Give an unencoded space in legacy Thai the source font's space fallback

A space that no subset of the face encodes was drawn as a quarter-em gap,
narrower than the text-space fallback the source font uses elsewhere.
Keep each subset's source space with it and use that for the gap.

crates/edit/src/tests.rs

@@ -13112,6 +13112,25 @@ fn editing_legacy_thai_shapes_a_run_across_subsets_of_the_same_face() {
     assert_eq!(pdfcraft_cos::write_full(&doc, &Default::default()).unwrap(), before, "a refused edit changes nothing");
 }
 
+/// A space no subset of the face encodes is the gap the source font's text-space fallback gives
+/// (`source_space`, a third of an em here), as wrapping measures it everywhere else.
+#[test]
+fn an_unencoded_space_in_legacy_thai_takes_the_source_space_fallback() {
+    let mut doc = text_page("BT /F1 16 Tf 50 650 Td <00d6> Tj ET");
+    let font = legacy_thai_font(&mut doc, "ABCDEF+THSarabunPSK", &legacy_thai_program(), LEGACY_THAI_UNICODE);
+    doc.set(pdfcraft_cos::ObjRef::new(5, 0), Object::Dict(font));
+    let style = BlockStyle { width: Some(400.0), ..Default::default() };
+    text::rewrite_block(&mut doc, 0, 0, Some("ก ก"), &style).unwrap();
+    // After the first ก (500 units), the second starts 330 units further: the gap.
+    let gaps: Vec<f64> = pdfcraft_content::parse(&page_content_bytes(&doc, 0))
+        .ops
+        .iter()
+        .filter(|op| op.is("TJ"))
+        .flat_map(|op| op.operands.first().and_then(Object::as_array).into_iter().flatten().filter_map(Object::as_f64))
+        .collect();
+    assert!(gaps.iter().any(|g| (g + 330.0).abs() < 0.01), "{gaps:?}");
+}
+
 /// Every resource name of one THSarabunPSK font object shares its program, decoded once. Distinct
 /// subsets of the face are read within one budget for the edit: past it the edit is refused with
 /// the reason, and the document is unchanged.

crates/edit/src/text_regions/styled.rs

@@ -112,6 +112,8 @@ struct LegacyFace {
     style: FontStyle,
     /// The font object the resource names, when it is given by reference.
     object: Option<pdfcraft_cos::ObjRef>,
+    /// Its text-space fallback ([`source_space`]), in ems.
+    space: f64,
 }
 
 /// What one edit may decode, and so keep, of the font programs of its THSarabunPSK subsets
@@ -860,15 +862,13 @@ fn legacy_thai_font(
         let Some(font) = doc.dict(resource) else { continue };
         let object = resource.as_ref();
         let face = match object.and_then(|o| cache.values().find(|face| face.object == Some(o))) {
-            Some(alias) => LegacyFace { glyphs: alias.glyphs.clone(), metrics: alias.metrics.clone(), style: alias.style, object },
+            Some(alias) => LegacyFace { glyphs: alias.glyphs.clone(), metrics: alias.metrics.clone(), object, ..*alias },
             None => {
                 let metrics = Metrics::from_dict(doc, &font);
-                let glyphs = pdfcraft_fonts::pdf::LegacyThaiFont::from_pdf(doc, &font, source_space(doc, fonts, &metrics), budget)
-                    .ok()
-                    .flatten()
-                    .map(std::rc::Rc::new);
+                let space = source_space(doc, fonts, &metrics);
+                let glyphs = pdfcraft_fonts::pdf::LegacyThaiFont::from_pdf(doc, &font, space, budget).ok().flatten().map(std::rc::Rc::new);
                 let style = FontStyle::of(doc, &font, &metrics);
-                LegacyFace { glyphs, metrics, style, object }
+                LegacyFace { glyphs, metrics, style, object, space }
             }
         };
         cache.insert(name, face);
@@ -996,7 +996,7 @@ fn shape_existing_thai(cache: &HashMap<String, LegacyFace>, primary: &str, text:
                         let code = f.metrics.encode(" ")?;
                         Some(f.metrics.width(f.metrics.codes(&code).first()?.0))
                     })
-                    .unwrap_or(0.25);
+                    .unwrap_or(first.space);
                 (primary, Vec::new(), width)
             } else {
                 return Err(EditError::Invalid(format!(