Auto merge of #15768 - ferjm:issue-15592-document-elementsFromPoint, r=emilio

Trigger reflow on document.elementsFromPoint

As [suggested](https://github.com/servo/servo/issues/15592#issuecomment-280379805) by @jdm `Document::nodes_from_point` now triggers a reflow.

I added a new reftest that panics with `ERROR:servo: Tried to hit test without a DisplayList` if this patch is not applied.

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15592.
- [X] There are tests for these changes

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components/script/dom/document.rs

@@ -1877,7 +1877,13 @@ impl Document {
             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)
+        if !self.window.reflow(ReflowGoal::ForScriptQuery,
+                               ReflowQueryType::NodesFromPoint(page_point, *client_point),
+                               ReflowReason::Query) {
+            return vec!();
+        };
+
+        self.window.layout().nodes_from_point_response()
     }
 }
 

components/script/dom/window.rs

@@ -1811,6 +1811,7 @@ fn debug_reflow_events(id: PipelineId, goal: &ReflowGoal, query_type: &ReflowQue
         ReflowQueryType::ContentBoxQuery(_n) => "\tContentBoxQuery",
         ReflowQueryType::ContentBoxesQuery(_n) => "\tContentBoxesQuery",
         ReflowQueryType::HitTestQuery(..) => "\tHitTestQuery",
+        ReflowQueryType::NodesFromPoint(..) => "\tNodesFromPoint",
         ReflowQueryType::NodeGeometryQuery(_n) => "\tNodeGeometryQuery",
         ReflowQueryType::NodeOverflowQuery(_n) => "\tNodeOverFlowQuery",
         ReflowQueryType::NodeScrollGeometryQuery(_n) => "\tNodeScrollGeometryQuery",