diff --git a/crates/engine/src/js.rs b/crates/engine/src/js.rs
index a2c444bd6..c80537fba 100644
--- a/crates/engine/src/js.rs
+++ b/crates/engine/src/js.rs
@@ -289,6 +289,12 @@ pub(crate) fn page_words(text: &pdfcraft_render::PageText, info: &pdfcraft_rende
         .collect()
 }
 
+#[cfg(test)]
+thread_local! {
+    /// Characters read to tell underscore runs from other words.
+    static UNDERSCORE_CHARS_READ: std::cell::Cell<usize> = const { std::cell::Cell::new(0) };
+}
+
 /// A page's words in reading order, their boxes mapped from view space by `map` (and ordered
 /// again, so that [x0, y0, x1, y1] holds whatever way `map` turns them).
 pub(crate) fn words_with(text: &pdfcraft_render::PageText, map: impl Fn(f32, f32) -> [f32; 2]) -> Vec<(String, [f32; 4])> {
@@ -296,6 +302,8 @@ pub(crate) fn words_with(text: &pdfcraft_render::PageText, map: impl Fn(f32, f32
     let mut last_line = u32::MAX;
     // A space glyph ends the word before it.
     let mut gap = false;
+    // Whether the last word is an underscore run (a form's blank), which other glyphs don't join.
+    let mut last_under = false;
     for (i, g) in text.glyphs.iter().enumerate() {
         let line = text.line_of.get(i).copied().unwrap_or(0);
         if g.text.trim().is_empty() {
@@ -303,10 +311,9 @@ pub(crate) fn words_with(text: &pdfcraft_render::PageText, map: impl Fn(f32, f32
             continue;
         }
         let under = g.text.chars().all(|c| c == '_');
-        let breaks = match words.last() {
-            None => true,
-            Some((w, _)) => gap || line != last_line || text.space_before.get(i).copied().unwrap_or(false) || w.chars().all(|c| c == '_') != under,
-        };
+        #[cfg(test)]
+        UNDERSCORE_CHARS_READ.with(|n| n.set(n.get() + g.text.len()));
+        let breaks = words.is_empty() || gap || line != last_line || text.space_before.get(i).copied().unwrap_or(false) || last_under != under;
         gap = false;
         last_line = line;
         match words.last_mut() {
@@ -317,7 +324,10 @@ pub(crate) fn words_with(text: &pdfcraft_render::PageText, map: impl Fn(f32, f32
                 r[2] = r[2].max(g.rect[2]);
                 r[3] = r[3].max(g.rect[3]);
             }
-            _ => words.push((g.text.clone(), g.rect)),
+            _ => {
+                words.push((g.text.clone(), g.rect));
+                last_under = under;
+            }
         }
     }
     words
@@ -409,6 +419,27 @@ impl Session {
 mod review_tests {
     use super::*;
 
+    /// A long run of underscores (a form's blank) is read in one pass: each glyph is compared with
+    /// the word so far without reading the word again.
+    #[test]
+    fn a_long_underscore_run_is_read_in_one_pass() {
+        let n = 10_000;
+        let glyph = |i: usize, t: &str| pdfcraft_render::TextGlyph {
+            text: t.into(),
+            rect: [i as f32, 0.0, i as f32 + 1.0, 10.0],
+            quad: [[0.0; 2]; 4],
+            direction: [1.0, 0.0],
+        };
+        let mut glyphs: Vec<_> = (0..n).map(|i| glyph(i, "_")).collect();
+        glyphs.push(glyph(n, "x"));
+        let text = pdfcraft_render::PageText { line_of: vec![0; glyphs.len()], space_before: vec![false; glyphs.len()], glyphs };
+        UNDERSCORE_CHARS_READ.with(|c| c.set(0));
+        let words = words_with(&text, |x, y| [x, y]);
+        assert_eq!(words.iter().map(|(w, _)| w.len()).collect::<Vec<_>>(), [n, 1]);
+        let read = UNDERSCORE_CHARS_READ.with(std::cell::Cell::get);
+        assert!(read <= 4 * n, "{read} characters read for {n} glyphs");
+    }
+
     #[test]
     fn duplicate_field_names_diff_against_first_original() {
         let first = pdfcraft_js::FieldState::new("duplicate", pdfcraft_js::FieldType::Text, vec!["first".into()]);