

This creates a buffer "*foo*" displaying a string in a left
margin area of width 2. The string consists of 2 characters but has a displ=
ay
width of 3 since the light bulb character has a display width of 2.

  (char-width ?=F0=9F=92=A1)
   =3D> 2

So, writing the left margin string to the terminal advances the output
cursor by 3 not 2. This confuses the display engine, as one can witness
by editing in the *foo* buffer, adding new lines etc.


In GNU Emacs 31.0.50 (build 1, aarch64-apple-darwin24.3.0, NS
 appkit-2575.40 Version 15.3.2 (Build 24D81)) of 2025-04-02 built on
 pro2
Repository revision: 71b3298c0e813ba1432e75370c460eea5caf72d5
Repository branch: master
System Description:  macOS 15.3.2

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To: Gerd =?UTF-8?Q?M=C3=B6llmann?= <gerd.moellmann@HIDDEN>
Subject: bug#77452: Acknowledgement (31.0.50; Wide characters in left
 margin on ttys)
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Subject: bug#77452: 31.0.50; Wide characters in left margin on ttys
Resent-From: Eli Zaretskii <eliz@HIDDEN>
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> From: Gerd Möllmann <gerd.moellmann@HIDDEN>
> Date: Wed, 02 Apr 2025 11:52:46 +0200
> 
> in emacs -Q -nw, eval 
> 
>   (defun foo ()
>     (interactive)
>     (with-current-buffer (get-buffer-create "*foo*")
>       (setq left-margin-width 2)
>       (erase-buffer)
>       (insert "abc")
>       (insert (propertize "def" 'display '((margin left-margin) "1💡")))))
> 
> and M-x foo.
> 
> This creates a buffer "*foo*" displaying a string in a left
> margin area of width 2. The string consists of 2 characters but has a display
> width of 3 since the light bulb character has a display width of 2.
> 
>   (char-width ?💡)
>    => 2
> 
> So, writing the left margin string to the terminal advances the output
> cursor by 3 not 2. This confuses the display engine, as one can witness
> by editing in the *foo* buffer, adding new lines etc.

The problem is not the char-width of 💡 nor with advancement of the
cursor, the problem is that the string "1💡" doesn't fit in the
2-column wide margin, and Emacs should have displayed only "1", since
we cannot clip glyphs on TTY frames.

Are you going to work on fixing this long-time problem?  Or should I?

Message sent to bug-gnu-emacs@HIDDEN:

Subject: bug#77452: 31.0.50; Wide characters in left margin on ttys
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To: Eli Zaretskii <eliz@HIDDEN>
Cc: 77452 <at> debbugs.gnu.org
From: Gerd =?UTF-8?Q?M=C3=B6llmann?= <gerd.moellmann@HIDDEN>
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Date: Wed, 02 Apr 2025 15:04:55 +0200
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Eli Zaretskii <eliz@HIDDEN> writes:

>> From: Gerd M=C3=B6llmann <gerd.moellmann@HIDDEN>
>> Date: Wed, 02 Apr 2025 11:52:46 +0200
>>=20
>> in emacs -Q -nw, eval=20
>>=20
>>   (defun foo ()
>>     (interactive)
>>     (with-current-buffer (get-buffer-create "*foo*")
>>       (setq left-margin-width 2)
>>       (erase-buffer)
>>       (insert "abc")
>>       (insert (propertize "def" 'display '((margin left-margin) "1=F0=9F=
=92=A1")))))
>>=20
>> and M-x foo.
>>=20
>> This creates a buffer "*foo*" displaying a string in a left
>> margin area of width 2. The string consists of 2 characters but has a di=
splay
>> width of 3 since the light bulb character has a display width of 2.
>>=20
>>   (char-width ?=F0=9F=92=A1)
>>    =3D> 2
>>=20
>> So, writing the left margin string to the terminal advances the output
>> cursor by 3 not 2. This confuses the display engine, as one can witness
>> by editing in the *foo* buffer, adding new lines etc.
>
> The problem is not the char-width of =F0=9F=92=A1 nor with advancement of=
 the
> cursor, the problem is that the string "1=F0=9F=92=A1" doesn't fit in the
> 2-column wide margin, and Emacs should have displayed only "1", since
> we cannot clip glyphs on TTY frames.

Exactly, that pretty much what I meant :-). The wide character may not
be output to the terminal, otherwise display and matrices get out of
sync.

> Are you going to work on fixing this long-time problem?  Or should I?

I tried to read and understand display_line etc. this morning, but I'm afra=
id
there is too much going on there for me to find the cause of this in
reasonable time.

