Large notebook trigger very many leak warnings

[jrieken]

Testing #210961

slow.ipynb.txt

• open dev tools console
• open the notebook from above
• 😱 notice leak warnings from various places

This is critical: REFUSES to accept new listeners because it exceeded its threshold by far because it means a listener will never be used, meaning a feature is likely not working.

This seems to be a widespread misconception and it seems each cell listens by itself. So, more cells, more listener, leak warning and refusal to accept more listeners. Generally, listener should be used sparingly, e.g instead of each cell being a listener there should be one per notebook which dispatches onto cell.

[amunger]

One such case is for the new cellDiagnostics that I added (off by default).
https://insiders.vscode.dev/github/microsoft/vscode/blob/aamunger/focusEditor/src/vs/workbench/contrib/notebook/browser/contrib/cellDiagnostics/cellDiagnostics.ts#L62