Bug 1782071 - Invalidate the undisplayed style cache on ContentRemoved. r=boris

See comment. Not sure how easy to test this is in practice since it
involves nodes getting cc'd.

I tried to repro (not too hard) with a crashtest running
SpecialPowers.gc() but that didn't cut it, looks like.

Differential Revision: https://phabricator.services.mozilla.com/D154891

layout/base/RestyleManager.cpp

@@ -376,8 +376,14 @@ void RestyleManager::ContentRemoved(nsIContent* aOldChild,
 
   // Computed style data isn't useful for detached nodes, and we'll need to
   // recompute it anyway if we ever insert the nodes back into a document.
-  if (aOldChild->IsElement()) {
-    RestyleManager::ClearServoDataFromSubtree(aOldChild->AsElement());
+  if (auto* element = Element::FromNode(aOldChild)) {
+    RestyleManager::ClearServoDataFromSubtree(element);
+    // If this element is undisplayed or may have undisplayed descendants, we
+    // need to invalidate the cache, since there's the unlikely event of those
+    // elements getting destroyed and their addresses reused in a way that we
+    // look up the cache with their address for a different element before it's
+    // invalidated.
+    IncrementUndisplayedRestyleGeneration();
   }
 
   // The container might be a document or a ShadowRoot.