Reproduce steps:

Open two VS Code instances with same folder (Stable + Insiders)

• Instance A
  • Open notebook_1
• Instance B
  • Open notebook_1
  • Modify cell_1
  • Save
• Instance A
  • Make sure cell_1 is refreshed
  • Update cell_2
  • cell_2 is now never saved to disk

It seems that the auto reload on file change on disk triggers this, the UI is attached to a new text buffer/model, however the saving/snapshot code is using another one.

Slightly confused by those steps. Do you mean the same notebook in both instances and not notebook_1 and notebook_2? Also, should the 3. step really happen in instance B and back in instance A?

edit fixed the steps above because that's how I could reproduce

This is a problem with the NotebookCellTextModel. It uses two competing data sources: TextBuffer and TextModel and in this case the TextModel is correctly updated but the TextBuffer isn't. The bug occurs because for saving we use the textual value of the latter

Having these two data sources doesn't seem healthy and IMO the presence of a resolved text model should always shadow the text buffer so that there is only one source of truth

@jrieken when we create the text model, we actually use the text buffer from the cell as the source of truth

not sure how the disconnect of textmodel <-> textbuffer happens. I'll dig into this a bit more.

When we reload the window, we would create a notebook text model with new cell text models, and try to link alive monaco text models back to the cell models, but we didn't reuse the text buffer from them

Glad we nailed it before release, good catch and thanks all for the help.