diff --git a/crates/ui-egui/src/prefs_dialog.rs b/crates/ui-egui/src/prefs_dialog.rs
index b91c149f4..20502f0f6 100644
--- a/crates/ui-egui/src/prefs_dialog.rs
+++ b/crates/ui-egui/src/prefs_dialog.rs
@@ -109,6 +109,12 @@ fn canvas_color(key: &str) -> Option<egui::Color32> {
 
 /// Per frame: push UI-side preferences into egui / the renderer when they change.
 pub fn apply_runtime(app: &mut VectorcraftApp, ctx: &egui::Context) {
+    apply(app, ctx, false);
+}
+
+/// [`apply_runtime`]; with `force`, the preferences are pushed into `ctx` even when nothing changed
+/// since the last push.
+fn apply(app: &mut VectorcraftApp, ctx: &egui::Context, force: bool) {
     let p = &app.session.prefs;
     if let Some(b) = Brightness::parse(&p.ui_brightness)
         && b != app.ui.brightness
@@ -127,7 +133,7 @@ pub fn apply_runtime(app: &mut VectorcraftApp, ctx: &egui::Context) {
     };
     let id = applied_id();
     let prev: Option<Applied> = ctx.data(|d| d.get_temp::<Option<Applied>>(id)).flatten();
-    if prev != Some(want) {
+    if force || prev != Some(want) {
         theme::apply_with(ctx, want.brightness, want.accent, want.scheme);
         // General › Show Tool Tips: off, no button or field shows its tool tip (they never come
         // due), whichever widget asks for one.
@@ -165,11 +171,11 @@ fn applied_id() -> egui::Id {
     egui::Id::unique("dc-applied-prefs")
 }
 
-/// [`apply_runtime`] with nothing recorded as applied, so the theme (brightness, accent, color
-/// scheme and canvas color) and the other UI-side preferences are pushed into `ctx` again.
+/// [`apply_runtime`] even when nothing changed, so the theme (brightness, accent, color scheme and
+/// canvas color) and the other UI-side preferences are pushed into `ctx` again. The record of what
+/// was last applied stays, so a Render Threads change since then still restarts the renderer.
 pub(crate) fn reapply(app: &mut VectorcraftApp, ctx: &egui::Context) {
-    ctx.data_mut(|d| d.remove::<Option<Applied>>(applied_id()));
-    apply_runtime(app, ctx);
+    apply(app, ctx, true);
 }
 
 pub fn show(app: &mut VectorcraftApp, ctx: &egui::Context) {
diff --git a/crates/ui-egui/src/prefs_dialog/tests.rs b/crates/ui-egui/src/prefs_dialog/tests.rs
index 2e4711505..cbc429b5f 100644
--- a/crates/ui-egui/src/prefs_dialog/tests.rs
+++ b/crates/ui-egui/src/prefs_dialog/tests.rs
@@ -79,6 +79,23 @@ fn saved_scheme_accent_and_canvas_color_show_from_the_first_frame() {
     }
 }
 
+/// A Render Threads change made after the preferences were applied, and before the first UI pass
+/// applies them again, still restarts the canvas renderer.
+#[test]
+fn reapply_restarts_the_renderer_after_a_render_threads_change() {
+    let mut app = app();
+    let ctx = egui::Context::default();
+    apply_runtime(&mut app, &ctx);
+    app.canvas.worker_started = true;
+    // The automatic count as an explicit value: the process-wide count other tests read stays the same.
+    let threads = vectorcraft_render::default_threads();
+    app.run("prefs.set", json!({"key": "renderThreads", "value": threads})).unwrap();
+    reapply(&mut app, &ctx);
+    let restarted = !app.canvas.worker_started;
+    app.run("prefs.set", json!({"key": "renderThreads", "value": -1})).unwrap();
+    assert!(restarted, "the canvas renderer kept its old thread count");
+}
+
 fn app() -> VectorcraftApp {
     VectorcraftApp::new(Session::new(), Services::default())
 }