diff --git a/components/layout_thread/lib.rs b/components/layout_thread/lib.rs
index 76146989fc1..1bfcf112c45 100644
--- a/components/layout_thread/lib.rs
+++ b/components/layout_thread/lib.rs
@@ -1318,6 +1318,7 @@ impl LayoutThread {
                                                          &mut layout_context);
         }
 
+        self.first_reflow.set(false);
         self.respond_to_query_if_necessary(&data.reflow_goal,
                                            &mut *rw_data,
                                            &mut layout_context,
@@ -1596,6 +1597,7 @@ impl LayoutThread {
         });
 
         self.perform_post_main_layout_passes(data,
+                                             root_flow,
                                              reflow_goal,
                                              document,
                                              rw_data,
@@ -1604,30 +1606,28 @@ impl LayoutThread {
 
     fn perform_post_main_layout_passes(&self,
                                        data: &Reflow,
+                                       mut root_flow: &mut FlowRef,
                                        reflow_goal: &ReflowGoal,
                                        document: Option<&ServoLayoutDocument>,
                                        rw_data: &mut LayoutThreadData,
                                        layout_context: &mut LayoutContext) {
         // Build the display list if necessary, and send it to the painter.
-        if let Some(mut root_flow) = self.root_flow.borrow().clone() {
-            self.compute_abs_pos_and_build_display_list(data,
-                                                        reflow_goal,
-                                                        document,
-                                                        FlowRef::deref_mut(&mut root_flow),
-                                                        &mut *layout_context,
-                                                        rw_data);
-            self.first_reflow.set(false);
+        self.compute_abs_pos_and_build_display_list(data,
+                                                    reflow_goal,
+                                                    document,
+                                                    FlowRef::deref_mut(&mut root_flow),
+                                                    &mut *layout_context,
+                                                    rw_data);
 
-            if opts::get().trace_layout {
-                layout_debug::end_trace(self.generation.get());
-            }
-
-            if opts::get().dump_flow_tree {
-                root_flow.print("Post layout flow tree".to_owned());
-            }
-
-            self.generation.set(self.generation.get() + 1);
+        if opts::get().trace_layout {
+            layout_debug::end_trace(self.generation.get());
         }
+
+        if opts::get().dump_flow_tree {
+            root_flow.print("Post layout flow tree".to_owned());
+        }
+
+        self.generation.set(self.generation.get() + 1);
     }
 
     fn reflow_all_nodes(flow: &mut Flow) {
diff --git a/tests/wpt/mozilla/meta/MANIFEST.json b/tests/wpt/mozilla/meta/MANIFEST.json
index 75e8b9a0a4a..8ea28218101 100644
--- a/tests/wpt/mozilla/meta/MANIFEST.json
+++ b/tests/wpt/mozilla/meta/MANIFEST.json
@@ -14546,6 +14546,12 @@
      {}
     ]
    ],
+   "mozilla/first-reflow-sheet-assert.html": [
+    [
+     "/_mozilla/mozilla/first-reflow-sheet-assert.html",
+     {}
+    ]
+   ],
    "mozilla/focus_blur.html": [
     [
      "/_mozilla/mozilla/focus_blur.html",
@@ -27925,6 +27931,10 @@
    "f2ec5f89521fb4eda64a5e70249bbfb8850c4278",
    "testharness"
   ],
+  "mozilla/first-reflow-sheet-assert.html": [
+   "8a0cf9152c3f2421d3e418f824f02c915092d78f",
+   "testharness"
+  ],
   "mozilla/focus_blur.html": [
    "2735c5c482e38743b0976789af858075c54408c1",
    "testharness"
diff --git a/tests/wpt/mozilla/tests/mozilla/first-reflow-sheet-assert.html b/tests/wpt/mozilla/tests/mozilla/first-reflow-sheet-assert.html
new file mode 100644
index 00000000000..268af6d333f
--- /dev/null
+++ b/tests/wpt/mozilla/tests/mozilla/first-reflow-sheet-assert.html
@@ -0,0 +1,16 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>Issue 18631: Assertion failure when double-adding the UA sheets to the page</title>
+<style>
+* { display: none }
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+// Trigger two forced reflows.
+test(function() {
+  document.documentElement.offsetTop;
+  document.documentElement.offsetTop;
+  assert_true(true, "Didn't assert");
+}, "Passes if it doesn't assert");
+</script>