render: optionally enable heuristics that deal with private usage area chars

Try to detect double-width *glyphs* for single-width *characters*, and
allow them to overflow into the next cell.

This is only done for single-width chars with a glyph width that is at
least 1.5 cells wide, but at most 3 cells.

The feature is gated by the new
‘tweak.allow-overflowing-double-width-glyphs’, and is disabled by
default.

Closes #116

CHANGELOG.md

@@ -45,6 +45,10 @@
   Ps r` (https://codeberg.org/dnkl/foot/issues/91).
 * `$COLORTERM` is now set to `truecolor` at startup, to indicate
   support for 24-bit RGB colors.
+* Experimental support for rendering double-width glyphs with a
+  character width of 1. Must be explicitly enabled with
+  `tweak.allow-overflowing-double-width-glyphs`
+  (https://codeberg.org/dnkl/foot/issues/116).
 
 
 ### Deprecated

config.c

@@ -1358,7 +1358,10 @@ parse_section_tweak(
     const char *key, const char *value, struct config *conf,
     const char *path, unsigned lineno)
 {
-    if (strcmp(key, "render-timer") == 0) {
+    if (strcmp(key, "allow-overflowing-double-width-glyphs") == 0)
+        conf->tweak.allow_overflowing_double_width_glyphs = str_to_bool(value);
+
+    else if (strcmp(key, "render-timer") == 0) {
         if (strcmp(value, "none") == 0) {
             conf->tweak.render_timer_osd = false;
             conf->tweak.render_timer_log = false;
@@ -1792,6 +1795,7 @@ config_load(struct config *conf, const char *conf_path, bool errors_are_fatal)
         .hold_at_exit = false,
 
         .tweak = {
+            .allow_overflowing_double_width_glyphs = false,
             .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

@@ -144,6 +144,7 @@ struct config {
     bool hold_at_exit;
 
     struct {
+        bool allow_overflowing_double_width_glyphs;
         bool render_timer_osd;
         bool render_timer_log;
         uint64_t delayed_render_lower_ns;

doc/foot.ini.5.scd

@@ -440,6 +440,23 @@ foot.
 When reporting bugs, please mention if, and to what, you have changed
 any of these options.
 
+*allow-overflowing-double-width-glyphs*
+
+	When enabled, double width glyphs with a character width of 1 are
+	allowed to overflow into the neighbouring cell.
+	
+	One use case for this is fonts "icon" characters in the Unicode
+	private usage area, e.g. Nerd Fonts, or Powerline Fonts. Without
+	this option, such glyphs will appear "cut off".
+
+	Another use case are legacy emoji characters like *WHITE FROWNING
+	FACE*.
+	
+	Note: this feature uses _heuristics_ to determine *which* glyphs
+	should be allowed to overflow.
+	
+	Default: _false_.
+
 *render-timer*
 	Enables a frame rendering timer, that prints the time it takes to
 	render each frame, in microseconds, either on-screen, to stderr,

render.c

@@ -433,6 +433,31 @@ render_cell(struct terminal *term, pixman_image_t *pix,
     const int cols_left = term->cols - col;
     int cell_cols = glyph != NULL ? max(1, min(glyph->cols, cols_left)) : 1;
 
+    /*
+     * Hack!
+     *
+     * Deal with double-width glyphs for which wcwidth() returns
+     * 1. Typically Unicode private usage area characters,
+     * e.g. powerline, or nerd hack fonts.
+     *
+     * Users can enable a tweak option that lets this glyphs
+     * overflow/bleed into the neighbouring cell.
+     *
+     * We only apply this workaround if:
+     *  - the user has explicitly enabled this feature
+     *  - the *character* width is 1
+     *  - the *glyph* width is at least 1.5 cells
+     *  - the *glyph* width is less than 3 cells
+     */
+    if (term->conf->tweak.allow_overflowing_double_width_glyphs &&
+        glyph != NULL &&
+        glyph->cols == 1 &&
+        glyph->width >= term->cell_width * 15 / 10 &&
+        glyph->width < 3 * term->cell_width)
+    {
+        cell_cols = min(2, cols_left);
+    }
+
     pixman_region32_t clip;
     pixman_region32_init_rect(
         &clip, x, y,