diff --git a/ROADMAP.md b/ROADMAP.md
index 93bc931e..542f7cd0 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -137,6 +137,7 @@ app's JSON control channel (`deckcraft --control PORT`).
 | 2026-10-17 | Edit Points: turning a shape into a freeform is one undo step, so Undo brings the original shape back (it used to stay a freeform) |
 | 2026-10-17 | While a placeholder's text is being edited, a click on a shape in front of it selects that shape instead of moving the caret, as in PowerPoint (#931) |
 | 2026-10-15 | Native saves keep an imported chart's embedded workbook, drawings and relationships; headless PPTX → edit → native save/reopen → PPTX tests verify the payloads and stale-workbook rejection |
+| 2026-10-20 | Tests: the Options dialog's Cancel, Esc and close button put the settings back the way they were (#878) |
 | 2026-10-15 | PPTX trigger sequences keep separate On Click steps, including paragraph builds; synthetic import/edit/save/reopen and headless show regressions |
 | 2026-10-16 | Export: PNG with a transparent background — the slide background is left out, only the slide's content is drawn (Export… dialog, File › Export, web download, `file.export`/`file.render` `transparentBackground`, `deckcraft-cli render --transparent`) (#784) |
 | 2026-10-16 | Arrange ▸ Align: Align to Slide and Align Selected Objects, as in PowerPoint. One object aligns to the slide and several to each other until the other choice is ticked; the choice holds for that selection and also drives Distribute |
diff --git a/crates/ui-egui/src/dialogs.rs b/crates/ui-egui/src/dialogs.rs
index 2393bd61..db345066 100644
--- a/crates/ui-egui/src/dialogs.rs
+++ b/crates/ui-egui/src/dialogs.rs
@@ -3684,3 +3684,7 @@ mod spelling_tests;
 #[cfg(test)]
 #[path = "paragraph_dialog_tests.rs"]
 mod paragraph_dialog_tests;
+
+#[cfg(test)]
+#[path = "preferences_tests.rs"]
+mod preferences_tests;
diff --git a/crates/ui-egui/src/preferences_tests.rs b/crates/ui-egui/src/preferences_tests.rs
new file mode 100644
index 00000000..8740b6de
--- /dev/null
+++ b/crates/ui-egui/src/preferences_tests.rs
@@ -0,0 +1,95 @@
+//! The Preferences (Options) dialog: controls apply as they change and Cancel, Esc and the
+//! window's close button put the settings back the way they were (#752, reported as #878).
+
+use deckcraft_engine::Session;
+use egui_kittest::kittest::{NodeT, Queryable};
+use egui_kittest::{Harness, Node};
+use serde_json::json;
+
+use crate::theme::Brightness;
+use crate::{Services, SlideApp};
+
+/// The slider itself, not the label drawn beside it.
+fn interface_size<'a>(h: &'a Harness<'_, SlideApp>) -> Node<'a> {
+    h.get_all_by_label("Interface size").find(|n| n.accesskit_node().role() == egui::accesskit::Role::Slider).expect("the slider is there")
+}
+
+fn harness() -> Harness<'static, SlideApp> {
+    let mut h = Harness::builder().with_size(egui::vec2(1400.0, 900.0)).build_ui_state(
+        |ui, app: &mut SlideApp| {
+            app.logic(ui.ctx());
+            app.ui(ui);
+        },
+        SlideApp::new(Session::with_new(), Services::default()),
+    );
+    // The app keeps asking for frames, so step instead of run().
+    h.run_steps(3);
+    h
+}
+
+fn open_options(h: &mut Harness<'_, SlideApp>) {
+    h.state_mut().run("app.preferences", json!({})).expect("open Options");
+    h.run_steps(3);
+}
+
+fn dark(app: &SlideApp) -> bool {
+    app.ui.brightness == Brightness::Dark
+}
+
+/// A checkbox applies as it is toggled and Cancel puts the old value back.
+#[test]
+fn options_cancel_undoes_the_changes() {
+    let mut h = harness();
+    let was = h.state().session.prefs.smart_guides;
+    open_options(&mut h);
+
+    h.query_by_label("Show smart guides").expect("the checkbox is there").click();
+    h.run_steps(2);
+    assert_eq!(h.state().session.prefs.smart_guides, !was, "the change applies as it is made");
+
+    h.query_by_label("Cancel").expect("the Cancel button is there").click();
+    h.run_steps(2);
+    assert_eq!(h.state().session.prefs.smart_guides, was, "Cancel restores the setting");
+    assert!(h.state().dialog.is_none(), "and the dialog closes");
+}
+
+/// The appearance toggle and the interface size go back on Cancel, too; Esc acts as Cancel.
+#[test]
+fn options_cancel_puts_the_appearance_and_scale_back() {
+    let mut h = harness();
+    let (dark_before, scale_before) = (dark(h.state()), h.state().ui.ui_scale);
+    open_options(&mut h);
+
+    h.query_by_label("Dark appearance").expect("the checkbox is there").click();
+    h.run_steps(2);
+    assert!(dark(h.state()), "the appearance applies as it is toggled");
+
+    // Clicking the slider's middle sets it near the middle of its range, away from the default.
+    interface_size(&h).click();
+    h.run_steps(2);
+    let scale_in_dialog = h.state().ui.ui_scale;
+    assert!((scale_in_dialog - scale_before).abs() > 1e-3, "the slider applied a new size");
+
+    h.key_press(egui::Key::Escape);
+    h.run_steps(2);
+    assert_eq!(dark(h.state()), dark_before, "Esc restores the appearance");
+    assert!((h.state().ui.ui_scale - scale_before).abs() < 1e-3, "Esc restores the size");
+    assert!(h.state().dialog.is_none(), "and the dialog closes");
+}
+
+/// The window's close button is the same as Cancel: the settings go back.
+#[test]
+fn options_close_button_restores_the_settings() {
+    let mut h = harness();
+    let was = h.state().session.prefs.smart_guides;
+    open_options(&mut h);
+
+    h.query_by_label("Show smart guides").expect("the checkbox is there").click();
+    h.run_steps(2);
+    assert_eq!(h.state().session.prefs.smart_guides, !was, "the change applies as it is made");
+
+    h.get_all_by_label("Close window").next().expect("the window's close button is there").click();
+    h.run_steps(2);
+    assert_eq!(h.state().session.prefs.smart_guides, was, "the close button restores the setting");
+    assert!(h.state().dialog.is_none(), "and the dialog closes");
+}