Edit text: do not group or join by an empty font name (#3661)

Review follow-up. `Metrics::from_dict` leaves `base_font` empty when a font
has no `/BaseFont`, and a bare subset tag such as `/ABCDEF+` left an empty
name after stripping. Two consequences:

- `same_style` split one paragraph into separate reading blocks for the
  exporters when both names were empty, though they grouped before subset
  joining;
- `/ABCDEF+` and `/GHIJKL+` compared equal after stripping, so unrelated
  malformed fonts could join into one line.

`same_base_font` now strips both tags first and requires a non-empty, equal
name, so a missing name or a bare tag matches nothing. `same_style` keeps
grouping lines whose fonts both lack a name, since the resource or fallback
face `same_font` already matched is their whole identity.

Tests: a page in a font without `/BaseFont` still reads as one block (it
split into two before this change), and a bare tag matches no other name.

crates/edit/src/tests.rs

@@ -883,7 +883,7 @@ fn text_page(content: &str) -> Document {
     let objs: Vec<String> = vec![
         "<< /Type /Catalog /Pages 2 0 R >>".into(),
         "<< /Type /Pages /Kids [3 0 R] /Count 1 >>".into(),
-        "<< /Type /Page /Parent 2 0 R /MediaBox [0 0 600 800] /Contents 4 0 R /Resources << /Font << /F1 5 0 R /F2 6 0 R /F3 7 0 R /F4 8 0 R >> >> >>".into(),
+        "<< /Type /Page /Parent 2 0 R /MediaBox [0 0 600 800] /Contents 4 0 R /Resources << /Font << /F1 5 0 R /F2 6 0 R /F3 7 0 R /F4 8 0 R /F5 9 0 R >> >> >>".into(),
         format!("<< /Length {} >>\nstream\n{content}\nendstream", content.len()),
         "<< /Type /Font /Subtype /Type1 /BaseFont /Helvetica /Encoding /WinAnsiEncoding >>".into(),
         // Subset: glyphs for a (97) and b (98) only.
@@ -894,6 +894,8 @@ fn text_page(content: &str) -> Document {
             .into(),
         // Helvetica again, in a second resource.
         "<< /Type /Font /Subtype /Type1 /BaseFont /Helvetica /Encoding /WinAnsiEncoding >>".into(),
+        // No `/BaseFont`: `Metrics::from_dict` leaves its base-font name empty.
+        "<< /Type /Font /Subtype /Type1 /Encoding /WinAnsiEncoding >>".into(),
     ];
     let mut out = b"%PDF-1.7\n".to_vec();
     let mut offs = Vec::new();
@@ -1760,6 +1762,22 @@ fn different_fonts_start_a_new_line() {
     assert_eq!(text::text_lines(&doc, 0).unwrap().len(), 2);
 }
 
+/// A font without `/BaseFont` has no name to compare. Its lines still group by resource, as they
+/// did before subset joining: an empty name must not split one paragraph into two blocks.
+#[test]
+fn lines_in_a_font_without_a_base_font_stay_one_block() {
+    let doc = text_page("BT /F5 12 Tf 72 700 Td (First) Tj 0 -15 Td (second) Tj ET");
+    let lines = text::text_lines(&doc, 0).unwrap();
+    assert_eq!(lines.len(), 2);
+    assert!(lines.iter().all(|l| l.base_font.is_empty()), "{:?}", lines.iter().map(|l| &l.base_font).collect::<Vec<_>>());
+    let blocks = text::text_blocks(&doc, 0).unwrap();
+    assert_eq!(blocks.len(), 1);
+    assert_eq!(blocks[0].text, "First second");
+    let reading = text::reading_blocks(&doc, 0).unwrap();
+    assert_eq!(reading.len(), 1, "{:?}", reading.iter().map(|b| &b.text).collect::<Vec<_>>());
+    assert_eq!(reading[0].text, "First second");
+}
+
 #[test]
 fn text_lines_are_found_and_replaced_in_place() {
     let mut doc =

crates/edit/src/text.rs

@@ -1466,8 +1466,10 @@ fn same_font(line: &TextLine, font: &[u8], base_font: &str, face: Option<&str>)
 
 /// Whether two `/BaseFont` names name the same font: a subset's six-letter tag and `+` prefix do
 /// not change the font (ISO 32000-2 §9.6.4), so `ABCDEF+Arial` and `GHIJKL+Arial` are one font.
+/// A name that is missing, or only a tag (`ABCDEF+`), names no font and matches nothing.
 fn same_base_font(a: &str, b: &str) -> bool {
-    !a.is_empty() && (a == b || strip_subset_tag(a) == strip_subset_tag(b))
+    let (a, b) = (strip_subset_tag(a), strip_subset_tag(b));
+    !a.is_empty() && a == b
 }
 
 /// `ABCDEF+Arial` → `Arial`. A name without a subset tag is returned unchanged.
@@ -2804,6 +2806,18 @@ pub(crate) fn translate_blocks(doc: &mut Document, page: usize, indexes: &[usize
 mod tests {
     use super::*;
 
+    #[test]
+    fn a_subset_tag_is_the_only_difference_between_subsets_of_one_font() {
+        assert!(same_base_font("ABCDEF+Arial", "GHIJKL+Arial"));
+        assert!(same_base_font("Arial", "ABCDEF+Arial"));
+        assert!(same_base_font("Arial", "Arial"));
+        assert!(!same_base_font("ABCDEF+Arial", "GHIJKL+Helvetica"));
+        // A tag with no name after it, or no name at all, names no font.
+        assert!(!same_base_font("ABCDEF+", "GHIJKL+"));
+        assert!(!same_base_font("", ""));
+        assert!(!same_base_font("Arial", ""));
+    }
+
     #[test]
     fn paragraph_wrapping_keeps_ideographic_spaces() {
         let width = |s: &str| s.chars().count() as f64;

crates/edit/src/text_regions.rs

@@ -52,7 +52,10 @@ fn invisible(line: &TextLine) -> bool {
 
 fn same_style(a: &TextLine, b: &TextLine) -> bool {
     same_font(a, b.font.as_bytes(), &b.base_font, b.face.as_deref())
-        && same_base_font(&a.base_font, &b.base_font)
+        // With no `/BaseFont` (or only a subset tag) there is no name to compare: the resource or
+        // fallback face `same_font` matched is the whole identity, so keep the lines together.
+        && (same_base_font(&a.base_font, &b.base_font)
+            || (strip_subset_tag(&a.base_font).is_empty() && strip_subset_tag(&b.base_font).is_empty()))
         && a.bold == b.bold
         && a.italic == b.italic
         && a.color == b.color