bug#51821: 29.0.50; Suggest add variable or frame parameter: line-height

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 16 Nov 2021 18:45:16 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: tumashu@163.com, 51821@debbugs.gnu.org
> 
> > From: Lars Ingebrigtsen <larsi@gnus.org>
> > Cc: tumashu@163.com,  51821@debbugs.gnu.org
> > Date: Tue, 16 Nov 2021 16:20:52 +0100
> > 
> > > That shouldn't happen, normally.  Emacs selects fonts of the same size
> > > for a face.
> > 
> > No, not really.  Here's a trivial example:
> > 
> > 
> > Note how the lines with jidanni are taller than the rest.
> 
> OK, let me see what can be done about that.

Please try the patch below.  I have a very limited number of cases
with which I can test it, so I think users of CJK scripts should say
what they think.  Basically, set the new variable line-height-factor
to something like 1.2 (more if you enlarge the non-ASCII font to make
double-width characters take close to 2 columns), and see if you get
the desired effect.

If this is what people want, I will install it and document it.

diff --git a/src/window.c b/src/window.c
index e801ff8..a368b6d 100644
--- a/src/window.c
+++ b/src/window.c
@@ -5246,7 +5246,10 @@ grow_mini_window (struct window *w, int delta)
 {
   struct frame *f = XFRAME (w->frame);
   int old_height = window_body_height (w, true);
-  int min_height = FRAME_LINE_HEIGHT (f);
+  int min_height =
+    FLOATP (Vline_height_factor)
+    ? FRAME_LINE_HEIGHT (f) * XFLOAT_DATA (Vline_height_factor)
+    : FRAME_LINE_HEIGHT (f);
 
   eassert (MINI_WINDOW_P (w));
 
@@ -5279,7 +5282,11 @@ grow_mini_window (struct window *w, int delta)
 shrink_mini_window (struct window *w)
 {
   struct frame *f = XFRAME (w->frame);
-  int delta = window_body_height (w, true) - FRAME_LINE_HEIGHT (f);
+  int min_line_height =
+    FLOATP (Vline_height_factor)
+    ? FRAME_LINE_HEIGHT (f) * XFLOAT_DATA (Vline_height_factor)
+    : FRAME_LINE_HEIGHT (f);
+  int delta = window_body_height (w, true) - min_line_height;
 
   eassert (MINI_WINDOW_P (w));
 
diff --git a/src/xdisp.c b/src/xdisp.c
index ef49297..d1f4043 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -22073,6 +22073,18 @@ append_space_for_newline (struct it *it, bool default_face_p)
 		  it->descent = it->override_descent;
 		  boff = it->override_boff;
 		}
+	      else
+		{
+		  /* Enlarge the ascent to make the row higher by
+		     line-height-factor if needed.  */
+		  if (face == face->ascii_face && it->voffset == 0
+		      && FLOATP (Vline_height_factor))
+		    {
+		      int enlarged_height = ((it->ascent + it->descent)
+					     * XFLOAT_DATA (Vline_height_factor));
+		      it->ascent = enlarged_height - it->descent;
+		    }
+		}
 	      if (EQ (height, Qt))
 		extra_line_spacing = 0;
 	      else
@@ -30587,9 +30599,9 @@ gui_produce_glyphs (struct it *it)
 	  /* When no suitable font is found, display this character by
 	     the method specified in the first extra slot of
 	     Vglyphless_char_display.  */
-	      Lisp_Object acronym = lookup_glyphless_char_display (-1, it);
+	  Lisp_Object acronym = lookup_glyphless_char_display (-1, it);
 
