config: add grapheme-width-method=default

This is the new default mode, and tries to do "the right thing". That
is, recognize and handle:

* Variation Selector 16 (graphical presentation)
* ZWJ sequences
* Skin tones and hair-styles
* Country flags

Things known to be incorrect:

* "Invalid" country letter combinations are still treated as
  flags. That is, the combined width is 2, even though it should be 4.

config.c

@@ -2371,7 +2371,7 @@ parse_section_tweak(struct context *ctx)
 
         return value_to_enum(
             ctx,
-            (const char *[]){"wcswidth", "double-width", "max", NULL},
+            (const char *[]){"default", "wcswidth", "double-width", "max", NULL},
             (int *)&conf->tweak.grapheme_width_method);
     }
 
@@ -2977,7 +2977,7 @@ config_load(struct config *conf, const char *conf_path,
 #if defined(FOOT_GRAPHEME_CLUSTERING) && FOOT_GRAPHEME_CLUSTERING
             .grapheme_shaping = fcft_caps & FCFT_CAPABILITY_GRAPHEME_SHAPING,
 #endif
-            .grapheme_width_method = GRAPHEME_WIDTH_WCSWIDTH,
+            .grapheme_width_method = GRAPHEME_WIDTH_DEFAULT,
             .delayed_render_lower_ns = 500000,         /* 0.5ms */
             .delayed_render_upper_ns = 16666666 / 2,   /* half a frame period (60Hz) */
             .max_shm_pool_size = 512 * 1024 * 1024,

config.h

@@ -326,6 +326,7 @@ struct config {
         bool overflowing_glyphs;
         bool grapheme_shaping;
         enum {
+            GRAPHEME_WIDTH_DEFAULT,
             GRAPHEME_WIDTH_WCSWIDTH,
             GRAPHEME_WIDTH_DOUBLE,
             GRAPHEME_WIDTH_MAX,

doc/foot.ini.5.scd

@@ -1229,21 +1229,63 @@ any of these options.
 
 *grapheme-width-method*
 	Selects which method to use when calculating the width
-	(i.e. number of columns) of a grapheme cluster. One of
-	*wcswidth*, *double-width* and *max*.
+	(i.e. number of columns) of a grapheme cluster. One of *default*,
+	*wcswidth*, *double-width* and *max*. See details below.
 	
-	*wcswidth* simply adds together the individual width of all
-	codepoints making up the cluster.
+	Background:
+	
+	Glyphs rendered in the terminal may be 1 or 2 columns wide. The
+	terminal emulator and the application running in the terminal must
+	agree what that width is, or you will see cursor
+	de-synchronization. This typically manifests itself as rendering
+	glitches and the cursor "jumping" when moving it over problematic
+	characters.
+	
+	Most glyphs consists of a single Unicode codepoint. In this case,
+	there is usually no issues.
+	
+	Some glyphs however consists of multiple codepoints. These are
+	called grapheme clusters, and can often be problematic.
+	
+	While there is a standard function to calculate the width of a
+	single codepoint (*wcwidth*(3p)), there is no such function for
+	calculating the width of a grapheme cluster.
+	
+	The closest thing we have is *wcswidth*(3p), that calculates the
+	width of a string. It is usually implemented by adding together
+	the widths of all codepoints that make up the string. This is not
+	correct for grapheme clusters. Take 👩🏾‍🚀for example (WOMAN
+	ASTRONAUT: MEDIUM-DARK SKIN TONE). It consists of 4 codepoints:
+	WOMAN + MEDIUM-DARK SKIN TONE + ZWJ + ROCKET. The sum of the
+	widths is 6.
+	
+	Some applications are better at calculating correct grapheme
+	cluster widths than other. The *grapheme-width-method* option lets
+	you choose the method that works best for your use case.
+	
+	*default* attempts to do "the right thing". When used with
+	applications that does not handle Unicode grapheme clusters
+	correctly, you will see cursor de-synchronization.
+	
+	*wcswidth* simply adds together the individual widths of all
+	codepoints making up the cluster. This is typically the most
+	compatible mode, at least for "older" applications.
+	
+	It will allocate too few cells for legacy emojis with an explicit
+	variation selector.
+	
+	It will allocate too many cells for complex emojis consisting of
+	multiple codepoints.
 	
 	*double-width* does the same, but limits the maximum number of
-	columns to 2. This is more correct, but may break some
-	applications since applications typically use *wcswidth*(3)
-	internally to calculate the width. This results in cursor
-	de-synchronization issues.
+	columns to 2. This is usually more correct, but like *default*,
+	will result in cursor de-synchronization when used with
+	applications that are not Unicode grapheme cluster
+	aware.
 	
 	*max* uses the width of the largest codepoint in the cluster.
 	
-	Default: _wcswidth_
+	Default: _default_
 
 *font-monospace-warn*
 	Boolean. When enabled, foot will use heuristics to try to verify

vt.c

@@ -831,6 +831,32 @@ action_utf8_print(struct terminal *term, char32_t wc)
                 composed != NULL ? composed->width : base_width;
 
             switch (term->conf->tweak.grapheme_width_method) {
+            case GRAPHEME_WIDTH_DEFAULT:
+                if (last == 0x200d) {
+                    /* Last character is ZWJ - ignore this characters width */
+                    width = grapheme_width;
+                } else if ((wc >= 0x1f3fb && wc <= 0x1f3ff  /* skin-tone */) ||
+                           (wc >= 0x1f9b0 && wc <= 0x1f9b3  /* hair-style */))
+                {
+                    width = max(2, grapheme_width);
+                } else if (last >= 0x1f1e6 && last <= 0x1f1ff &&
+                           wc >= 0x1f1e6 && wc <= 0x1f1ff)
+                {
+                    /* Last character and this character are country
+                     * letters, meaning the grapheme is a flag */
+                    /* TODO: only do this for valid flag combinations */
+                    width = 2;
+                } else if (unlikely(wc == 0xfe0f)) {
+                    /* Variation selector 16 - graphical presentation */
+                    width = 2;
+                } else {
+                    /* Emulate wcswidth() */
+                    width += grapheme_width;
+                }
+
+                new_cc->width = width;
+                break;
+
             case GRAPHEME_WIDTH_MAX:
                 new_cc->width = max(grapheme_width, width);
                 break;