--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -10598,40 +10598,57 @@ void nsCSSFrameConstructor::CreateNeeded
                               pseudoStyle,
                               true, nullptr);
   newItem->mIsAllInline = true;
   newItem->mChildItems.SetParentHasNoXBLChildren(true);
   iter.InsertItem(newItem);
 }
 
 #ifdef DEBUG
+/**
+ * Returns true iff aFrame should be wrapped in an anonymous flex/grid item,
+ * rather than being a direct child of aContainerFrame.
+ *
+ * NOTE: aContainerFrame must be a flex or grid container - this function is
+ * purely for sanity-checking the children of these container types.
+ * NOTE: See also NeedsAnonFlexOrGridItem(), for the non-debug version of this
+ * logic (which operates a bit earlier, on FCData instead of frames).
+ */
 static bool
-FrameWantsToBeInAnonymousItem(const nsIAtom* aParentType, const nsIFrame* aFrame)
-{
-  MOZ_ASSERT(aParentType == nsGkAtoms::flexContainerFrame ||
-             aParentType == nsGkAtoms::gridContainerFrame);
-
-  return aFrame->IsFrameOfType(nsIFrame::eLineParticipant);
+FrameWantsToBeInAnonymousItem(const nsIFrame* aContainerFrame,
+                              const nsIFrame* aFrame)
+{
+  nsIAtom* containerType = aContainerFrame->GetType();
+  MOZ_ASSERT(containerType == nsGkAtoms::flexContainerFrame ||
+             containerType == nsGkAtoms::gridContainerFrame);
+
+  // Any line-participant frames (e.g. text) definitely want to be wrapped in
+  // an anonymous flex/grid item.
+  if (aFrame->IsFrameOfType(nsIFrame::eLineParticipant)) {
+    return true;
+  }
+
+  return false;
 }
 #endif
 
 static void
 VerifyGridFlexContainerChildren(nsIFrame* aParentFrame,
                                 const nsFrameList& aChildren)
 {
 #ifdef DEBUG
   auto parentType = aParentFrame->GetType();
   if (parentType != nsGkAtoms::flexContainerFrame &&
       parentType != nsGkAtoms::gridContainerFrame) {
     return;
   }
 
   bool prevChildWasAnonItem = false;
   for (const nsIFrame* child : aChildren) {
-    MOZ_ASSERT(!FrameWantsToBeInAnonymousItem(parentType, child),
+    MOZ_ASSERT(!FrameWantsToBeInAnonymousItem(aParentFrame, child),
                "frame wants to be inside an anonymous item, but it isn't");
     if (IsAnonymousFlexOrGridItem(child)) {
       AssertAnonymousFlexOrGridItemParent(child, aParentFrame);
       MOZ_ASSERT(!prevChildWasAnonItem, "two anon items in a row");
       nsIFrame* firstWrappedChild = child->PrincipalChildList().FirstChild();
       MOZ_ASSERT(firstWrappedChild, "anonymous item shouldn't be empty");
       prevChildWasAnonItem = true;
     } else {