Order of output mime types not respected by Notebook Output renderer

Does this issue occur when all extensions are disabled?: Yes/No

• VS Code Version: Stable
• OS Version: Any

Steps to Reproduce:

1. Create a Jupyter notebook with the following content

Sample ipynb file

{
"cells": [
  {
    "cell_type": "code",
    "execution_count": 10,
    "metadata": {},
    "outputs": [
      {
        "data": {
          "application/json": {
            "hello": 134
          },
          "text/html": [
            "<div>1234</div><ul><li>1</li><li>2</li><li>3</li></ul>"
          ],
          "text/plain": [
            "<__main__.Something at 0x131c3d5b0>"
          ]
        },
        "execution_count": 10,
        "metadata": {},
        "output_type": "execute_result"
      }
    ],
    "source": [
      "class Something():\n",
      "\tone = 1234\n",
      "\tdef _repr_html_(self):\n",
      "\t\treturn '<div>1234</div><ul><li>1</li><li>2</li><li>3</li></ul>'\n",
      "\n",
      "\tdef _repr_json_(self):\n",
      "\t\treturn {'hello':134}\n",
      "\n",
      "\n",
      "Something()"
    ]
  }
],
"metadata": {
  "kernelspec": {
    "display_name": "Python 3.9.13 ('.venvWidgets': venv)",
    "language": "python",
    "name": "python3"
  },
  "language_info": {
    "codemirror_mode": {
      "name": "ipython",
      "version": 3
    },
    "file_extension": ".py",
    "mimetype": "text/x-python",
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
    "version": "3.9.13"
  },
  "orig_nbformat": 4
},
"nbformat": 4,
"nbformat_minor": 2
}

1. Open the file in VS Code
2. The JSON output is displayed instead of HTML, this is wrong, the HTML output is richer and should be given preference

@connor4312 @mjbvz based on the code we seem to have some preference hardcoded in VS Code for mime types See here https://github.com/microsoft/vscode/blob/issue164304/src/vs/workbench/contrib/notebook/common/notebookCommon.ts#L43

export const NOTEBOOK_DISPLAY_ORDER: readonly string[] = [
	'application/json',
	'application/javascript',
	'text/html',
	'image/svg+xml',
	Mimes.latex,
	Mimes.markdown,
	'image/png',
	'image/jpeg',
	Mimes.text
];

I’m not entirely sure why this is there or what what it does, but my understanding of display order of mime types was:

• Extension needs to sort the output items in the order we would like them displayed. I.e. if the output has 2 mime types, then exnesion needs to ensure the mime types are sorted so that the first item is displayed.

Right now this results in a poor UX as users cannot see the HTML output instead end up with JSON which doesn’t necesssarily contain the best output for viewing.

[mjbvz]

I also don’t have any context on this. Let’s revisit it in November

[connor4312]

I agree that this seems outside the responsibility of the serializer to know about. Not having worked with the notebook API in a little while and just looking at it, it doesn’t look like a notebook type even needs a serializer. I think json should be just above ‘text’ at the end of the prioritization list, however.

In testing’s API, I had a similar use case around the TestRunProfile. In that case allowed extensions to set isDefault on a profile to determine if that profile should be used to run tests by default, provided the user hasn’t chosen different profile(s) for that test controller already. If API is warranted for notebooks here, adding a similar flag on NotebookCellOutputItem’s might be a more minimal approach than a priority list, and also allows extensions to control display on a per-output basis.