it: test natural scrolling

src/it/test_utils/test_container_node_ext.rs

@@ -0,0 +1,20 @@
+use {
+    crate::{
+        it::test_error::TestResult,
+        tree::{ContainerNode, ToplevelNode},
+    },
+    std::rc::Rc,
+};
+
+pub trait TestContainerExt {
+    fn first_toplevel(&self) -> TestResult<Rc<dyn ToplevelNode>>;
+}
+
+impl TestContainerExt for ContainerNode {
+    fn first_toplevel(&self) -> TestResult<Rc<dyn ToplevelNode>> {
+        match self.children.first() {
+            None => bail!("container does not have children"),
+            Some(c) => Ok(c.node.clone()),
+        }
+    }
+}