diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index 587379a85..680cbff22 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -246,6 +246,18 @@ fn text_shape_mask_matte_parent() {
     assert_eq!(info["layers"].as_array().unwrap().len(), 2);
 }
 
+/// A shape layer the size of the comp and placed at its default position covers the frame: the
+/// Shape Tool's double-click (#1284) makes one this way, for backgrounds, mattes and overlays.
+#[test]
+fn a_comp_sized_shape_layer_sits_on_the_comp_centre() {
+    let mut s = Session::default();
+    s.execute("comp.new", json!({"width": 400, "height": 300, "duration": 2})).unwrap();
+    let sh = s.execute("layer.newShape", json!({"kind": "rect", "size": [400, 300], "fill": "#ff0000"})).unwrap()["layer"].as_u64().unwrap();
+    let l = s.active_comp().unwrap().layer(effectcraft_project::LayerId(sh)).unwrap().clone();
+    let pos = l.props.prop("transform/position").unwrap().value.clone();
+    assert!(matches!(pos, effectcraft_keyframe::Value::Vec3(p) if (p[0] - 200.0).abs() < 1e-9 && (p[1] - 150.0).abs() < 1e-9), "{pos:?}");
+}
+
 /// The toolbar's Fill / Stroke change a selected shape layer's paint (#205): a shape drawn
 /// without a stroke gets one, later edits change it, and the Contents "Add:" items (#206) include
 /// an empty Path and a Gradient Stroke that render.
diff --git a/crates/ui-egui/src/header.rs b/crates/ui-egui/src/header.rs
index 1260c9c9e..0c6a9d169 100644
--- a/crates/ui-egui/src/header.rs
+++ b/crates/ui-egui/src/header.rs
@@ -82,6 +82,15 @@ pub fn show(app: &mut EffectcraftApp, ui: &mut egui::Ui, rect: Rect) {
         {
             app.ui.status = e;
         }
+        // Double-click a shape tool: a shape layer filling the composition, centred, for
+        // backgrounds, mattes and overlays (as After Effects does).
+        if cur.is_shape()
+            && resp.double_clicked()
+            && let Some((w, h)) = app.session.active_comp().map(|c| (c.width, c.height))
+            && let Err(e) = crate::menus::invoke(app, ui.ctx(), "layer.newShape", json!({"kind": crate::panels::viewer::shape_kind(cur), "size": [w, h]}))
+        {
+            app.ui.status = e;
+        }
         if slot.len() > 1 {
             let held_for = resp.is_pointer_button_down_on().then(|| ui.input(|i| i.pointer.press_start_time().map(|s| i.time - s))).flatten();
             if let Some(seconds) = held_for
diff --git a/crates/ui-egui/src/panels/viewer.rs b/crates/ui-egui/src/panels/viewer.rs
index 5fe7697c9..8410bb78c 100644
--- a/crates/ui-egui/src/panels/viewer.rs
+++ b/crates/ui-egui/src/panels/viewer.rs
@@ -2138,7 +2138,7 @@ struct ShapeDrag {
 }
 
 /// The kind `tool` draws, as `shape.newShape` / `layer.addMask` name it.
-fn shape_kind(tool: Tool) -> &'static str {
+pub(crate) fn shape_kind(tool: Tool) -> &'static str {
     match tool {
         Tool::Rectangle => "rect",
         Tool::RoundedRect => "rounded",