Bugzilla

There seems to be a time window between `AppShutdownConfirmed` and `AppShutdown` where we already notified existing processes that they will shutdown but continue to ask for new processes from events still in flight (and explicitly enforced to be processed before we advance the phase). Simply anticipating the existing checks inside `BeginSubprocessLaunch` and `CreateBrowser` to `AppShutdownConfirmed` however seemed to create some fallout that needs investigation. 

We might want to verify if we can better add some shutdown check at a higher level of the stack, like `DocShell` creation or `ChangeRemoteness` execution.

There seems to be a time window between `AppShutdownConfirmed` and `AppShutdown` where we already notified existing processes that they will shutdown but continue to ask for new processes from events still in flight (and explicitly enforced to be processed before we advance the phase). 

Bug 1810666 avoids that those requests will try to recycle existing processes that have been already slated for removal, but that means we will now try to create new processes that are immediately torn down again. Simply anticipating the existing checks inside `BeginSubprocessLaunch` and `CreateBrowser` to `AppShutdownConfirmed` however seemed to create some fallout that needs investigation. 

We might want to verify if we can better add some shutdown check at a higher level of the stack, like `DocShell` creation or `ChangeRemoteness` execution.