[1.22] Fix teardown when 'allowHeadless' is enabled

[carlos-zamora]

Summary of the Pull Request

Fixes a bug where Monarch::_peasants would not be updated when the last window was closed and compatibility.allowHeadless was enabled.

Detailed Description of the Pull Request / Additional comments

The bug was originally caused by Monarch::_peasants still holding on to a reference to itself in the list. Normally, a Peasant was removed from the list via the WindowManager::SignalClose() (aka Monarch::SignalClose()) function. However, AppHost::_CloseRequested() would omit that call when the last window was closing.

Normally, that makes sense. It's the last window, so let's call AppHost::_quit() --> WindowManager::QuitAll() --> Monarch::QuitAll(), where we just call Peasant::Quit() on all the peasants and the monarch and we call it a day. Everything should be cleaned up and if we need a new session, we'll create a new Monarch anyways.

Problem is, when compatibility.allowHeadless is enabled, the WindowEmperor doesn't actually close! We want the WindowEmperor to stick around because that's the "headless" part we're allowing. See WindowEmperor::_decrementWindowCount():

terminal/src/cascadia/WindowsTerminal/WindowEmperor.cpp

Lines 296 to 308 in bbd6427

	void WindowEmperor::_decrementWindowCount()
	{
	// When we run out of windows, exit our process if and only if:
	// * We're not allowed to run headless OR
	// * we've explicitly been told to "quit", which should fully exit the Terminal.
	const bool quitWhenLastWindowExits{ !_app.Logic().AllowHeadless() };
	const bool noMoreWindows{ _windowThreadInstances.fetch_sub(1, std::memory_order_relaxed) == 1 };
	if (noMoreWindows &&
	(_quitting || quitWhenLastWindowExits))
	{
	_close();
	}
	}

The fix here was to avoid the QuitAll() code path mentioned above if compatibility.allowHeadless is enabled. Just treat this as a normal teardown otherwise so that the WindowEmperor creates a new Monarch when we need it.

Validation Steps Performed

✅ The scenario from the bug works:

• enable compatibility.allowHeadless
• set windowingBehavior to useExisting
• Open Terminal
• Close the only open window
• Attempting to open Terminal again should open a new window
  ✅ when all windows are closed, we still have the background process

Closes #18827

[DHowett]

Thanks! + @lhecker will you take a look too? deceptively easy fix

[DHowett]

I think the description is subtly wrong.

We only ever have one Monarch, and it lives forever.

[lhecker]

I... think? this should work. I also found the intricate dependencies in the old code somewhat confusing.