diff --git a/crates/ui-egui/src/dialogs/revolve.rs b/crates/ui-egui/src/dialogs/revolve.rs
index 5966cf7d4..f3c7d7e35 100644
--- a/crates/ui-egui/src/dialogs/revolve.rs
+++ b/crates/ui-egui/src/dialogs/revolve.rs
@@ -98,14 +98,13 @@ pub(crate) fn controls(app: &VectorcraftApp, ui: &mut egui::Ui, d: &mut Dialog)
     });
     number(ui, d, "offset", tl!("Offset"), (0.0, 100000.0), "", 1, Some(unit));
     ui.horizontal(|ui| {
-        let mut cap = d.bool("cap");
+        let cap = flag(d, "cap", true);
         let tip = tl!("Close the circular ends of open profiles. Partial sweeps keep their cut sides open.");
-        if widgets::check_tip(ui, c3("Cap"), cap, true, tip) {
-            cap = !cap;
-            d.fields.insert("cap".into(), json!(cap));
+        if widgets::check3_tip(ui, c3("Cap"), cap, true, tip) {
+            d.fields.insert("cap".into(), json!(cap != Some(true)));
         }
         ui.add_space((ui.available_width() - 24.0).max(0.0));
-        if reset_dot(ui, "cap", !cap, "true") {
+        if reset_dot(ui, "cap", cap != Some(true), "true") {
             d.fields.insert("cap".into(), json!(true));
         }
     });
@@ -123,17 +122,17 @@ pub(crate) fn controls(app: &VectorcraftApp, ui: &mut egui::Ui, d: &mut Dialog)
         }
         ui.add_space(6.0);
         ui.horizontal(|ui| {
-            let visible = d.fields.get("expandVisibleOnly").and_then(Value::as_bool).unwrap_or(true);
+            let visible = flag(d, "expandVisibleOnly", true);
             let tip = tl!("Trim covered geometry on Expand Appearance. Live preview keeps the full surface. Transparent paints and surfaces exceeding visibility limits retain all faces.");
             let toggled = ui.allocate_ui_with_layout(
                 vec2((ui.available_width() - 30.0).max(100.0), 24.0), egui::Layout::left_to_right(egui::Align::Center), |ui| {
-                    widgets::check_tip(ui, tl!("Keep visible surfaces only when expanding"), visible, true, tip)
+                    widgets::check3_tip(ui, tl!("Keep visible surfaces only when expanding"), visible, true, tip)
                 },
             ).inner;
             if toggled {
-                d.fields.insert("expandVisibleOnly".into(), json!(!visible));
+                d.fields.insert("expandVisibleOnly".into(), json!(visible != Some(true)));
             }
-            if reset_dot(ui, "expandVisibleOnly", !visible, "true") {
+            if reset_dot(ui, "expandVisibleOnly", visible != Some(true), "true") {
                 d.fields.insert("expandVisibleOnly".into(), json!(true));
             }
         });
@@ -253,7 +252,9 @@ fn view_controls(ui: &mut egui::Ui, d: &mut Dialog) {
 fn choice(ui: &mut egui::Ui, d: &mut Dialog, key: &str, name: &str, options: &[(Value, &str)]) {
     let id = d.str("__effect");
     let default = vectorcraft_effects::default_params(&id).and_then(|p| p.get(key).cloned()).unwrap_or(Value::Null);
-    let value = d.fields.get(key).cloned().unwrap_or_else(|| default.clone());
+    // A mixed setting (null) shows its default, with its reset dot filled.
+    let mixed = d.fields.get(key).is_some_and(Value::is_null);
+    let value = d.fields.get(key).filter(|v| !v.is_null()).cloned().unwrap_or_else(|| default.clone());
     ui.horizontal(|ui| {
         ui.spacing_mut().item_spacing.x = 6.0;
         label(ui, name);
@@ -266,39 +267,48 @@ fn choice(ui: &mut egui::Ui, d: &mut Dialog, key: &str, name: &str, options: &[(
             d.fields.insert(key.into(), v.clone());
         }
         let default_label = options.iter().find(|(v, _)| *v == default).map_or("", |(_, l)| *l);
-        if reset_dot(ui, key, value != default, default_label) {
+        if reset_dot(ui, key, mixed || value != default, default_label) {
             d.fields.insert(key.into(), default.clone());
         }
     });
 }
 
-/// A checkbox for `key`, with its reset dot.
+/// The checkbox setting `key`: `None` when it is null (the 3D and Materials panel's selected
+/// effects hold it with different values), else its value, or `default` when it has none.
