I pushed a change to narrow the problem. I remembered vividly that almost 2 or 3 years ago @bpasero reported a similar issue with the same root cause. What happens is

• Mouse down on title bar item
  • open notebook editor
  • we restore view state, focus the editor in the cell
• Mouse up
  • the active element becomes tab control again, triggers a blur event
  • we set the cell focus mode to container (which means it should focus the container other than the inner editor) Even untitled files are not dirty until you start typing.

I think @roblourens made it work pretty well but not sure when it broke and how ...

A few things going on, so we do listen on onDidBlurEditorWidget and fix the cell's focusMode when the editor blurs. This doesn't take effect here because the FocusTracker delays firing its blur event with setTimeout, so it doesn't come until after the editor has had focus() called on it, so at that point, we think the cell is focused. But changing that would actually not be the right fix, because if we set focusMode to container, then we will not try to put focus back in the editor when the notebook editor gets focus. focusMode needs to be whether the cell editor is 'supposed to' have focus, not whether it actually does. So for now I'll just fix this by removing the check for whether focusMode actually changed before we fire the change event.

Maybe I should also change CodeCell/MarkdownCell's updateFocusMode to also check where focus went after onDidBlurEditorWidget. If it's inside the notebook editor, we should set focusMode to Container. If it's outside the notebook editor, it should stay as Editor. Also, maybe it would make sense for that to be a property on the editor instead of individual cells.