Make the width of the thin space configurable (#8070)

Adds `FontTweak::thin_space_width` and `FontTweak::tab_size`

crates/epaint/src/text/font.rs

@@ -432,8 +432,7 @@ impl FontFace {
             && let Some(space) = self.glyph_info(' ')
         {
             let glyph_info = GlyphInfo {
-                advance_width_unscaled: (crate::text::TAB_SIZE as f32
-                    * space.advance_width_unscaled.0)
+                advance_width_unscaled: (self.tweak.tab_size * space.advance_width_unscaled.0)
                     .into(),
                 ..space
             };
@@ -441,21 +440,18 @@ impl FontFace {
             return Some(glyph_info);
         }
 
-        if c == '\u{2009}' {
-            // Thin space, often used as thousands deliminator: 1 234 567 890
-            // https://www.compart.com/en/unicode/U+2009
-            // https://en.wikipedia.org/wiki/Thin_space
-
-            if let Some(space) = self.glyph_info(' ') {
-                let em = self.font.borrow_dependent().metrics.units_per_em as f32;
-                let advance_width = f32::min(em / 6.0, space.advance_width_unscaled.0 * 0.5); // TODO(emilk): make configurable
-                let glyph_info = GlyphInfo {
-                    advance_width_unscaled: advance_width.into(),
-                    ..space
-                };
-                self.glyph_info_cache.insert(c, glyph_info);
-                return Some(glyph_info);
-            }
+        if (c == '\u{2009}' || c == '\u{202F}')
+            && let Some(space) = self.glyph_info(' ')
+        {
+            // Thin space (U+2009) and narrow no-break space (U+202F),
+            // often used as thousands separator: 1 234 567 890
+            let advance_width = self.tweak.thin_space_width * space.advance_width_unscaled.0;
+            let glyph_info = GlyphInfo {
+                advance_width_unscaled: advance_width.into(),
+                ..space
+            };
+            self.glyph_info_cache.insert(c, glyph_info);
+            return Some(glyph_info);
         }
 
         if invisible_char(c) {

crates/epaint/src/text/fonts.rs

@@ -193,6 +193,19 @@ pub struct FontTweak {
 
     /// Override the font's default variation coordinates.
     pub coords: VariationCoords,
+
+    /// Width of a thin space (`\u{2009}`) and narrow no-break space (`\u{202F}`),
+    /// as a fraction of the normal space width.
+    ///
+    /// Thin space is often used as a thousands separator: `1 234 567`.
+    ///
+    /// Default: `0.5` (half a normal space).
+    pub thin_space_width: f32,
+
+    /// Width of a tab character (`\t`), measured in number of space widths.
+    ///
+    /// Default: `4.0`.
+    pub tab_size: f32,
 }
 
 impl Default for FontTweak {
@@ -203,6 +216,8 @@ impl Default for FontTweak {
             y_offset: 0.0,
             hinting_override: None,
             coords: VariationCoords::default(),
+            thin_space_width: 0.5,
+            tab_size: 4.0,
         }
     }
 }

crates/epaint/src/text/mod.rs

@@ -6,9 +6,6 @@ mod fonts;
 mod text_layout;
 mod text_layout_types;
 
-/// One `\t` character is this many spaces wide.
-pub const TAB_SIZE: usize = 4;
-
 pub use {
     fonts::{
         FontData, FontDefinitions, FontFamily, FontId, FontInsert, FontPriority, FontTweak, Fonts,

crates/epaint/src/text/text_layout.rs

@@ -8,7 +8,6 @@ use crate::{
     Color32, Mesh, Stroke, Vertex,
     stroke::PathStroke,
     text::{
-        TAB_SIZE,
         font::{StyledMetrics, UvRect, is_cjk, is_cjk_break_allowed},
         fonts::FontFaceKey,
     },
@@ -250,11 +249,23 @@ fn layout_shaped_run(
             .unwrap_or('\u{FFFD}'); // Unicode Replacement Character
 
         // Tab is a layout concept, not a glyph — the shaper doesn't know about tab stops.
-        // Override the advance width to TAB_SIZE × space width.
+        // Override the advance width using the font's configured tab size.
         if chr == '\t' {
+            let tweak = font.fonts_by_id.get(&run.font_key).map(|ff| ff.tweak());
+            let tab_size = tweak.map_or(4.0, |t| t.tab_size);
             let (_, space_info) = font.glyph_info(' ');
             let space_width_px = space_info.advance_width_unscaled.0 * px_scale;
-            advance_width_px = TAB_SIZE as f32 * space_width_px;
+            advance_width_px = tab_size * space_width_px;
+        }
+
+        // Thin space (U+2009) and narrow no-break space (U+202F):
+        // override the shaper's advance width with the configured fraction of a space.
+        if chr == '\u{2009}' || chr == '\u{202F}' {
+            let tweak = font.fonts_by_id.get(&run.font_key).map(|ff| ff.tweak());
+            let thin_space_width = tweak.map_or(0.5, |t| t.thin_space_width);
+            let (_, space_info) = font.glyph_info(' ');
+            let space_width_px = space_info.advance_width_unscaled.0 * px_scale;
+            advance_width_px = thin_space_width * space_width_px;
         }
 
         // Apply extra_letter_spacing only at cluster boundaries,