diff --git a/crates/engine/src/cmd/component.rs b/crates/engine/src/cmd/component.rs
index 6a5b1012..6b0e3e17 100644
--- a/crates/engine/src/cmd/component.rs
+++ b/crates/engine/src/cmd/component.rs
@@ -217,14 +217,14 @@ pub fn specs() -> Vec<CommandSpec> {
         spec!(
             "component.addProperty",
             "Add component property",
-            "{id? (component, set, or a layer inside a component: it gets bound), name, type: boolean|text|instanceSwap|variant|slot, default?, layer? (node bound to it; a frame for slots), preferred? (instance swap and slot properties: component or set ids offered first)}",
+            "{id? (component, set, or a layer inside a component: it gets bound), name, type: boolean|text|instanceSwap|variant|slot, default? (boolean: the bound layer's visibility in the main; without it, the layer's current visibility), layer? (node bound to it; a frame for slots), preferred? (instance swap and slot properties: component or set ids offered first)}",
             true,
             add_prop
         ),
         spec!(
             "component.editProperty",
             "Edit component property",
-            "{id? (component or set), name, newName?, default? (variant properties: that variant takes the top-left place), value? + newValue? (rename a variant value), index? (variant properties: move to this place; the variants' names follow), preferred? (instance swap and slot properties: component or set ids offered first)}",
+            "{id? (component or set), name, newName?, default? (variant properties: that variant takes the top-left place; boolean: the main's bound layers show or hide to match), value? + newValue? (rename a variant value), index? (variant properties: move to this place; the variants' names follow), preferred? (instance swap and slot properties: component or set ids offered first)}",
             true,
             edit_prop
         ),
@@ -1141,7 +1141,8 @@ fn add_prop_for(cmd: &str, s: &mut Session, p: &Value) -> Result<Value> {
     }
     let layer_node = layer.and_then(|l| doc.node(l)).cloned();
     let def = match t.to_ascii_lowercase().as_str() {
-        "boolean" | "bool" => PropDef::Boolean { default: boolean(p, "default").unwrap_or(true) },
+        // Without a default, a bound layer keeps showing as it does: its visibility is the default.
+        "boolean" | "bool" => PropDef::Boolean { default: boolean(p, "default").or_else(|| layer_node.as_ref().map(|n| n.visible)).unwrap_or(true) },
         "text" => PropDef::Text {
             default: string(p, "default")
                 .or_else(|| layer_node.as_ref().and_then(|n| n.kind.text()).map(|t| t.characters.clone()))
@@ -1245,7 +1246,12 @@ fn bind(cmd: &str, doc: &mut Document, layer: NodeId, property: Option<&str>, fi
         }
         _ => {}
     }
-    doc.get_mut(layer)?.prop_refs.insert(field.to_string(), property.to_string());
+    let n = doc.get_mut(layer)?;
+    n.prop_refs.insert(field.to_string(), property.to_string());
+    // On the main, a boolean property's value is the visibility of the layers it controls.
+    if let PropDef::Boolean { default } = def {
+        n.visible = *default;
+    }
     Ok(())
 }
 
@@ -1414,6 +1420,10 @@ fn edit_prop(s: &mut Session, p: &Value) -> Result<Value> {
             rename_prop_refs(doc, *c, &name, Some(nn))?;
         }
     }
+    // The main's bound layers show the new default.
+    if let PropDef::Boolean { default } = new_def {
+        set_boolean_default(doc, &comps, new_name.as_deref().unwrap_or(&name), default);
+    }
     ok()
 }
 
diff --git a/crates/engine/src/components.rs b/crates/engine/src/components.rs
index 67259d03..98fd85f1 100644
--- a/crates/engine/src/components.rs
+++ b/crates/engine/src/components.rs
@@ -1110,9 +1110,73 @@ fn geometry_derived(doc: &Document, before: &Document, id: NodeId, group: &str)
     false
 }
 
+/// The components sharing the properties of component `c`: the variants of its set, or `c` alone.
+pub fn property_owners(doc: &Document, c: NodeId) -> Vec<NodeId> {
+    match variant_set(doc, c).and_then(|s| doc.node(s)) {
+        Some(set) => set.children.iter().copied().filter(|v| doc.node(*v).is_some_and(|n| matches!(n.kind, NodeKind::Component { .. }))).collect(),
+        None => vec![c],
+    }
+}
+
+/// Make `value` the default of boolean property `name` on components `comps`, showing or hiding
+/// their layers bound to it: on a main component, a boolean property's value is the visibility of
+/// its layers. Returns the components and layers it changed.
