script: Fix MouseOver handling

Now we only query for the topmost node, and apply the hover state to all
of the parent elements.

This fixes things like #9705, where the hover state was applied only to
the children.

This also makes us more conformant with other browsers in the case of
taking in account margins and paddings.

For example, prior to this PR, when your mouse was over the inner
element, in the bottom part, `hover` styles didn't apply to the parent.

```html
<style>
div {
  padding: 10px;
  margin: 10px;
  height: 15px;
  background: blue;
}

div:hover {
  background: red;
}
</style>

<div>
  <div></div>
</div>
```

Fixes #9705

1 files changed, 2 insertions, 1 deletions

diff --git a/components/script/layout_interface.rs b/components/script/layout_interface.rs
index 17f553d43e8..4ad7c241f14 100644
--- a/components/script/layout_interface.rs
+++ b/components/script/layout_interface.rs
@@ -106,6 +106,7 @@ pub trait LayoutRPC {
     fn node_geometry(&self) -> NodeGeometryResponse;
     /// Requests the node containing the point of interest
     fn hit_test(&self, point: Point2D<f32>) -> Result<HitTestResponse, ()>;
+    /// Query layout for the topmost node under the mouse.
     fn mouse_over(&self, point: Point2D<f32>) -> Result<MouseOverResponse, ()>;
     /// Query layout for the resolved value of a given CSS property
     fn resolved_style(&self) -> ResolvedStyleResponse;
@@ -139,7 +140,7 @@ pub struct NodeGeometryResponse {
     pub client_rect: Rect<i32>,
 }
 pub struct HitTestResponse(pub UntrustedNodeAddress);
-pub struct MouseOverResponse(pub Vec<UntrustedNodeAddress>);
+pub struct MouseOverResponse(pub UntrustedNodeAddress);
 pub struct ResolvedStyleResponse(pub Option<String>);
 
 #[derive(Clone)]