diff --git a/crates/fj-viewer/src/viewer.rs b/crates/fj-viewer/src/viewer.rs
index 0ba724dc2..45cf01926 100644
--- a/crates/fj-viewer/src/viewer.rs
+++ b/crates/fj-viewer/src/viewer.rs
@@ -77,7 +77,7 @@ pub struct ViewerHandle {
 
 impl ViewerHandle {
     /// # Open a new window
-    pub fn open_window(&mut self) -> WindowHandle {
+    pub fn open_window(&self) -> WindowHandle {
         // Use a conservative ordering, just to be on the safe side. This code
         // shouldn't be performance-sensitive anyway.
         let id = self.next_window_id.fetch_add(1, Ordering::SeqCst);
diff --git a/crates/fj/src/instance.rs b/crates/fj/src/instance.rs
index ad8f35bbc..6146b42ce 100644
--- a/crates/fj/src/instance.rs
+++ b/crates/fj/src/instance.rs
@@ -99,7 +99,7 @@ impl Instance {
             return Ok(());
         }
 
-        make_viewer_and_spawn_thread(|mut viewer| {
+        make_viewer_and_spawn_thread(|viewer| {
             viewer.open_window().display_mesh(tri_mesh);
         })?;