diff --git a/crates/ui-egui/src/grid.rs b/crates/ui-egui/src/grid.rs
index be8a578..54e8387 100644
--- a/crates/ui-egui/src/grid.rs
+++ b/crates/ui-egui/src/grid.rs
@@ -2722,8 +2722,9 @@ pub(crate) fn paint_overlays(
         p.rect_stroke(r, 2.0, Stroke::new(1.0, Color32::from_gray(160)), StrokeKind::Inside);
         crate::icons::paint(p, r.shrink(3.0), crate::icons::Icon::Chevron, Color32::from_gray(60));
     }
-    // Input message of the active cell's validation.
+    // Input message of the active cell's validation, hidden while its list is open: it would cover the items.
     if app.editor.is_none()
+        && app.grid.list_picker != Some(sel.active)
         && let Some(dv) = sh.validations.iter().find(|d| d.ranges.iter().any(|r| r.contains(sel.active)))
         && dv.show_input
         && !(dv.input_title.is_empty() && dv.input_message.is_empty())
diff --git a/crates/ui-egui/tests/validation_input_message.rs b/crates/ui-egui/tests/validation_input_message.rs
new file mode 100644
index 0000000..e05dbce
--- /dev/null
+++ b/crates/ui-egui/tests/validation_input_message.rs
@@ -0,0 +1,56 @@
+//! A list validation's input message must not cover the dropdown it describes (#505).
+
+use egui::{Event, Key, Modifiers, PointerButton};
+use egui_kittest::kittest::Queryable;
+use gridcraft_engine::Session;
+use gridcraft_engine::core::CellRef;
+use gridcraft_ui_egui::{SheetApp, grid};
+use serde_json::json;
+
+type Harness = egui_kittest::Harness<'static, SheetApp>;
+
+const MESSAGE: &str = "Pick a colour from the list";
+
+fn harness() -> Harness {
+    let mut session = Session::new();
+    session.new_workbook();
+    session.execute("data.validation", json!({"range": "B2", "type": "list", "formula1": "\"Red,Green,Blue\"", "inputMessage": MESSAGE})).unwrap();
+    session.execute("selection.set", json!({"cell": "B2"})).unwrap();
+    let mut h = egui_kittest::Harness::builder().with_size(egui::vec2(1200.0, 800.0)).build_ui_state(
+        |ui, app: &mut SheetApp| {
+            app.logic(ui.ctx());
+            app.ui(ui);
+        },
+        SheetApp::new(session, Default::default()),
+    );
+    h.run_steps(4);
+    h
+}
+
+fn message_shown(h: &Harness) -> bool {
+    h.query_by_label(MESSAGE).is_some()
+}
+
+#[test]
+fn input_message_hides_while_the_list_is_open() {
+    let mut h = harness();
+    assert!(message_shown(&h), "the selected cell's input message is shown");
+
+    let app = h.state();
+    let sh = app.session.active().unwrap().wb.active().unwrap();
+    let geo = grid::Geo::new(sh, app.grid.rect.unwrap(), app.view().scroll);
+    let pos = grid::validation_arrow(sh, &geo, CellRef::parse("B2").unwrap()).unwrap().center();
+    h.input_mut().events.push(Event::PointerMoved(pos));
+    h.input_mut().events.push(Event::PointerButton { pos, button: PointerButton::Primary, pressed: true, modifiers: Modifiers::NONE });
+    h.input_mut().events.push(Event::PointerButton { pos, button: PointerButton::Primary, pressed: false, modifiers: Modifiers::NONE });
+    h.run_steps(2);
+
+    assert!(h.state().grid.list_picker.is_some(), "the dropdown arrow opens the list");
+    assert!(h.query_by_label("Green").is_some(), "the list shows its items");
+    assert!(!message_shown(&h), "the input message must not cover the open list");
+
+    h.input_mut().events.push(Event::Key { key: Key::Escape, physical_key: None, pressed: true, repeat: false, modifiers: Modifiers::NONE });
+    h.run_steps(2);
+    assert!(h.state().grid.list_picker.is_none(), "Escape closes the list");
+    assert!(message_shown(&h), "the input message comes back once the list is closed");
+}