+fn flag(d: &Dialog, key: &str, default: bool) -> Option<bool> {
+    match d.fields.get(key) {
+        Some(Value::Null) => None,
+        value => Some(value.and_then(Value::as_bool).unwrap_or(default)),
+    }
+}
+
+/// A checkbox for `key`, with its reset dot. A mixed setting shows a dash, and a click turns it on.
 fn toggle(ui: &mut egui::Ui, d: &mut Dialog, key: &str, name: &str, default: bool) {
     ui.horizontal(|ui| {
-        let on = d.bool(key);
-        if widgets::check(ui, name, on, ui.is_enabled()) {
-            d.fields.insert(key.into(), json!(!on));
+        let on = flag(d, key, default);
+        if widgets::check3(ui, name, on, ui.is_enabled()) {
+            d.fields.insert(key.into(), json!(on != Some(true)));
         }
         ui.add_space((ui.available_width() - 24.0).max(0.0));
-        let mixed = d.fields.get(key).is_some_and(Value::is_null);
-        if reset_dot(ui, key, mixed || on != default, if default { "true" } else { "false" }) {
+        if reset_dot(ui, key, on != Some(default), if default { "true" } else { "false" }) {
             d.fields.insert(key.into(), json!(default));
         }
     });
 }
 
 fn lighting_controls(ui: &mut egui::Ui, d: &mut Dialog) {
+    let shade = flag(d, "shade", true);
     ui.horizontal(|ui| {
-        let shade = d.bool("shade");
-        if widgets::check(ui, tl!("Shading"), shade, true) {
-            d.fields.insert("shade".into(), json!(!shade));
+        if widgets::check3(ui, tl!("Shading"), shade, true) {
+            d.fields.insert("shade".into(), json!(shade != Some(true)));
         }
         ui.add_space((ui.available_width() - 24.0).max(0.0));
-        if reset_dot(ui, "shade", !shade, "true") {
+        if reset_dot(ui, "shade", shade != Some(true), "true") {
             d.fields.insert("shade".into(), json!(true));
         }
     });
-    ui.add_enabled_ui(d.bool("shade"), |ui| {
+    // A mixed Shading leaves the light settings on, for the effects that shade.
+    ui.add_enabled_ui(flag(d, "shade", true) != Some(false), |ui| {
         lighting_presets(ui, d);
         light_list(ui, d);
         let mut selected = selected_light(d);
@@ -419,8 +429,10 @@ fn light_list(ui: &mut egui::Ui, d: &mut Dialog) {
             super::revolve_gizmo::clear(ui.ctx());
         }
     });
+    // Add Light is off while a setting of the single light is mixed (null).
+    let mixed = explicit_lights(d).is_none() && LIGHT_KEYS.iter().any(|(_, key)| d.fields.get(*key).is_some_and(Value::is_null));
     ui.horizontal(|ui| {
-        if ui.add_enabled(list.len() < vectorcraft_three_d::MAX_LIGHTS, egui::Button::new(tl!("Add Light"))).clicked() {
+        if ui.add_enabled(list.len() < vectorcraft_three_d::MAX_LIGHTS && !mixed, egui::Button::new(tl!("Add Light"))).clicked() {
             let mut added = selected_light(d);
             let defaults = vectorcraft_effects::default_params(&d.str("__effect")).unwrap_or(json!({}));
             for (_, key) in LIGHT_KEYS {
@@ -464,7 +476,8 @@ fn light_color(ui: &mut egui::Ui, d: &mut Dialog) {
         }
         ui.label(format!("#{:02X}{:02X}{:02X}", rgb[0], rgb[1], rgb[2]));
         ui.add_space((ui.available_width() - 24.0).max(0.0));
-        if reset_dot(ui, "lightColor", rgb != [255; 3], "#FFFFFF") {
+        let mixed = d.fields.get("lightColor").is_some_and(Value::is_null);
+        if reset_dot(ui, "lightColor", mixed || rgb != [255; 3], "#FFFFFF") {
             d.fields.insert("lightColor".into(), json!("#ffffff"));
         }
     });
diff --git a/crates/ui-egui/src/panels/tests_three_d.rs b/crates/ui-egui/src/panels/tests_three_d.rs
index 44ed256f6..3c94bf88a 100644
--- a/crates/ui-egui/src/panels/tests_three_d.rs
+++ b/crates/ui-egui/src/panels/tests_three_d.rs
@@ -337,6 +337,70 @@ fn a_drag_ends_when_the_panel_is_no_longer_drawn() {
     assert_eq!(effect(&app).1["depth"], 90.0);
 }
 
+/// The panel's controls, drawn once with the accessibility tree on.
