From cd88f6de4be1f8eba1db038b371d769f584be53b Mon Sep 17 00:00:00 2001
From: YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>
Date: Tue, 27 Sep 2022 09:20:58 +0900
Subject: [PATCH] Adjust zero-width grapheme clusters so they are displayed
 (Bug#50951)

* src/composite.c (composition_gstring_adjust_zero_width): New function.
* src/composite.h: Declare it.
* src/font.c (Ffont_shape_gstring): Use it before putting gstring to cache.
---
 src/composite.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
 src/composite.h |  1 +
 src/font.c      |  1 +
 3 files changed, 46 insertions(+)

diff --git a/src/composite.c b/src/composite.c
index 249d7587f61..0417bc866d4 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -800,6 +800,50 @@ composition_gstring_width (Lisp_Object gstring, ptrdiff_t from, ptrdiff_t to,
   return width;
 }
 
+/* Adjust the width of each grapheme cluster of GSTRING because
+   zero-width grapheme clusters are not displayed.  If the width is
+   zero, then the width of the last glyph in the cluster is
+   incremented.  */
+
+void
+composition_gstring_adjust_zero_width (Lisp_Object gstring)
+{
+  ptrdiff_t from = 0;
+  int width = 0;
+
+  for (ptrdiff_t i = 0; ; i++)
+    {
+      Lisp_Object glyph;
+
+      if (i == LGSTRING_GLYPH_LEN (gstring)
+	  || (glyph = LGSTRING_GLYPH (gstring, i),
+	      (NILP (glyph) || from != LGLYPH_FROM (glyph))))
+	{
+	  eassert (i > 0);
+	  Lisp_Object last = LGSTRING_GLYPH (gstring, i - 1);
+
+	  if (width == 0)
+	    {
+	      if (NILP (LGLYPH_ADJUSTMENT (last)))
+		LGLYPH_SET_ADJUSTMENT (last,
+				       CALLN (Fvector,
+					      make_fixnum (0), make_fixnum (0),
+					      make_fixnum (LGLYPH_WIDTH (last)
+							   + 1)));
+	      else
+		ASET (LGLYPH_ADJUSTMENT (last), 2,
+		      make_fixnum (LGLYPH_WADJUST (last) + 1));
+	    }
+	  if (i == LGSTRING_GLYPH_LEN (gstring) || NILP (glyph))
+	    break;
+	  from = LGLYPH_FROM (glyph);
+	  width = 0;
+	}
+      width += (NILP (LGLYPH_ADJUSTMENT (glyph))
+		? LGLYPH_WIDTH (glyph) : LGLYPH_WADJUST (glyph));
+    }
+}
+
 
 static Lisp_Object gstring_work;
 static Lisp_Object gstring_work_headers;
diff --git a/src/composite.h b/src/composite.h
index d77dd0d5062..8a6fd203d09 100644
--- a/src/composite.h
+++ b/src/composite.h
@@ -340,6 +340,7 @@ extern Lisp_Object composition_gstring_from_id (ptrdiff_t);
 extern bool composition_gstring_p (Lisp_Object);
 extern int composition_gstring_width (Lisp_Object, ptrdiff_t, ptrdiff_t,
                                       struct font_metrics *);
+extern void composition_gstring_adjust_zero_width (Lisp_Object);
 
 extern bool find_automatic_composition (ptrdiff_t, ptrdiff_t, ptrdiff_t,
 					ptrdiff_t *, ptrdiff_t *,
diff --git a/src/font.c b/src/font.c
index 413cb381eeb..defbb5084b5 100644
--- a/src/font.c
+++ b/src/font.c
@@ -4678,6 +4678,7 @@ GSTRING.  */)
       from = LGLYPH_FROM (glyph);
       to = LGLYPH_TO (glyph);
     }
+  composition_gstring_adjust_zero_width (gstring);
   return composition_gstring_put_cache (gstring, XFIXNUM (n));
 
  shaper_error:
-- 
2.39.2