1. In the other cases (reactions and edit message) we include targetMessageID in the MessageInfo / RawMessageInfo. Can you explain why you aren't including it here?
2. Should this be a required parameter in the MessageData? It's only used on the keyserver side, so it shouldn't be risky. It's okay to make this change later (after you refactor keyserver code to always include this), but I think it should be a required parameter at the end of your stack

I think we don't need to include targetMessageID in *MessageInfo for sidebar source, because we are still keeping it in the content column, and sending it to the client via sourceMessage field. Reactions and edits don't do that, so they need to have targetMessageID in *MessageInfo.

An assumption I am working under: We don't want to rewrite the way we handle sidebar source messages (how we obtain the ID of their source message). We just want to have the target_message column filled.

Alternatively, if having both sourceMessage and targetMessageID in SidebarSourceMessageData is redundant and not a good pattern, I could just extract targetMessageID` from message.sourceMessage inside of messageCreator (here)

like so

let targetMessageID = null;
if (messageData.targetMessageID) {
  targetMessageID = messageData.targetMessageID;
} else if (messageData.sourceMessage) {
  targetMessageID = messageData.sourceMessage.id;
}

This would require no other changes in the code.