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> From: Gerd Möllmann <gerd.moellmann@HIDDEN>
> Cc: 77452 <at> debbugs.gnu.org
> Date: Wed, 02 Apr 2025 15:04:55 +0200
> 
> Eli Zaretskii <eliz@HIDDEN> writes:
> 
> > The problem is not the char-width of 💡 nor with advancement of the
> > cursor, the problem is that the string "1💡" doesn't fit in the
> > 2-column wide margin, and Emacs should have displayed only "1", since
> > we cannot clip glyphs on TTY frames.
> 
> Exactly, that pretty much what I meant :-). The wide character may not
> be output to the terminal, otherwise display and matrices get out of
> sync.
> 
> > Are you going to work on fixing this long-time problem?  Or should I?
> 
> I tried to read and understand display_line etc. this morning, but I'm afraid
> there is too much going on there for me to find the cause of this in
> reasonable time.

Does the below look right to you?

diff --git a/src/xdisp.c b/src/xdisp.c
index 2c676c0..1c54e7c 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -25685,7 +25685,7 @@ #define RECORD_MAX_MIN_POS(IT)					\
 
       /* Now, get the metrics of what we want to display.  This also
 	 generates glyphs in `row' (which is IT->glyph_row).  */
-      n_glyphs_before = row->used[TEXT_AREA];
+      n_glyphs_before = row->used[it->area];
       x = it->current_x;
 
       /* Remember the line height so far in case the next element doesn't
@@ -25732,6 +25732,7 @@ #define RECORD_MAX_MIN_POS(IT)					\
 	 the next one.  */
       if (it->area != TEXT_AREA)
 	{
+	  enum glyph_row_area area = it->area;
 	  row->ascent = max (row->ascent, it->max_ascent);
 	  row->height = max (row->height, it->max_ascent + it->max_descent);
 	  row->phys_ascent = max (row->phys_ascent, it->max_phys_ascent);
@@ -25740,6 +25741,14 @@ #define RECORD_MAX_MIN_POS(IT)					\
 	  row->extra_line_spacing = max (row->extra_line_spacing,
 					 it->max_extra_line_spacing);
 	  set_iterator_to_next (it, true);
+	  /* If we exhausted the glyphs of the marginal area...	 */
+	  if (it->area != area
+	      /* ...and the last character was multi-column...	*/
+	      && it->nglyphs > 1
+	      /* ...and not all of its glyphs fit in the marginal area... */
+	      && row->used[area] < n_glyphs_before + it->nglyphs)
+	      /* ...then reset back to the previous character.	*/
+	    row->used[area] = n_glyphs_before;
 	  /* If we didn't handle the line/wrap prefix above, and the
 	     call to set_iterator_to_next just switched to TEXT_AREA,
 	     process the prefix now.  */

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> Cc: 77452 <at> debbugs.gnu.org
> Date: Wed, 02 Apr 2025 17:47:47 +0300
> From: Eli Zaretskii <eliz@HIDDEN>
> 
> > I tried to read and understand display_line etc. this morning, but I'm afraid
> > there is too much going on there for me to find the cause of this in
> > reasonable time.
> 
> Does the below look right to you?

Sorry, please try the slightly more safe change below:

diff --git a/src/xdisp.c b/src/xdisp.c
index 2c676c0..33d7314 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -25685,7 +25685,7 @@ #define RECORD_MAX_MIN_POS(IT)					\
 
       /* Now, get the metrics of what we want to display.  This also
 	 generates glyphs in `row' (which is IT->glyph_row).  */
-      n_glyphs_before = row->used[TEXT_AREA];
+      n_glyphs_before = row->used[it->area];
       x = it->current_x;
 
       /* Remember the line height so far in case the next element doesn't
@@ -25732,6 +25732,7 @@ #define RECORD_MAX_MIN_POS(IT)					\
 	 the next one.  */
       if (it->area != TEXT_AREA)
 	{
+	  enum glyph_row_area area = it->area;
 	  row->ascent = max (row->ascent, it->max_ascent);
 	  row->height = max (row->height, it->max_ascent + it->max_descent);
 	  row->phys_ascent = max (row->phys_ascent, it->max_phys_ascent);
@@ -25740,6 +25741,15 @@ #define RECORD_MAX_MIN_POS(IT)					\
 	  row->extra_line_spacing = max (row->extra_line_spacing,
 					 it->max_extra_line_spacing);
 	  set_iterator_to_next (it, true);
+	  if (!FRAME_WINDOW_P (it->f)
+	      /* If we exhausted the glyphs of the marginal area...	 */
+	      it->area != area
+	      /* ...and the last character was multi-column...	*/
+	      && it->nglyphs > 1
+	      /* ...and not all of its glyphs fit in the marginalk area... */
+	      && row->used[area] < n_glyphs_before + it->nglyphs)
+	      /* ...then reset back to the previous character.	*/
+	    row->used[area] = n_glyphs_before;
 	  /* If we didn't handle the line/wrap prefix above, and the
 	     call to set_iterator_to_next just switched to TEXT_AREA,
 	     process the prefix now.  */

Last modified: Wed, 2 Apr 2025 15:00:03 UTC

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Copyright (C) 1999 Darren O. Benham, 1997 nCipher Corporation Ltd, 1994-97 Ian Jackson.