fix: unify animated presentation and stacking

compositor/src/stacking.rs

@@ -0,0 +1,48 @@
+use ferese_layout::WindowId;
+
+/// Persistent back-to-front order. Geometry updates never change this order.
+#[derive(Default)]
+pub(crate) struct WindowStack {
+    order: Vec<WindowId>,
+}
+
+impl WindowStack {
+    pub fn insert(&mut self, id: WindowId) {
+        if !self.order.contains(&id) {
+            self.order.push(id);
+        }
+    }
+
+    pub fn remove(&mut self, id: WindowId) {
+        self.order.retain(|candidate| *candidate != id);
+    }
+
+    pub fn raise(&mut self, id: WindowId) {
+        self.remove(id);
+        self.insert(id);
+    }
+
+    pub fn rank(&self, id: WindowId) -> usize {
+        self.order
+            .iter()
+            .position(|candidate| *candidate == id)
+            .unwrap_or(usize::MAX)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn remapping_neighbours_does_not_change_order() {
+        let mut stack = WindowStack::default();
+        stack.insert(WindowId(1));
+        stack.insert(WindowId(2));
+        stack.raise(WindowId(1));
+        stack.insert(WindowId(2));
+        assert!(stack.rank(WindowId(1)) > stack.rank(WindowId(2)));
+        stack.remove(WindowId(1));
+        assert_eq!(stack.rank(WindowId(2)), 0);
+    }
+}