-	      eassert (it->what == IT_GLYPHLESS);
+	  eassert (it->what == IT_GLYPHLESS);
 	  produce_glyphless_glyph (it, true,
 				   STRINGP (acronym) ? acronym : Qnil);
 	  goto done;
@@ -30613,6 +30625,15 @@ gui_produce_glyphs (struct it *it)
  	    {
  	      it->ascent = FONT_BASE (font) + boff;
  	      it->descent = FONT_DESCENT (font) - boff;
+	      /* If this glyph uses the ASCII face, enlarge the ascent
+		 to make the row higher by line-height-factor.  */
+	      if (face == face->ascii_face && it->voffset == 0
+		  && FLOATP (Vline_height_factor))
+		{
+		  int enlarged_height = ((it->ascent + it->descent)
+					 * XFLOAT_DATA (Vline_height_factor));
+		  it->ascent = enlarged_height - it->descent;
+		}
  	    }
 
 	  if (get_char_glyph_code (it->char_to_display, font, &char2b))
@@ -30763,6 +30784,13 @@ gui_produce_glyphs (struct it *it)
 		{
 		  it->ascent = FONT_BASE (font) + boff;
 		  it->descent = FONT_DESCENT (font) - boff;
+		  if (face == face->ascii_face && it->voffset == 0
+		      && FLOATP (Vline_height_factor))
+		    {
+		      int enlarged_height = ((it->ascent + it->descent)
+					     * XFLOAT_DATA (Vline_height_factor));
+		      it->ascent = enlarged_height - it->descent;
+		    }
 		}
 	    }
 
@@ -30886,6 +30914,13 @@ gui_produce_glyphs (struct it *it)
 		{
 		  it->ascent = FONT_BASE (font) + boff;
 		  it->descent = FONT_DESCENT (font) - boff;
+		  if (face == face->ascii_face && it->voffset == 0
+		      && FLOATP (Vline_height_factor))
+		    {
+		      int enlarged_height = ((it->ascent + it->descent)
+					     * XFLOAT_DATA (Vline_height_factor));
+		      it->ascent = enlarged_height - it->descent;
+		    }
 		}
 	      it->phys_ascent = it->ascent;
 	      it->phys_descent = it->descent;
@@ -31243,6 +31278,24 @@ gui_produce_glyphs (struct it *it)
 	  if (it->glyph_row
 	      && (metrics.lbearing < 0 || metrics.rbearing > metrics.width))
 	    it->glyph_row->contains_overlapping_glyphs_p = true;
+	  if (it->voffset == 0 && FLOATP (Vline_height_factor))
+	    {
+	      Lisp_Object font_object = LGSTRING_FONT (gstring);
+	      struct font *gstring_font = XFONT_OBJECT (font_object);
+
+	      /* This is for when the grapheme cluster displays some
+		 ligature using ASCII font: if the height is smaller
+		 than line-height-factor says, enlarge the ascent.  */
+	      if (face == face->ascii_face
+		  && face->ascii_face->font == gstring_font)
+		{
+		  int enlarged_height =
+		    ((FONT_BASE (gstring_font) + FONT_DESCENT (gstring_font))
+		     * XFLOAT_DATA (Vline_height_factor));
+		  if (metrics.ascent + metrics.descent < enlarged_height)
+		    metrics.ascent = enlarged_height - metrics.descent;
+		}
+	    }
 	  it->ascent = it->phys_ascent = metrics.ascent;
 	  it->descent = it->phys_descent = metrics.descent;
 	}
@@ -35581,6 +35634,16 @@ syms_of_xdisp (void)
 mini-window frame's default font's height.  */);
   Vmax_mini_window_height = make_float (0.25);
 
+  DEFVAR_LISP ("line-height-factor", Vline_height_factor,
+    doc: /* Factor for enlarging the height of lines that use the default font.
+The value should be a float number greater than 1.  It determines how
+much will Emacs enlarge the height of a screen line that shows only
+characters displayed with the default face's font for ASCII characters.
+This is to avoid differences in height between lines that use the
+ASCII font and those which use non-ASCII (for example, Chinese)
+font, which is typically higher than the ASCII one.  */);
+  Vline_height_factor = Qnil;
+
   DEFVAR_LISP ("resize-mini-windows", Vresize_mini_windows,
     doc: /* How to resize mini-windows (the minibuffer and the echo area).
 A value of nil means don't automatically resize mini-windows.