Fix the DECTCEM reset position in the conpty stream (#17148)

## Summary of the Pull Request

When the conpty renderer determines that it needs to hide the cursor,
it does so by inserting a `DECTCEM` reset sequence at the start of the
output buffer, assuming that is the start of the frame. But when the
`_noFlushOnEnd` flag is set, you can have multiple frames pending in the
buffer, and the `DECTCEM` sequence will then end up in the wrong place.

This PR fixes the issue by saving the buffer size at the start of the
frame, and using that saved offset as the insert position for the
`DECTCEM` sequence.

## Validation Steps Performed

I have a game that was frequently affected by this issue (the cursor
would be visible when it was meant to be hidden). With this PR applied,
it now works perfectly.

## PR Checklist
- [x] Closes #15449

src/renderer/vt/XtermEngine.cpp

@@ -45,6 +45,7 @@ XtermEngine::XtermEngine(_In_ wil::unique_hfile hPipe,
     // visible, then PaintCursor will be called, and we'll set this to true
     // during the frame.
     _nextCursorIsVisible = false;
+    _startOfFrameBufferIndex = _buffer.size();
 
     // Do not perform synchronization clearing in passthrough mode.
     // In passthrough, the terminal leads and we follow what it is
@@ -112,7 +113,7 @@ XtermEngine::XtermEngine(_In_ wil::unique_hfile hPipe,
         // by prepending a cursor off.
         if (_lastCursorIsVisible != Tribool::False)
         {
-            _buffer.insert(0, "\x1b[?25l");
+            _buffer.insert(_startOfFrameBufferIndex, "\x1b[?25l");
             _lastCursorIsVisible = Tribool::False;
         }
         // If the cursor was NOT previously visible, then that's fine! we don't

src/renderer/vt/state.cpp

@@ -158,6 +158,7 @@ void VtEngine::_flushImpl() noexcept
     {
         const auto fSuccess = WriteFile(_hFile.get(), _buffer.data(), gsl::narrow_cast<DWORD>(_buffer.size()), nullptr, nullptr);
         _buffer.clear();
+        _startOfFrameBufferIndex = 0;
         if (!fSuccess)
         {
             LOG_LAST_ERROR();

src/renderer/vt/vtrenderer.hpp

@@ -93,6 +93,7 @@ namespace Microsoft::Console::Render
     protected:
         wil::unique_hfile _hFile;
         std::string _buffer;
+        size_t _startOfFrameBufferIndex = 0;
 
         std::string _formatBuffer;
         std::string _conversionBuffer;