webdriver: Check if container is obscured for "Element Click" (#38497)

Implement step 7 of [Element
Click](https://w3c.github.io/webdriver/#element-click): check whether
container is obscured.

Testing: `/webdriver/tests/classic/element_click/interactability.py` can
now fully pass. Other changes are combined effect with
17a269a8ad due to script execution, and
exposes new problem:
https://github.com/servo/servo/pull/38497#discussion_r2257866612

---------

Signed-off-by: Euclid Ye <euclid.ye@huawei.com>

components/script/webdriver_handlers.rs

@@ -1846,8 +1846,18 @@ pub(crate) fn handle_element_click(
                     return Err(ErrorStatus::ElementNotInteractable);
                 }
 
-                // Step 7
-                // TODO: return error if obscured
+                // Step 7. If element's container is obscured by another element,
+                // return error with error code element click intercepted.
+                // https://w3c.github.io/webdriver/#dfn-obscuring
+                // An element is obscured if the pointer-interactable paint tree is empty,
+                // or the first element in this tree is not an inclusive descendant of itself.
+                // `paint_tree` is guaranteed not empty as element is "in view".
+                if !container
+                    .upcast::<Node>()
+                    .Contains(Some(paint_tree[0].upcast::<Node>()))
+                {
+                    return Err(ErrorStatus::ElementClickIntercepted);
+                }
 
                 // Step 8 for <option> element.
                 match element.downcast::<HTMLOptionElement>() {