Auto merge of #12813 - emilio:hit-test, r=notriddle

dom: getElementsFromPoint does the hit testing on viewport coordinates.

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We got this wrong (I think it wasn't my fault actually), I was just writing a test for #12777 when I found this.

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components/script/dom/document.rs

@@ -1583,12 +1583,12 @@ impl Document {
         self.browsing_context.is_none() || !url_has_network_scheme(&self.url)
     }
 
-    pub fn nodes_from_point(&self, page_point: &Point2D<f32>) -> Vec<UntrustedNodeAddress> {
-        let client_point =
-            Point2D::new(page_point.x - self.window.PageXOffset() as f32,
-                         page_point.y - self.window.PageYOffset() as f32);
+    pub fn nodes_from_point(&self, client_point: &Point2D<f32>) -> Vec<UntrustedNodeAddress> {
+        let page_point =
+            Point2D::new(client_point.x + self.window.PageXOffset() as f32,
+                         client_point.y + self.window.PageYOffset() as f32);
 
-        self.window.layout().nodes_from_point(*page_point, client_point)
+        self.window.layout().nodes_from_point(page_point, *client_point)
     }
 }