Message timestamp flickers `undefined` when streaming

[dlqqq]

Description

The timestamp next to each message (e.g. 01:23) flickers to undefined when the message is being frequently updated, e.g. when Jupyter AI streams a reply back to the user.

This is visible from the demo in jupyterlab/jupyter-ai#1043, and I have reproduced this locally on the v3-dev branch as well.