+fn controls(app: &mut VectorcraftApp, ctx: &egui::Context) -> Vec<egui::accesskit::Node> {
+    let mut out = ctx.run_ui(egui::RawInput::default(), |ui| show(app, ui));
+    out.textures_delta.clear();
+    out.platform_output.accesskit_update.map(|u| u.nodes.into_iter().map(|(_, n)| n).collect()).unwrap_or_default()
+}
+
+fn named<'a>(nodes: &'a [egui::accesskit::Node], label: &str) -> &'a egui::accesskit::Node {
+    nodes.iter().find(|n| n.label() == Some(label)).unwrap_or_else(|| panic!("no {label:?}"))
+}
+
+/// A checkbox setting the selected effects hold with different values shows a dash, leaves the
+/// controls it guards available, and another change leaves it as each effect has it.
+#[test]
+fn a_mixed_checkbox_shows_a_dash_and_another_change_keeps_each_effects_value() {
+    use crate::panels::tests_appearance::{click, frame_events, text_rect};
+    let mut app = app();
+    let a = app.session.active().unwrap().selection.objects[0].0;
+    set_type(&mut app, "extrude").unwrap();
+    let b = app.run("shape.rectangle", json!({"x": 250, "y": 100, "width": 100, "height": 100})).unwrap()["id"].as_u64().unwrap();
+    app.run("effect.apply", json!({"effect": "threeD.extrude", "item": null, "params": {"shade": false}})).unwrap();
+    app.run("select.set", json!({"ids": [a, b]})).unwrap();
+    let ctx = egui::Context::default();
+    crate::theme::install_fonts(&ctx);
+    ctx.enable_accesskit();
+    let texts = frame_events(&ctx, &mut app, vec![], show);
+    click(&ctx, &mut app, text_rect(&texts, "Lighting").center(), show);
+    let nodes = controls(&mut app, &ctx);
+    assert_eq!(named(&nodes, "Shading").toggled(), Some(egui::accesskit::Toggled::Mixed), "a mixed Shading shows a dash");
+    let behind = named(&nodes, "Light behind the object");
+    assert!(!behind.is_disabled(), "a mixed Shading leaves the light settings available");
+    assert!(!named(&nodes, "Add Light").is_disabled());
+    let at = behind.bounds().map(|r| egui::pos2(((r.x0 + r.x1) / 2.0) as f32, ((r.y0 + r.y1) / 2.0) as f32)).unwrap();
+    let history = undo_len(&app);
+    click(&ctx, &mut app, at, show);
+    assert_eq!(undo_len(&app), history + 1);
+    let params = |id: u64| app.session.active().unwrap().doc.node(vectorcraft_doc::NodeId(id)).unwrap().appearance.effects[0].params.clone();
+    let merged = |id: u64| vectorcraft_effects::merged_params("threeD.extrude", &params(id));
+    assert_eq!((merged(a)["shade"].clone(), merged(b)["shade"].clone()), (json!(true), json!(false)));
+    assert_eq!((merged(a)["lightBehind"].clone(), merged(b)["lightBehind"].clone()), (json!(true), json!(true)));
+    // A single light whose settings differ: Add Light is off.
+    app.run("select.set", json!({"ids": [b]})).unwrap();
+    app.run("effect.setParams", json!({"index": 0, "item": null, "params": {"lightIntensity": 40.0}})).unwrap();
+    app.run("select.set", json!({"ids": [a, b]})).unwrap();
+    assert!(named(&controls(&mut app, &ctx), "Add Light").is_disabled());
+}
+
+/// Revolve's Cap checkbox shows a dash when the selected effects disagree on it.
+#[test]
+fn a_mixed_revolve_cap_shows_a_dash() {
+    let mut app = app();
+    let a = app.session.active().unwrap().selection.objects[0].0;
+    set_type(&mut app, "revolve").unwrap();
+    let b = app.run("shape.rectangle", json!({"x": 250, "y": 100, "width": 100, "height": 100})).unwrap()["id"].as_u64().unwrap();
+    app.run("effect.apply", json!({"effect": "threeD.revolve", "item": null, "params": {"cap": false}})).unwrap();
+    app.run("select.set", json!({"ids": [a, b]})).unwrap();
+    let ctx = egui::Context::default();
+    crate::theme::install_fonts(&ctx);
+    ctx.enable_accesskit();
+    let history = undo_len(&app);
+    assert_eq!(named(&controls(&mut app, &ctx), "Cap").toggled(), Some(egui::accesskit::Toggled::Mixed));
+    assert_eq!(undo_len(&app), history);
+}
+
 #[test]
 fn changed_settings_lists_only_new_values() {
     let before = json!({"depth": 50.0, "cap": true, "bevel": "none"});
diff --git a/crates/ui-egui/src/panels/three_d.rs b/crates/ui-egui/src/panels/three_d.rs
index cf6515396..25dfaa684 100644
--- a/crates/ui-egui/src/panels/three_d.rs
+++ b/crates/ui-egui/src/panels/three_d.rs
@@ -246,7 +246,7 @@ pub fn show(app: &mut VectorcraftApp, ui: &mut Ui) {
 
 /// Sets to null each setting in `fields` (the first selected object's 3D effect) that the 3D effect
 /// of another selected object has with another value. The controls then show it at its default
-/// with its reset dot filled, and its reset dot, a menu choice, a checkbox click, or a number other
+/// (a checkbox shows a dash) with its reset dot filled, and its reset dot, a menu choice, a checkbox click, or a number other
 /// than the default shown is a change sent to every effect. The light list (`lights`, where null
 /// is the single light) is left as it is.
