From eae8d21428acdb3fb842df091ded0eb276bcaaaa Mon Sep 17 00:00:00 2001
From: janekptacijarabaci <janekptacijarabaci@seznam.cz>
Date: Tue, 24 Apr 2018 21:39:13 +0200
Subject: moebius#138: Optimize operations on root of deeply-nested frame tree

https://github.com/MoonchildProductions/moebius/pull/138
---
 dom/base/Element.cpp | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

(limited to 'dom/base/Element.cpp')

diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index 8b9808aa3..092755590 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -1844,6 +1844,24 @@ Element::UnbindFromTree(bool aDeep, bool aNullParent)
     SetParentIsContent(false);
   }
 
+#ifdef DEBUG
+  // If we can get access to the PresContext, then we sanity-check that
+  // we're not leaving behind a pointer to ourselves as the PresContext's
+  // cached provider of the viewport's scrollbar styles.
+  if (document) {
+    nsIPresShell* presShell = document->GetShell();
+    if (presShell) {
+      nsPresContext* presContext = presShell->GetPresContext();
+      if (presContext) {
+        MOZ_ASSERT(this !=
+                   presContext->GetViewportScrollbarStylesOverrideNode(),
+                   "Leaving behind a raw pointer to this node (as having "
+                   "propagated scrollbar styles) - that's dangerous...");
+      }
+    }
+  }
+#endif
+
   // Ensure that CSS transitions don't continue on an element at a
   // different place in the tree (even if reinserted before next
   // animation refresh).
-- 
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