Incremental text output is broken

[ellisonbg]

Describe the bug

In JupyterLab master, if you run code that adds new text output, you get the existing output appending again and again.

To Reproduce

Open notebook and type the code:

Expected behavior

What is really weird is that if you open the same notebook in classic, with the output saved, it is fixed. Somehow, the output data is the same, it is how we display it.

[ellisonbg]

And if you save, close and open the same notebook in Lab is renders fine. So this is only while the code is running.

[jasongrout]

Is this the cause behind #6497?

[blink1073]

Maybe related to #6304?

[ellisonbg]

Yes I think this is related to the pymc issue, and likely caused by the recent change.I am going to focus on the UX work so if others can look into this that would be great :-)

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