PDF: track the first text run with a flag

text_rotated counted the Tf operators written so far on every run, which is
quadratic for text that alternates between WinAnsi and CID characters.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/pdf/src/lib.rs

@@ -743,8 +743,9 @@ impl Page {
             if let Some(f) = self.fonts.get_mut(slot) {
                 *f = true;
             }
+            let first_run = runs.is_empty();
             let _ = writeln!(runs, "/F{} {} Tf", slot + 1, size);
-            if runs.matches("Tf\n").count() == 1 {
+            if first_run {
                 runs.push_str(&tm);
             }
             let _ = writeln!(runs, "{string} Tj");

crates/pdf/src/tests.rs

@@ -873,6 +873,18 @@ fn cjk_text_is_not_replaced_by_question_marks() -> Result<(), Box<dyn Error>> {
     Ok(())
 }
 
+#[test]
+fn many_alternating_runs_share_one_text_matrix() {
+    let mut doc = PdfDoc::new();
+    let p = doc.add_page(612.0, 792.0);
+    p.text(10.0, 20.0, 10.0, Font::Helvetica, [0, 0, 0], &"a\u{4e2d}".repeat(2000));
+    let pdf = doc.finish();
+    let count = |needle: &[u8]| pdf.windows(needle.len()).filter(|w| *w == needle).count();
+    assert_eq!(count(b" Tm\n"), 1, "the position is set once, before the first run");
+    assert_eq!(count(b" Tf\n"), 4000, "every run selects its font");
+    assert_eq!(count(b" Tj\n"), 4000);
+}
+
 #[test]
 fn symbol_font_draws_maths_in_its_own_encoding() {
     let mut doc = PdfDoc::new();