 pub(crate) fn mark_mixed(app: &VectorcraftApp, fields: &mut Map<String, Value>) {
diff --git a/crates/ui-egui/src/widgets.rs b/crates/ui-egui/src/widgets.rs
index 1d0031e4e..689f28ac5 100644
--- a/crates/ui-egui/src/widgets.rs
+++ b/crates/ui-egui/src/widgets.rs
@@ -70,7 +70,7 @@ pub use text::{
     exact_text_field, exact_text_field_named, plain_field, plain_field_named, search_field, text_field, text_field_named, text_presets,
     text_presets_named,
 };
-pub use toggles::{check, check_box, check_name, check_tip, check3, radio, radio_button};
+pub use toggles::{check, check_box, check_name, check_tip, check3, check3_tip, radio, radio_button};
 
 /// A widget id salt that needs only [`std::hash::Hash`]. egui 0.37's [`egui::AsIdSalt`] also
 /// requires `Debug` for its debug-only id-source registry; VectorCraft's public field helpers do
diff --git a/crates/ui-egui/src/widgets/toggles.rs b/crates/ui-egui/src/widgets/toggles.rs
index 8dac694ec..5cac2023e 100644
--- a/crates/ui-egui/src/widgets/toggles.rs
+++ b/crates/ui-egui/src/widgets/toggles.rs
@@ -26,7 +26,12 @@ pub fn check(ui: &mut Ui, label: &str, value: bool, enabled: bool) -> bool {
 
 /// [`check`] with tooltip `tip` on the box and its label. Returns true when toggled.
 pub fn check_tip(ui: &mut Ui, label: &str, value: bool, enabled: bool, tip: &str) -> bool {
-    let (clicked, resp) = check_row(ui, tl!(label), Some(value), enabled);
+    check3_tip(ui, label, Some(value), enabled, tip)
+}
+
+/// [`check3`] with tooltip `tip` on the box and its label. Returns true when clicked.
+pub fn check3_tip(ui: &mut Ui, label: &str, value: Option<bool>, enabled: bool, tip: &str) -> bool {
+    let (clicked, resp) = check_row(ui, tl!(label), value, enabled);
     resp.on_hover_text(tip);
     clicked
 }
diff --git a/docs/three-d.md b/docs/three-d.md
index 93e831e01..bb0995315 100644
--- a/docs/three-d.md
+++ b/docs/three-d.md
@@ -33,11 +33,13 @@ pages describe it.
   and ranges, showing the 3D effect of the first selected object (its own effects first, then those of its fills
   and strokes). A changed setting goes to the 3D effect of each selected object whose effect takes that setting;
   other effects and objects without a 3D effect do not change. A setting the selected 3D effects hold with
-  different values shows its default with its reset dot filled. Its reset dot, a choice from its menu or a click
-  on its checkbox sets it on all of them; a typed or dragged number sets it when it differs from the default
-  shown.
+  different values shows its default with its reset dot filled; a checkbox shows a dash instead, and a mixed
+  Shading leaves the light settings available. Its reset dot, a choice from its menu or a click on its checkbox
+  (which turns a mixed one on) sets it on all of them; a typed or dragged number sets it when it differs from the
+  default shown. Other changes leave it as each effect has it.
   The list of lights shows the first object's lights, and an edit to it (Add Light, Delete Light, or a setting
-  of one light in a list) reaches only the objects whose list of lights matches the first object's.
+  of one light in a list) reaches only the objects whose list of lights matches the first object's. Add Light is
+  unavailable while a setting of a single light differs between the selected effects.
 - Each change is one undo step. Dragging a value or a slider previews it on the artboard and records one step
   when the pointer is released; a typed value is recorded when it is entered. Drawing the panel records nothing.
 - The Materials tab, shadows and ray-traced rendering are not built (see below). The rotation and light rings on