diff --git a/apps/effectcraft-web/src/persist.rs b/apps/effectcraft-web/src/persist.rs
index 6735d066..6ee2c86b 100644
--- a/apps/effectcraft-web/src/persist.rs
+++ b/apps/effectcraft-web/src/persist.rs
@@ -167,7 +167,7 @@ pub fn restore(s: &mut Session) -> bool {
     }
     if meta.dirty {
         // Unsaved changes stay unsaved.
-        s.bump();
+        s.mark_unsaved();
     }
     SNAP.with(|c| c.set((s.revision, 0.0)));
     log::info!("restored the last session ({})", meta.path.as_deref().unwrap_or("untitled"));
diff --git a/crates/engine/src/lib.rs b/crates/engine/src/lib.rs
index 17fa491d..ca93f29b 100644
--- a/crates/engine/src/lib.rs
+++ b/crates/engine/src/lib.rs
@@ -635,6 +635,13 @@ impl Session {
         self.saved_revision = self.revision;
     }
 
+    /// The project has changes its file doesn't (a session restored with unsaved changes): it
+    /// stays modified until it is saved, whatever is undone.
+    pub fn mark_unsaved(&mut self) {
+        self.saved_project = Arc::new(Project::default());
+        self.bump();
+    }
+
     pub fn active_comp(&self) -> Option<&Comp> {
         self.project.comp(self.state.active_comp?)
     }
diff --git a/crates/engine/src/tests_history.rs b/crates/engine/src/tests_history.rs
index d90ca7d0..c67e7c14 100644
--- a/crates/engine/src/tests_history.rs
+++ b/crates/engine/src/tests_history.rs
@@ -175,3 +175,18 @@ fn modified_mark_follows_undo_and_no_op_edits_record_nothing() {
     assert!(!s.is_dirty(), "rolled back");
     let _ = std::fs::remove_file(path);
 }
+
+#[test]
+fn a_session_restored_with_unsaved_changes_stays_modified() {
+    let mut s = Session::default();
+    s.execute("comp.new", json!({"name": "H", "width": 64, "height": 64, "duration": 2})).unwrap();
+    let p = (*s.project).clone();
+    // The web app reopens the last visit's project, which had unsaved changes.
+    s.replace_project(p, None);
+    assert!(!s.is_dirty());
+    s.mark_unsaved();
+    assert!(s.is_dirty());
+    solid(&mut s, "A");
+    s.execute("edit.undo", json!({})).unwrap();
+    assert!(s.is_dirty(), "undo doesn't reach a saved state");
+}