Ensure TermControl is not closing when firing UIA events (#14714)

The `SignalTextChanged` crash seems to be occurring due to the `TermControlAutomationPeer` being destructed by the time the UIA event is actually dispatched. Even though we're already checking if TCAP and TermControl still exist, it could be that the TermControl is being closed as text is being output.

The proposed fix here is to record when the closing process starts and exposing that information directly to the TCAP. If TCAP sees that we're in the process of closing, don't bother sending a UIA event.

Closes #13978

(cherry picked from commit 3dd4079)
Service-Card-Id: 87728855
Service-Version: 1.16

src/cascadia/TerminalControl/TermControl.cpp

@@ -1983,6 +1983,11 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         if (!_IsClosing())
         {
             _closing = true;
+            if (_automationPeer)
+            {
+                auto autoPeerImpl{ winrt::get_self<implementation::TermControlAutomationPeer>(_automationPeer) };
+                autoPeerImpl->Close();
+            }
 
             _RestorePointerCursorHandlers(*this, nullptr);
 

src/cascadia/TerminalControl/TermControlAutomationPeer.cpp

@@ -117,6 +117,13 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         }
     }
 
+    void TermControlAutomationPeer::Close()
+    {
+        // GH#13978: If the TermControl has already been removed from the UI tree, XAML might run into weird bugs.
+        // This will prevent the `dispatcher.RunAsync` calls below from raising UIA events on the main thread.
+        _termControl = {};
+    }
+
     // Method Description:
     // - Signals the ui automation client that the terminal's selection has changed and should be updated
     // Arguments:

src/cascadia/TerminalControl/TermControlAutomationPeer.h

@@ -49,6 +49,7 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         void UpdateControlBounds();
         void SetControlPadding(const Core::Padding padding);
         void RecordKeyEvent(const WORD vkey);
+        void Close();
 
 #pragma region FrameworkElementAutomationPeer
         hstring GetClassNameCore() const;