Ending the active session gives undefined activeStackItem even if there is another session

[sandy081]

Testing #213603

Here is the extension code listening to activeStackItem

context.subscriptions.push(
        vscode.debug.onDidChangeActiveStackItem(session => {
            console.log('Active debug stack item changed: ', vscode.debug.activeStackItem);
        })
    );

Steps

• Open a workspace with two python files
• Install Python Extension
• Open file 1 with content print ('Hello File 1'). Keep a break point on this line and start debugging this file. As expected break point hits and the debugging extension logs this stackItem.
• Open file 2 with content print ('Hello File 2'). Keep a break point on this line and start debugging this file. As expected break point hits and the debugging extension logs this stackItem.
• Resume the second session to continue.

🐛 Extension logs undefined first for activeStackItem and after it logs the stack item of first session

[connor4312]

Thanks for the bug. This actually represents the view state accurately at the moment: we focus the session, not a thread, and this is not represented in the API. I'll see if consumers run into a case where they need this info, but unless I hear something from consumers I plan to leave as-is.