--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -7669,16 +7669,19 @@ nsCSSFrameConstructor::ContentAppended(n
   // where frames can be moved around, determine if the list is for the
   // start or end of the block.
   if (nsLayoutUtils::GetAsBlock(parentFrame) && !haveFirstLetterStyle &&
       !haveFirstLineStyle && !parentIBSplit) {
     items.SetLineBoundaryAtStart(!prevSibling ||
                                  !prevSibling->IsInlineOutside() ||
                                  prevSibling->IsBrFrame());
     // :after content can't be <br> so no need to check it
+    //
+    // FIXME(emilio): A display: contents sibling could! Write a test-case and
+    // fix.
     items.SetLineBoundaryAtEnd(
         !nextSibling || !nextSibling->IsInlineOutside());
   }
   // To suppress whitespace-only text frames, we have to verify that
   // our container's DOM child list matches its flattened tree child list.
   items.SetParentHasNoXBLChildren(haveNoXBLChildren);
 
   nsFrameItems frameItems;