diff --git a/crates/ui/src/docking/desktop.rs b/crates/ui/src/docking/desktop.rs
new file mode 100644
index 00000000..2047a641
--- /dev/null
+++ b/crates/ui/src/docking/desktop.rs
@@ -0,0 +1,69 @@
+//! Which of the application's windows lies under a point on the desktop (feature
+//! `native-windows`). Points and rects are in desktop logical points.
+
+use egui::{Pos2, Rect};
+
+/// Where a desktop point falls.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub(super) enum Over {
+    /// The floating native window with this [`super::Floating::id`], at this point in its own
+    /// coordinates.
+    Window(u64, Pos2),
+    /// The main window, at this point in its coordinates.
+    Main(Pos2),
+    /// None of the application's windows.
+    Desktop,
+}
+
+/// The window under `point`: the first of `windows` (floating native windows by id and desktop
+/// rect, front first) that holds it, then `main` (the main window's content). The window
+/// `dragged` follows the pointer by its bar and is never its own target.
+pub(super) fn window_at(point: Pos2, windows: &[(u64, Rect)], main: Option<Rect>, dragged: Option<u64>) -> Over {
+    if !point.is_finite() {
+        return Over::Desktop;
+    }
+    if let Some((id, rect)) = windows.iter().find(|(id, rect)| Some(*id) != dragged && rect.contains(point)) {
+        return Over::Window(*id, point - rect.min.to_vec2());
+    }
+    match main {
+        Some(main) if main.contains(point) => Over::Main(point - main.min.to_vec2()),
+        _ => Over::Desktop,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use egui::{pos2, vec2};
+
+    use super::*;
+
+    fn rect(x: f32, y: f32, w: f32, h: f32) -> Rect {
+        Rect::from_min_size(pos2(x, y), vec2(w, h))
+    }
+
+    #[test]
+    fn the_front_window_wins_then_the_main_window_then_the_desktop() {
+        let main = Some(rect(100.0, 50.0, 800.0, 600.0));
+        // Window 2 lies over window 1, and both over the main window.
+        let windows = [(2, rect(300.0, 100.0, 200.0, 200.0)), (1, rect(250.0, 80.0, 200.0, 200.0))];
+        assert_eq!(window_at(pos2(350.0, 150.0), &windows, main, None), Over::Window(2, pos2(50.0, 50.0)));
+        assert_eq!(window_at(pos2(260.0, 90.0), &windows, main, None), Over::Window(1, pos2(10.0, 10.0)));
+        assert_eq!(window_at(pos2(120.0, 60.0), &windows, main, None), Over::Main(pos2(20.0, 10.0)));
+        assert_eq!(window_at(pos2(20.0, 20.0), &windows, main, None), Over::Desktop);
+        // A floating window outside the main window.
+        let windows = [(3, rect(1000.0, 0.0, 100.0, 100.0))];
+        assert_eq!(window_at(pos2(1050.0, 40.0), &windows, main, None), Over::Window(3, pos2(50.0, 40.0)));
+        // Unknown main window, a point that is not one.
+        assert_eq!(window_at(pos2(120.0, 60.0), &[], None, None), Over::Desktop);
+        assert_eq!(window_at(pos2(f32::NAN, 60.0), &windows, main, None), Over::Desktop);
+    }
+
+    #[test]
+    fn the_dragged_window_is_looked_through() {
+        let main = Some(rect(0.0, 0.0, 800.0, 600.0));
+        let windows = [(2, rect(300.0, 100.0, 200.0, 200.0)), (1, rect(250.0, 80.0, 200.0, 200.0))];
+        assert_eq!(window_at(pos2(350.0, 150.0), &windows, main, Some(2)), Over::Window(1, pos2(100.0, 70.0)));
+        assert_eq!(window_at(pos2(350.0, 150.0), &windows, main, Some(1)), Over::Window(2, pos2(50.0, 50.0)));
+        assert_eq!(window_at(pos2(480.0, 280.0), &windows, main, Some(2)), Over::Main(pos2(480.0, 280.0)));
+    }
+}
diff --git a/crates/ui/src/docking/mod.rs b/crates/ui/src/docking/mod.rs
index 7c1f4585..6fefc004 100644
--- a/crates/ui/src/docking/mod.rs
+++ b/crates/ui/src/docking/mod.rs
@@ -8,6 +8,8 @@
 pub mod chrome;
 #[cfg(feature = "serde")]
 mod command;
+#[cfg(feature = "native-windows")]
+mod desktop;
 mod dock_model;
 mod hits;
 mod labels;