Notebook execution is stored in "internal metadata" That's fine but the problem is this source of truth is held in the extension host and not in the renderer. You can run into all kinds of trouble, e.g

• while a cell executes, select "F1 > restart extension host"
• execute the same cell from different extension host
• have a bad extension that never end an execution

Instead there should be one central place, maybe INotebookKernelService or a new service, that owns the truth about all executions and "projects" that truth onto internal metadata. The mainThreadXYZ parts should interact with that service and handle things like restart ext host etc