diff --git a/crates/engine/src/link_watch.rs b/crates/engine/src/link_watch.rs
index 51ffbd098..44b1bd86a 100644
--- a/crates/engine/src/link_watch.rs
+++ b/crates/engine/src/link_watch.rs
@@ -17,7 +17,8 @@
 //! update, or answering No, leaves the link modified until the file changes again.
 
 use std::collections::{BTreeSet, HashMap, HashSet};
-use std::sync::{Arc, Mutex, PoisonError};
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::{Arc, Condvar, Mutex, PoisonError};
 
 use serde_json::{Value, json};
 
@@ -27,8 +28,21 @@ use crate::{Result, Session};
 pub type Stamp = (u64, u128);
 
 /// Stamps of a document's linked files taken on a worker thread: (document uid, [(file,
-/// object ids, stamp)]).
-pub type LinkScan = Arc<Mutex<Option<(u64, Vec<(String, Vec<u64>, Option<Stamp>)>)>>>;
+/// object ids, stamp)]), the condition the worker signals when it ends, and whether it has.
+pub type LinkScan = Arc<(Mutex<Option<(u64, Vec<(String, Vec<u64>, Option<Stamp>)>)>>, Condvar, AtomicBool)>;
+
+/// Held by a look's worker: marks the look as ended and wakes [`Session::wait_link_scan`] when
+/// dropped, whether the worker returned or unwound.
+struct Ended(LinkScan);
+
+impl Drop for Ended {
+    fn drop(&mut self) {
+        let (done, finished, ended) = &*self.0;
+        let _held = done.lock().unwrap_or_else(PoisonError::into_inner);
+        ended.store(true, Ordering::SeqCst);
+        finished.notify_all();
+    }
+}
 
 /// The stamp of the linked file stored as `path` in the document saved at `doc_path`; `None` when
 /// there is no file, its path isn't followed ([`crate::cmd::fileio::document_path`]) or
@@ -79,12 +93,14 @@ impl Session {
         let out = slot.clone();
         let scope = crate::file_access::ScopeHandle::capture();
         let spawned = std::thread::Builder::new().name("link-watch".into()).spawn(move || {
+            // Marks the look as ended when the worker returns or unwinds.
+            let _ended = Ended(out.clone());
             let stamps = scope.run(|| {
                 crate::file_access::confine(crate::file_access::current().or(opened_by).as_ref(), || {
                     files.into_iter().map(|(p, ids)| (p.clone(), ids, stamp(&p, doc_path.as_deref()))).collect()
                 })
             });
-            *out.lock().unwrap_or_else(PoisonError::into_inner) = Some((uid, stamps));
+            *out.0.lock().unwrap_or_else(PoisonError::into_inner) = Some((uid, stamps));
         });
         if spawned.is_ok() {
             self.link_scan = Some(slot);
@@ -94,11 +110,21 @@ impl Session {
     /// Apply a finished look (`None` while there is none, or it is still running): see
     /// [`Session::apply_link_stamps`].
     pub fn poll_link_scan(&mut self) -> Option<Result<Value>> {
-        let (uid, stamps) = self.link_scan.as_ref()?.lock().unwrap_or_else(PoisonError::into_inner).take()?;
+        let (uid, stamps) = self.link_scan.as_ref()?.0.lock().unwrap_or_else(PoisonError::into_inner).take()?;
         self.link_scan = None;
         Some(self.apply_link_stamps(uid, stamps, false))
     }
 
