it: test natural scrolling

src/it/test_utils/test_toplevel_node_ext.rs

@@ -0,0 +1,12 @@
+use crate::tree::ToplevelNode;
+
+pub trait TestToplevelNodeExt {
+    fn center(&self) -> (i32, i32);
+}
+
+impl TestToplevelNodeExt for dyn ToplevelNode {
+    fn center(&self) -> (i32, i32) {
+        let rect = self.node_absolute_position();
+        ((rect.x1() + rect.x2()) / 2, (rect.y1() + rect.y2()) / 2)
+    }
+}