Outputs do not show up on collaborators screens

[krassowski]

Description

Here print(1 + 1) was executed by a collaborator, the output does not show

But an error shows up:

TypeError: Cannot read properties of undefined (reading 'streamText')
    at d._add (jlab_core.abd83dd7cfacd6da4d00.js?v=abd83dd7cfacd6da4d00:1:1264450)
    at d.add (jlab_core.abd83dd7cfacd6da4d00.js?v=abd83dd7cfacd6da4d00:1:1263879)
    at jlab_core.abd83dd7cfacd6da4d00.js?v=abd83dd7cfacd6da4d00:1:238322
    at 1168.8e5df3139aa020a611cb.js?v=8e5df3139aa020a611cb:1:551
    at q._onSharedModelChanged (jlab_core.abd83dd7cfacd6da4d00.js?v=abd83dd7cfacd6da4d00:1:238119)
    at m (jlab_core.abd83dd7cfacd6da4d00.js?v=abd83dd7cfacd6da4d00:1:1838868)
    at Object.l [as emit] (jlab_core.abd83dd7cfacd6da4d00.js?v=abd83dd7cfacd6da4d00:1:1838544)
    at a.emit (jlab_core.abd83dd7cfacd6da4d00.js?v=abd83dd7cfacd6da4d00:1:1836381)
    at Array._modelObserver (1168.8e5df3139aa020a611cb.js?v=8e5df3139aa020a611cb:1:7524)
    at Object.r (383.db345dbeef5ef774e50c.js?v=db345dbeef5ef774e50c:1:936)
e.exceptionHandler @ jlab_core.abd83dd7cfacd6da4d00.js?v=abd83dd7cfacd6da4d00:1
m @ jlab_core.abd83dd7cfacd6da4d00.js?v=abd83dd7cfacd6da4d00:1
l @ jlab_core.abd83dd7cfacd6da4d00.js?v=abd83dd7cfacd6da4d00:1
emit @ jlab_core.abd83dd7cfacd6da4d00.js?v=abd83dd7cfacd6da4d00:1
_modelObserver @ 1168.8e5df3139aa020a611cb.js?v=8e5df3139aa020a611cb:1
r @ 383.db345dbeef5ef774e50c.js?v=db345dbeef5ef774e50c:1
ji @ 383.db345dbeef5ef774e50c.js?v=db345dbeef5ef774e50c:1
(anonymous) @ 383.db345dbeef5ef774e50c.js?v=db345dbeef5ef774e50c:1
r @ 383.db345dbeef5ef774e50c.js?v=db345dbeef5ef774e50c:1
$o @ 383.db345dbeef5ef774e50c.js?v=db345dbeef5ef774e50c:1
Jo @ 383.db345dbeef5ef774e50c.js?v=db345dbeef5ef774e50c:1
Ci @ 383.db345dbeef5ef774e50c.js?v=db345dbeef5ef774e50c:1
Ai @ 383.db345dbeef5ef774e50c.js?v=db345dbeef5ef774e50c:1
vi @ 383.db345dbeef5ef774e50c.js?v=db345dbeef5ef774e50c:1
M @ 994.c290665e94ae9503337f.js?v=c290665e94ae9503337f:1
(anonymous) @ 994.c290665e94ae9503337f.js?v=c290665e94ae9503337f:1
J.<computed> @ 994.c290665e94ae9503337f.js?v=c290665e94ae9503337f:1
q @ 994.c290665e94ae9503337f.js?v=c290665e94ae9503337f:1
K.t.onmessage @ 994.c290665e94ae9503337f.js?v=c290665e94ae9503337f:1

Reproduce

1. Install jupyter-collaboration and jupyterLab 4.3.0b0 or just use this binder
2. Open a notebook
3. Open a new view as a collaborator in a different browser
4. Execute some cells producing text output
5. See no output

Expected behavior

Output shows up

Context

• JupyterLab version: 4.3.0b0
• Most likely introduced by Improve stream output handling #16498
• Related to Does not show outputs with JupyterLab 4.3.0b0 datalayer/jupyter-server-nbmodel#26

[krassowski]

This error comes from:

jupyterlab/packages/outputarea/src/model.ts

Lines 334 to 337 in 67cb8f8

	// We append the new text to the current text.
	// This creates a text change event.
	const prev = this.list.get(this.length - 1) as IOutputModel;
	const curText = prev.streamText!;

It looks like the code assumes that there always is a previous output of some kind.