+pub fn set_boolean_default(doc: &mut Document, comps: &[NodeId], name: &str, value: bool) -> Vec<NodeId> {
+    let mut out = Vec::new();
+    for &c in comps {
+        let Some(NodeKind::Component { props, .. }) = doc.node(c).map(|n| &n.kind) else { continue };
+        let Some(PropDef::Boolean { default }) = props.get(name) else { continue };
+        if *default != value
+            && let Some(NodeKind::Component { props, .. }) = doc.node_mut(c).map(|n| &mut n.kind)
+        {
+            props.insert(name.to_string(), PropDef::Boolean { default: value });
+            out.push(c);
+        }
+        // The main's own layers (not those of instances nested in it, which follow their mains).
+        let layers: Vec<NodeId> = doc
+            .subtree(c)
+            .into_iter()
+            .filter(|d| *d != c && !inside_instance(doc, *d))
+            .filter(|d| doc.node(*d).is_some_and(|n| n.visible != value && n.prop_refs.get("visible").map(String::as_str) == Some(name)))
+            .collect();
+        for d in layers {
+            if let Some(n) = doc.node_mut(d) {
+                n.visible = value;
+                out.push(d);
+            }
+        }
+    }
+    out
+}
+
+/// A main component's layer bound to a boolean property was shown or hidden: that is the
+/// property's new default, which its other bound layers (in every variant) take too. Returns the
+/// components and layers that changed as a result.
+fn follow_boolean_defaults(doc: &mut Document, before: &Document, changed: &[NodeId]) -> Vec<NodeId> {
+    let mut out = Vec::new();
+    for &id in changed {
+        let (Some(n), Some(b)) = (doc.node(id), before.node(id)) else { continue };
+        if n.visible == b.visible || inside_instance(doc, id) {
+            continue;
+        }
+        let Some(name) = n.prop_refs.get("visible").cloned() else { continue };
+        let visible = n.visible;
+        let Some(c) = owning_component(doc, id).filter(|c| *c != id) else { continue };
+        out.extend(set_boolean_default(doc, &property_owners(doc, c), &name, visible));
+    }
+    out
+}
+
 /// After an edit: record overrides inside instances, then rebuild instances whose main changed,
 /// whose properties or main were switched, or whose overrides were reset.
 pub fn sync_after_edit(doc: &mut Document, before: &Document, changed: &[NodeId], touched: &BTreeSet<NodeId>) {
+    let mut changed = changed.to_vec();
+    for id in follow_boolean_defaults(doc, before, &changed) {
+        if !changed.contains(&id) {
+            changed.push(id);
+        }
+    }
+    let changed = changed.as_slice();
     // 0. A slot whose layers were added, removed or reordered holds the instance's own content
     //    from now on; copies of instance layers placed elsewhere stop mirroring their main.
     for &id in changed {
diff --git a/crates/engine/src/components_tests.rs b/crates/engine/src/components_tests.rs
index 209560cd..30254651 100644
--- a/crates/engine/src/components_tests.rs
+++ b/crates/engine/src/components_tests.rs
@@ -429,6 +429,90 @@ fn component_properties_apply_to_instances() {
     healthy(&s);
 }
 
+/// The default of boolean property `name` on component `c`.
+fn bool_default(s: &Session, c: NodeId, name: &str) -> Option<bool> {
+    match &node(s, c).kind {
+        NodeKind::Component { props, .. } => match props.get(name) {
+            Some(PropDef::Boolean { default }) => Some(*default),
+            _ => None,
+        },
+        _ => None,
+    }
+}
+
+#[test]
+fn boolean_property_default_is_the_mains_visibility() {
+    let mut s = Session::new();
+    let (c, bg, title, dot) = card(&mut s);
+    // #993: a `false` default hides the bound layer in the main, not only in instances.
+    ex(&mut s, "component.addProperty", json!({"id": c, "name": "Show dot", "type": "boolean", "default": false, "layer": dot}));
+    assert!(!node(&s, dot).visible);
+    let i = idv(&ex(&mut s, "instance.create", json!({"component": c, "x": 300, "y": 0})));
+    assert!(!node(&s, copy_of(&s, i, dot)).visible);
+    // Without a default, the property takes the layer's visibility.
+    ex(&mut s, "node.set", json!({"id": bg, "props": {"visible": false}}));
+    ex(&mut s, "component.addProperty", json!({"id": c, "name": "Show bg", "type": "boolean", "layer": bg}));
+    assert_eq!(bool_default(&s, c, "Show bg"), Some(false));
+    assert!(!node(&s, bg).visible);
+    // Binding a layer to an existing property shows it as the default says.
+    ex(&mut s, "component.bindProperty", json!({"layer": title, "property": "Show dot"}));
+    assert!(!node(&s, title).visible);
+    // A new default shows or hides every bound layer of the main; instances keep their values.
+    ex(&mut s, "instance.setProperties", json!({"ids": [i], "properties": {"Show dot": false}}));
+    ex(&mut s, "component.editProperty", json!({"id": c, "name": "Show dot", "default": true}));
+    assert!(node(&s, dot).visible && node(&s, title).visible);
+    assert!(!node(&s, copy_of(&s, i, dot)).visible, "the instance's own value stays");
+    // Renamed with a new default in one go.
