When editing the first message in a batch, all messages in the batch change (v5)

[graven]

To reproduce:

1. Write a batch of messages and make sure that they display in a single bubble.
2. Right-click on the topmost message and select "Edit message".
3. Change the text of the message.

Expected:
First message is changed to whatever is entered at step 3, the other messages in the batch are intact.

Actual:
All messages in the batch change to whatever is written in step 3.

Additional information:
Conversation parties see the changes in the batch as expected. Also, the same (erroneous) behavior manifests when a conversation party edits the first message of their batch.