diff --git a/crates/engine/src/lib.rs b/crates/engine/src/lib.rs
index 412b3afdf..71b7c736a 100644
--- a/crates/engine/src/lib.rs
+++ b/crates/engine/src/lib.rs
@@ -627,6 +627,12 @@ impl Session {
             }
             None => run.machine.begin(self),
         };
+        // Redraw what the command has added so far (LINE adds a segment per point), not only when it ends.
+        if let Ok(st) = self.state_mut()
+            && !Arc::ptr_eq(&run.before, &st.doc)
+        {
+            st.revision += 1;
+        }
         match step {
             Ok(Step::Continue) => {
                 self.running = Some(run);
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index 98b21bbf6..446639b8f 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -25,6 +25,26 @@ fn line_via_command_line() {
     assert_eq!(s.doc().unwrap().model.len(), 3);
 }
 
+#[test]
+fn edits_while_a_command_runs_redraw() {
+    // The canvas rebuilds when (revision, doc pointer) changes; each new segment must change it (issue #4).
+    fn key(s: &Session) -> (u64, usize) {
+        let st = s.state().unwrap();
+        (st.revision, Arc::as_ptr(&st.doc) as usize)
+    }
+    let mut s = Session::new();
+    for (cmd, pts) in [("line", ["0,0", "10,0", "10,5", "0,5"]), ("pline", ["20,0", "30,0", "30,5", "20,5"])] {
+        s.cmdline(cmd).unwrap();
+        s.cmdline(pts[0]).unwrap();
+        for p in &pts[1..] {
+            let before = key(&s);
+            s.cmdline(p).unwrap();
+            assert_ne!(key(&s), before, "{cmd}: segment to {p} not redrawn");
+        }
+        s.cmdline("").unwrap();
+    }
+}
+
 #[test]
 fn line_undo_option_and_aliases() {
     let mut s = Session::new();