Fix crash when closing panes very quickly

[PankajBhojwani]

#17358 introduced a bug where if you open/close panes very rapidly Terminal will crash. This was because _content was being set to null and then a Close event was being emitted, but several functions attempt to access the pane's _content as part of the close routine. For example, TerminalTab tries to update the TaskbarProgress every time a pane is closed and as part of that update sequence it queries the pane - which has null content now - for the taskbar progress, resulting in a crash. This PR fixes that crash.

Refs #17358

[PankajBhojwani]

This change works fully when animations are disabled, however when pane animations are enabled and you rapidly open/close panes, we could end up in a state like this, where the pane's content is gone but doesn't end up getting removed from the UI tree (not a crash though):

Still working on remedying that, @lhecker let me know if you have thoughts on how to approach this

[lhecker]

Thanks for looking into this!

[lhecker]

I think the old code should work even if _content == nullptr. It's like moving an empty vector<>.

[lhecker]

Ah lol, I started reviewing the PR straight from the mail notification and didn't notice your comment.

Did you try adding a co_await resume_foreground() into the CloseRequested callback here?

terminal/src/cascadia/TerminalApp/Pane.cpp

Line 1733 in a80539c

_closeRequestedRevoker = _content.CloseRequested(winrt::auto_revoke, [this](auto&&, auto&&) { Close(); });

Another thing I can think of is this place:

terminal/src/cascadia/TerminalApp/Pane.cpp

Lines 1724 to 1726 in a80539c

	if (const auto c = _takePaneContent())
	{
	c.Close();

What if the call into IContent*::Close results in a callback back into the pane? Now that the _content has been "taken" (= nullptr) it may result in those issues you're trying to address in this PR. If that's the case, maybe we should simply not "take" the _content when calling close? Something like this:

void Pane::_setPaneContent(IPaneContent content)
{
    // Prevent reentrancy with the extra check?
    // Need to add _deferredClose as a new member.
    if (_content && _deferredClose != _content)
    {
        _deferredClose = _content;
        _content.Close();
        if (_deferredClose == _content)
        {
            _deferredClose = nullptr;
        }
    }

    // ...
}