I still need to understand how to go through the testing suite.

You're in luck! For better or worse, this part of the code actually doesn't have any tests! So don't worry too much ☺️

You will likely need to run tools\runformat.cmd though, to get the automatic code-formatter to run 😄

I get this error when I try to run the code formatter - any suggestions for how to fix?

Also - any suggestions on what the icon should be @zadjii-msft - https://learn.microsoft.com/en-us/windows/apps/design/style/segoe-ui-symbol-font?

For whatever reason the cmd script wasn't working for me, but is this equivalent?

but is this equivalent

Yep looks like it! I bet the cmd version wasn't robust enough to be run from outside the solution root directory 🤷

Sorry, yea, this is an admittedly crazy obscure feature I don't think most folks have ever used:

Is there any place where we would know that there's only one pane left?

Update to this comment: The below suggestion I made does not work as ctl + shift + w uses ClosePane --> back to the drawing board :).

Does it make sense to call DetachPane instead of ClosePane in the menu item click?

closePaneMenuItem.Click([weakThis](auto&&, auto&&) { if (auto tab{ weakThis.get() }) { tab->DetachPane(); } });

Then in DetachPane it makes sense to me that we would know that only one pane is left.

std::shared_ptr<Pane>` TerminalTab::DetachPane()
    {
        // if we only have one pane, or the focused pane is the root, remove it
        // entirely and close this tab
        if (_rootPane == _activePane)
        {
            return DetachRoot();
        }
        // Attempt to remove the active pane from the tree
        if (const auto pane = _rootPane->DetachPane(_activePane))
        {
            // Just make sure that the remaining pane is marked active
            _UpdateActivePane(_rootPane->GetActivePane());

            if (_rootPane->GetLeafPaneCount() == 1)
            {
                this->_closePaneMenuItem.Visibility(WUX::Visibility::Collapsed);
            }

            return pane;
        }

        return nullptr;
    }

With these changes it seems like it resolves the edge case you mentioned. I'm very new to the codebase so if this is not quite the way you were thinking this should be implemented let me know :)

Not really sure what DetachPane vs. ClosePane does.

Is there any place where we would know that there's only one pane?

I'm not sure if this is what you were thinking but seems to work ok if the check is in _UpdateActivePane().