+    /// Block until the look in flight has ended (at once when none is running), so that the next
+    /// [`Session::poll_link_scan`] applies its stamps. A look whose worker stopped without stamps
+    /// ends too, and the poll then returns `None`. For tests that drive the look step by step.
+    pub fn wait_link_scan(&self) {
+        let Some(scan) = &self.link_scan else { return };
+        let (done, finished, ended) = &**scan;
+        let guard = done.lock().unwrap_or_else(PoisonError::into_inner);
+        drop(finished.wait_while(guard, |stamps| stamps.is_none() && !ended.load(Ordering::SeqCst)).unwrap_or_else(PoisonError::into_inner));
+    }
+
     /// Compare the stamps of document `uid`'s linked files with the ones last seen and act on
     /// the changed ones as Update Links says (`update`: read them again regardless). Returns
     /// `{updated: [ids], ask: [ids], modified: [ids]}`: read again, to offer (Ask When Modified),
diff --git a/crates/engine/src/tests/link_paths.rs b/crates/engine/src/tests/link_paths.rs
index 2c96be935..76bd7168a 100644
--- a/crates/engine/src/tests/link_paths.rs
+++ b/crates/engine/src/tests/link_paths.rs
@@ -77,11 +77,8 @@ fn linked_files_on_other_computers_are_never_read() {
         }
         s.execute("links.updateChanged", &json!({})).unwrap();
         s.start_link_scan();
-        let t0 = std::time::Instant::now();
-        while s.poll_link_scan().is_none() {
-            assert!(t0.elapsed().as_secs() < 10, "the look never finished");
-            std::thread::yield_now();
-        }
+        s.wait_link_scan();
+        assert!(s.poll_link_scan().is_some(), "no look ran");
     });
     assert_eq!(far_reads(&seen), Vec::<&String>::new());
     // The watch kept no stamp of them (`//tmp/…` would have one had it been looked at).
diff --git a/crates/engine/src/tests/linkwatch.rs b/crates/engine/src/tests/linkwatch.rs
index 8c866b8d9..f1192e02d 100644
--- a/crates/engine/src/tests/linkwatch.rs
+++ b/crates/engine/src/tests/linkwatch.rs
@@ -6,10 +6,13 @@ use serde_json::{Value, json};
 use crate::tests_links::{BLUE, Folder, RED, centre_colour, place, png, session};
 use crate::*;
 
-/// Write `bytes` to `path` so its modification time moves even on coarse file systems.
+/// Write `bytes` to `path`, dated 2 s after the file it replaces so its modification time moves
+/// even on coarse file systems.
 fn save(path: &str, bytes: &[u8]) {
-    std::thread::sleep(std::time::Duration::from_millis(30));
+    let stamp = std::fs::metadata(path).unwrap().modified().unwrap();
     std::fs::write(path, bytes).unwrap();
+    let later = std::fs::FileTimes::new().set_modified(stamp + std::time::Duration::from_secs(2));
+    std::fs::OpenOptions::new().write(true).open(path).unwrap().set_times(later).unwrap();
 }
 
 fn colour(s: &Session) -> [u8; 3] {
@@ -19,14 +22,8 @@ fn colour(s: &Session) -> [u8; 3] {
 /// One background look, waited for.
 fn look(s: &mut Session) -> Value {
     s.start_link_scan();
-    let t0 = std::time::Instant::now();
-    loop {
-        if let Some(r) = s.poll_link_scan() {
-            return r.unwrap();
-        }
-        assert!(t0.elapsed().as_secs() < 10, "the look never finished");
-        std::thread::yield_now();
-    }
+    s.wait_link_scan();
+    s.poll_link_scan().expect("a look ran").unwrap()
 }
 
 #[test]
diff --git a/crates/ui-egui/src/tests/place.rs b/crates/ui-egui/src/tests/place.rs
index 719baff96..e4c9d47ef 100644
--- a/crates/ui-egui/src/tests/place.rs
+++ b/crates/ui-egui/src/tests/place.rs
@@ -758,7 +758,8 @@ fn frame_at(app: &mut VectorcraftApp, ctx: &egui::Context, time: f64) -> std::ti
 /// for) or until `done` → whether it was.
 fn idle(app: &mut VectorcraftApp, ctx: &egui::Context, time: &mut f64, secs: f64, done: &dyn Fn(&VectorcraftApp) -> bool) -> bool {
     let end = *time + secs;
-    let real_end = std::time::Instant::now() + std::time::Duration::from_secs(20);
+    // The look last waited for: one that a frame after the wait still holds ended without stamps.
+    let mut waited: Option<vectorcraft_engine::link_watch::LinkScan> = None;
     loop {
         let wait = frame_at(app, ctx, *time);
         if done(app) {
@@ -769,10 +770,13 @@ fn idle(app: &mut VectorcraftApp, ctx: &egui::Context, time: &mut f64, secs: f64
             *time = end;
             return false;
         }
-        // The look runs on a worker thread, in real time: the clock waits for it, else a busy
-        // machine's slow look outlasts the simulated seconds and the test fails at random.
-        if app.session.link_scan.is_some() && std::time::Instant::now() < real_end {
-            std::thread::sleep(std::time::Duration::from_millis(5));
+        // The look runs on a worker thread, in real time: the clock waits for it to finish, else a
+        // busy machine's slow look outlasts the simulated seconds and the test fails at random.
+        if let Some(scan) = app.session.link_scan.clone()
+            && !waited.as_ref().is_some_and(|w| std::sync::Arc::ptr_eq(w, &scan))
+        {
+            app.session.wait_link_scan();
+            waited = Some(scan);
             continue;
         }
         *time = next;