+    ex(&mut s, "component.editProperty", json!({"id": c, "name": "Show dot", "newName": "Dot", "default": false}));
+    assert!(!node(&s, dot).visible && !node(&s, title).visible);
+    assert_eq!(bool_default(&s, c, "Dot"), Some(false));
+    // Showing a bound layer on the main is the property's new default, for its other layers too.
+    ex(&mut s, "node.set", json!({"id": dot, "props": {"visible": true}}));
+    assert_eq!(bool_default(&s, c, "Dot"), Some(true));
+    assert!(node(&s, title).visible);
+    let j = idv(&ex(&mut s, "instance.create", json!({"component": c, "x": 600, "y": 0})));
+    assert!(node(&s, copy_of(&s, j, dot)).visible && node(&s, copy_of(&s, j, title)).visible);
+    assert!(!node(&s, copy_of(&s, i, dot)).visible, "existing instances keep their values");
+    // Hiding a layer inside an instance is an override there, not a new default.
+    let jdot = copy_of(&s, j, dot);
+    ex(&mut s, "node.set", json!({"id": jdot, "props": {"visible": false}}));
+    assert_eq!(bool_default(&s, c, "Dot"), Some(true));
+    assert!(node(&s, dot).visible);
+    // Undo puts the default and the layers back together.
+    s.undo().unwrap();
+    s.undo().unwrap();
+    s.undo().unwrap();
+    assert_eq!(bool_default(&s, c, "Dot"), Some(false));
+    assert!(!node(&s, dot).visible && !node(&s, title).visible);
+    healthy(&s);
+}
+
+#[test]
+fn boolean_property_default_follows_across_variants() {
+    let mut s = Session::new();
+    let mut variants = Vec::new();
+    let mut labels = Vec::new();
+    for (k, state) in ["Default", "Hover"].iter().enumerate() {
+        let f =
+            idv(&ex(&mut s, "frame.create", json!({"x": k as f64 * 150.0, "y": 0, "width": 100, "height": 40, "name": format!("State={state}")})));
+        labels.push(idv(&ex(&mut s, "text.create", json!({"text": *state, "x": 10, "y": 10, "parent": f, "props": {"name": "Label"}}))));
+        variants.push(idv(&ex(&mut s, "component.create", json!({"ids": [f]}))));
+    }
+    let set = idv(&ex(&mut s, "component.combineVariants", json!({"ids": variants.clone()})));
+    ex(&mut s, "component.addProperty", json!({"id": set, "name": "Show label", "type": "boolean", "default": false, "layer": labels[0]}));
+    ex(&mut s, "component.bindProperty", json!({"layer": labels[1], "property": "Show label"}));
+    assert!(labels.iter().all(|l| !node(&s, *l).visible));
+    ex(&mut s, "component.editProperty", json!({"id": set, "name": "Show label", "default": true}));
+    assert!(labels.iter().all(|l| node(&s, *l).visible));
+    // Hiding the label in one variant is the set's default: the other variant's label follows.
+    ex(&mut s, "node.set", json!({"id": labels[1], "props": {"visible": false}}));
+    assert!(variants.iter().all(|v| bool_default(&s, *v, "Show label") == Some(false)));
+    assert!(!node(&s, labels[0]).visible);
+    let i = idv(&ex(&mut s, "instance.create", json!({"component": set, "x": 0, "y": 200})));
+    assert!(!node(&s, kids(&s, i)[0]).visible);
+    healthy(&s);
+}
+
 #[test]
 fn push_changes_to_main() {
     let mut s = Session::new();
diff --git a/docs/mcp.md b/docs/mcp.md
index dfea95cc..3d412727 100644
--- a/docs/mcp.md
+++ b/docs/mcp.md
@@ -713,6 +713,14 @@ most when it can't be reached).
   moves the chosen variant there, `{name, index}` reorders variant properties (the variants'
   names follow). `component.properties` on a set reports `conflicts`: variants sharing one
   combination of values.
+- **Boolean properties**: on the main component, a boolean property's value is the visibility of
+  the layers bound to it. `component.addProperty {type: "boolean", default, layer}`,
+  `component.bindProperty {field: "visible"}` and `component.editProperty {name, default}` show or
+  hide the main's bound layers to match the default (without `default`, `addProperty` takes the
+  bound layer's visibility). Showing or hiding a bound layer on the main sets the default, and the
+  property's other bound layers (in every variant of a set) follow. New instances start from the
+  default; existing instances keep the values they have. Files open as saved: a main whose bound
+  layer disagrees with the default stays that way until one of them is edited.
 - `component.pushChanges {id?}` copies the instance's overrides into its main (from a layer of a
   nested instance too: always into the outer instance's main).